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Chevrolet Caprice Classic 1991 - 1990-92 ENGINE PERFORMANCE 4.3L Self-Diagnostics - TBI Tests W/Codes
Chevrolet Caprice Classic 1991 - INTRODUCTION
Most engine control problems are the result of mechanical breakdowns, poor electrical connections or damaged vacuum hoses. Before considering the computer system as a possible cause of problems, perform checks and inspections covered in BASIC TESTING article in this section. Failure to do so may result in lost diagnostic time.
If no faults were found while performing BASIC DIAGNOSTIC PROCEDURES, proceed with DIAGNOSTIC PROCEDURE under SELF-DIAGNOSTIC SYSTEM. If no fault codes or only a non-running Code 12 is present and driveability problems exist, proceed to TESTS W/O CODES article in this section for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.). If only intermittent codes are present, see INTERMITTENTS in TESTS W/O CODES article in this section.
Chevrolet Caprice Classic 1991 - MODEL IDENTIFICATION
Chevrolet Caprice Classic 1991 MODEL IDENTIFICATION
Body Design Model Name "B" Caprice
Chevrolet Caprice Classic 1991 - SELF-DIAGNOSTIC SYSTEM
NOTE: Electronic Control Module (ECM) may also be referred to as Powertrain Control Module (PCM) in some diagnostic charts and figures. Terms are used interchangeably.
Control module is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, control module will illuminate SERVICE ENGINE SOON light located on instrument panel. When malfunction is detected and light is turned on, a corresponding trouble code will be stored in control module memory. To retrieve stored codes, see RETRIEVING CODES (NON-SCAN). Malfunctions are recorded as hard failures or as intermittent failures.
Chevrolet Caprice Classic 1991 - Hard Failures
Hard failures cause SERVICE ENGINE SOON light to glow and remain on until malfunction is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause of malfunction must be determined using diagnostic (code) charts. If a sensor fails, control module will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional, but it will most likely encounter degraded driveability.
Chevrolet Caprice Classic 1991 - Intermittent Failures
Intermittent failures cause SERVICE ENGINE SOON light to flicker or glow and go out about 10 seconds after intermittent fault goes away. Corresponding trouble code, however, will be retained in control module memory. If related fault does not reoccur within 50 engine starts, related trouble code will be erased from control module memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in TESTS W/O CODES article in this section.
Chevrolet Caprice Classic 1991 - DIAGNOSTIC PROCEDURE
Diagnosis of computerized engine control system should be performed in following order:
1) Ensure all engine systems not related to computer system are operating properly. DO NOT proceed with testing unless all other problems have been repaired. DIAGNOSTIC CIRCUIT CHECK must be performed before using trouble code charts. See BASIC TESTING article in this section.
2) If trouble codes were displayed (other than Code 12), determine whether codes are hard or intermittent trouble codes. Hard codes will cause SERVICE ENGINE SOON light to glow continuously while engine is running. See HARD OR INTERMITTENT TROUBLE CODE DETERMINATION. For diagnosing hard codes, proceed to appropriate trouble code chart. For diagnosing intermittent codes, proceed to INTERMITTENTS in TESTS W/O CODES article in this section. Exceptions are Code 13, 15, 24, 44 and 45 charts, which may be used to help diagnose intermittent codes.
3) If no trouble codes were displayed and a driveability problem exists, refer to SYMPTOMS in TESTS W/O CODES article in this section. Doing so will identify proper system or component to check in appropriate I - SYS/COMP TESTS article in this section.
4) After repairs are made, clear any trouble codes and perform FIELD SERVICE MODE CHECK in BASIC TESTING article in this section.
NOTE: For information on retrieving codes using a "Scan" tester, refer to user and reference manuals supplied with tester.
NOTE: Inserting jumper wire into test and ground terminals of ALDL connector with engine running will cause fuel injected vehicles to enter field service mode. Flashes of SERVICE ENGINE SOON light will not indicate codes if this is done. See FIELD SERVICE MODE CHECK in BASIC TESTING article in this section.
- Turn ignition on. DO NOT start engine. SERVICE ENGINE SOON light should glow. Locate Assembly Line Data Link (ALDL) connector attached to control module wiring harness. Most ALDL connectors are located under dash on driver side of vehicle. For exact location of ALDL, see COMPONENT LOCATIONS in appropriate I - SYS/COMP TESTS article in this section. Insert jumper wire from terminal "B" (diagnostic test terminal) to terminal "A" (ground) of ALDL connector. See Fig. 1 .
- SERVICE ENGINE SOON light should begin to flash codes. Each code will be repeated 3 times. If codes are not flashed or SERVICE ENGINE SOON light does not glow, perform DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article in this section. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.
Chevrolet Caprice Classic 1991 - READING TROUBLE CODES
Control module stores component failure information under a related trouble code which can be recalled for diagnosis and repair. Trouble codes may be read by counting flashes of SERVICE ENGINE SOON light or by reading digital display on a "Scan" tester. "Scan" tester is faster to use, more accurate and capable of reading information which otherwise would necessitate testing individual control module and sensor/solenoid connector terminals using a digital voltmeter. See SCAN TESTER USAGE and SCAN DATA.
NOTE: When using most "Scan" testers, a time delay exists between serial data updates. For instantaneous response, a digital voltmeter must be used.
If "Scan" tester is not available, reading flashes of SERVICE ENGINE SOON light is possible by grounding diagnostic test terminal "B" of ALDL with ignition on and engine off. For example, "FLASH, FLASH, pause, FLASH, longer pause" identifies Code 21. First series of flashes is first digit of trouble code. Second series of flashes is second digit of trouble code. Trouble codes are displayed starting with lowest numbered code. Each code is displayed 3 times. Codes will continue to repeat as long as ALDL test terminal is grounded.
NOTE: Trouble codes will be recorded at various operating times. Some codes require operation of that sensor or switch for 5 seconds; others require operation for 5 minutes or longer at normal operating temperature, vehicle speed and load. Therefore, some codes may not set in a service bay operational mode and may require road testing vehicle in order to duplicate condition under which code will set.
Chevrolet Caprice Classic 1991 - TROUBLE CODE DEFINITION
Chevrolet Caprice Classic 1991 ECM/PCM TROUBLE CODE DEFINITION
Code No. Circuit Affected 12 (1) No RPM Reference Pulse 13 Open Oxygen (O2) Sensor Circuit 14 CTS Signal Voltage Low 15 CTS Signal Voltage High 21 TPS Signal Voltage High 22 TPS Signal Voltage Low 23 MAT Sensor Signal Voltage High 24 Vehicle Speed Sensor Circuit 25 MAT Sensor Signal Voltage Low 26 Quad-Driver Error 32 EGR System Error 33 MAP Sensor Signal Voltage High 34 MAP Sensor Signal Voltage Low 35 IAC Idle Speed Error 42 EST Circuit Open Or Grounded 43 ESC Error 44 Lean Exhaust Indication 45 Rich Exhaust Indication 51 Faulty PROM, MEM-CAL Or ECM/PCM 52 Faulty/Missing CAL-PAC Or MEM-CAL 54 Fuel Pump Voltage Low 55 ECM/PCM Error
(1) Display of a Code 12 is normal when no reference pulses are received by control module (engine not running).
NOTE: Use trouble code charts only if SERVICE ENGINE SOON light is illuminated (indicating a current problem exists). Exceptions are Code 13, 15, 24, 44 and 45 charts, which may be used to help diagnose intermittent codes. Anytime control module-related Codes 51, 52 or 55 are displayed with another code, start with 50-series code, and then proceed to lower numbered codes.
Chevrolet Caprice Classic 1991 - HARD OR INTERMITTENT TROUBLE CODE DETERMINATION
During any diagnostic procedure, determine if codes are hard failure codes or intermittent failure codes. Diagnostic charts will not usually help analyze intermittent codes. To determine hard codes and intermittent codes:
- Manually enter diagnostic mode. Read and record all stored trouble codes. Exit diagnostic mode, and clear trouble codes. See CLEARING TROUBLE CODES.
- Apply parking brake, and place transmission in Neutral or Park. Block drive wheels, and start engine. SERVICE ENGINE SOON light should go out. Run warm engine at specified curb idle for 2 minutes and note SERVICE ENGINE SOON light.
- If SERVICE ENGINE SOON light comes on, manually enter diagnostic mode. Read and record trouble codes. This will reveal hard failure codes. Codes 13, 15, 24, 44 and 45 may require a road test to reset hard failure after trouble codes were cleared.
- If SERVICE ENGINE SOON light does not come on, all stored trouble codes were intermittent failures, except as noted above.
Chevrolet Caprice Classic 1991 - CLEARING TROUBLE CODES
Turn ignition switch to ON position, and ground diagnostic test terminal "B" at ALDL connector. Turn ignition switch to OFF position, and remove control module fuse from fuse block for 10 seconds. Replace fuse. Remove diagnostic terminal ground lead. If fuse cannot be located, pigtail at battery can be disconnected. When power to ECM is removed, degraded driveability may be exhibited until control module "relearns" optimum operational parameters.
Chevrolet Caprice Classic 1991 - ECM/PCM LOCATION
On most models, engine control module is located behind right or left side of dash or behind right or left kick panel. On Lumina, control module is located on right side of engine compartment. See COMPONENT LOCATIONS in appropriate I - SYS/COMP TESTS article in this section.
Chevrolet Caprice Classic 1991 - DIAGNOSTIC MATERIALS Diagnostic Aids
Diagnostic aids (located in many trouble code charts) are additional tips used to help diagnose trouble codes when inspected circuit is okay. Diagnostic aids may help lead to a definitive solution to trouble code problem.
Chevrolet Caprice Classic 1991 - Field Service Mode Check
If ALDL test terminal "B" is grounded with engine running, SERVICE ENGINE SOON light will indicate operational mode of engine. This test confirms proper operation of fuel system and verifies "closed loop" operation. Clear codes and perform this test after any repair is completed. Field service mode check can be found by proceeding to FIELD SERVICE MODE CHECK in BASIC DIAGNOSTIC PROCEDURES article.
Chevrolet Caprice Classic 1991 - SPECIAL TOOLS (DIAGNOSTIC)
NOTE: A "Scan" tester plugged into ALDL may be used to read trouble codes and check voltages in system on serial data line (terminal "E", or terminal "M" on P-4 systems). This can save a great deal of time. For additional information, see SCAN TESTER USAGE and SCAN DATA.
Computerized engine control system is most easily diagnosed using "Scan" tester; however, other tools may aid in diagnosing problems if a "Scan" tester is unavailable. These tools are a tachometer, test light, ohmmeter, digital voltmeter with 10-megohm input impedance (minimum), vacuum pump, vacuum gauge, fuel injector test lights (for both TBI and PFI) and 6 jumper wires 6" long (one wire with female connectors at both ends, one wire with male connectors at both ends and 4 wires with male and female connectors at opposite ends). A test light, rather than a voltmeter, must be used when indicated by a diagnostic chart.
Chevrolet Caprice Classic 1991 - SCAN TESTER USAGE
NOTE: Before connecting "Scan" tester to vehicle, diagnostic system should be checked to determine if system is operating properly and if information received will be accurate. This is done by performing DIAGNOSTIC CIRCUIT CHECK located in BASIC TESTING article in this section. If vehicle does not pass diagnostic circuit check, information received may be invalid.
"Scan" tester is a specialized tester which, when plugged into ALDL, can be used to diagnose on-board computer control systems by providing instant access to circuit voltage information without need to crawl under dash or hood to backprobe sensors and connectors.
"Scan" tester cuts down diagnostic time dramatically by furnishing input data (voltage signals) which can be compared to specification parameters. See SCAN DATA. They may also furnish information on output device (solenoids and motors) status. However, status parameters only indicate output signals have been sent to devices by control module; they do not indicate whether devices have responded properly to signal. Verify proper response at output device using a voltmeter or test light.
NOTE: Code 12 should always exist when ALDL is grounded with key on and engine not running, but it may not be indicated by all makes of "Scan" testers.
A problem may exist even if trouble codes are not present. About 80 percent of driveability problems occur without trouble codes. Sensors that are out of specification will not set a trouble code but will cause driveability problems.
Using "Scan" tester is easiest method of checking sensor specifications and other data parameters. Tester is also useful in finding intermittent wiring problems by wiggling wiring harnesses and connections (key on, engine off) while observing data parameters. See SCAN DATA.
NOTE: If erroneous voltage signals are suspected, verify tester information using a digital voltmeter and wiring schematic. If non-existent codes are displayed, turn ignition off, remove tester, turn ignition on and ground ALDL test terminal "B". Same codes flashed by SERVICE ENGINE SOON light should be indicated by "Scan" tester.
Chevrolet Caprice Classic 1991 - SCAN DATA
NOTE: Information contained in following tables is typical of readings taken on vehicle with engine idling, upper radiator hose hot, closed throttle, transmission in Park or Neutral, "closed loop" status achieved and all accessories off (except as noted in tables). Data parameters are updated every 1 1/4 seconds. On systems using P-4 computers, parameter updates are more frequent. Not all devices and systems are used on all models; following lists only represent most commonly used parameters. For additional information, refer to owner's manual furnished with tester.
Chevrolet Caprice Classic 1991 THROTTLE BODY INJECTION
Tester Position Units Measured Nominal Value A/C Clutch On/Off Off (On With A/C) A/C Request Yes/No No/Yes (With Request) AIR Divert Sol On/Off On (Air To Switching Sol.) Off (Air To Atmosphere) AIR Switching Sol. On/Off On (To Exhaust Manifold) Off (To Catalytic Converter) BARO Volts 3.0 - 4.5 Battery Voltage Volts 13.5 - 14.5 Block Learn Counts 118 - 138 (128 Normal) Brake Switch On/Off On When Engaged Canister Purge Sol On/Off On/Engine Cold (Idle Some) Clear Flood On/Off See Scan Tester Manual Coolant Fan On/Off Off Below 216?F (102?C) Coolant Temp. ?C 85 - 105? (Norm. Temperature) Crank RPM RPM 100 - 900 Cross Counts Counts 0 - 255 Cruise Cont. Sw. On/Off On When Engaged EGR Solenoid On/Off On When Energized EGR Duty Cycle 0 - 100% 0/Closed; 100/Fully Open Fan Relay On/Off On When Energized Fan Request On/Off On With Request Fuel Back-Up Yes/No Yes When Engaged IAC Counts 0 - 50 Ignition/Crank On/Off On With Ignition/Crank Injector Pulse Width Mil./Sec .8 - 3.0 INT (Integrator) Counts 110-145 (128 Normal) Knock Retard (ESC) Counts 0 - 255 Knock Signal Yes/No Yes When Knock Exists MAT ?C 10 - 90? MAP Volts 1.0 (Idle) to 4.5 (WOT) "Open/Closed Loop Status" Ol/Cl Closed/Open During Extended Idle O2 Sensor Millivolts 100 (Lean) To 999 (Rich) P/N Switch P/N/RDL Park/Neutral P/S Switch Norm/Hi Normal PROM I.D. PROM No. Original Factory Number RPM RPM Spec. +/-25 RPM Drive (A/T), Spec. +/-50 RPM Neut. (M/T) Spark Advance Degrees Varies TCC On/Off Off (On With Command) Throttle Angle 0-100% 0 (Idle) To 110 (WOT) TPS Volts 1.25 (Idle) To 5.00 (WOT) Trouble Codes Code No. No Codes Upshift Light (M/T) On/Off Off VSS Or MPH MPH 0 - Actual 3rd Gear Switch On/Off On/3rd & 4th Gear 4th Gear Switch On/Off On/4th Gear
Chevrolet Caprice Classic 1991 - PCM/BCM CODE CHARTS
NOTE: In following diagnostic flow charts, schematics and illustrations are courtesy of General Motors Corp.
Chevrolet Caprice Classic 1991 - CODE 13, OPEN OXYGEN (O2) SENSOR CIRCUIT
Chevrolet Caprice Classic 1991 CODE 13 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color O2 Signal D7 Purple O2 Ground D6 Tan
NOTE: Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
This tests if problem still exists. Vehicle cannot enter "closed loop" mode if O2 sensor circuit is open. Code 13 indicates an open in O2 sensor circuit. Code will set if:
- Engine is at normal operating temperature.
- Neither Code 21 nor Code 22 is stored.
- O2 sensor voltage is constant within a specified range (.34-.55 volt).
- Throttle angle is greater than idle.
- A precalibrated amount of time has elapsed since start-up.
- All conditions have existed for a precalibrated amount of time.
Chevrolet Caprice Classic 1991 - Test Number 2)
Determines if O2 sensor, wiring or control module is at fault. If wiring is good, grounding O2 sensor wire will cause .45 volt reference supplied by control module to pull low.
Chevrolet Caprice Classic 1991 - Test Number 3)
This tests O2 sensor circuit wiring. Use only a high impedance (10-megohm minimum) digital voltmeter.
NOTE: Caprice O2 sensor uses a heating element. Heating element is not controlled by ECM.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
Control module will not go into closed loop if Code 13 is set. Code 13 may set if vehicle runs out of fuel or stalls while vehicle is in motion. If O2 sensor ground becomes loose, a false O2 sensor reading will occur. This can result in a Code 13 being set.
Chevrolet Caprice Classic 1991 - CODE 14, COOLANT TEMP SENSOR, SIGNAL VOLTAGE LOW
Chevrolet Caprice Classic 1991 CODE 14 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color CTS Signal C10 Yellow CTS Ground A11 Black
NOTE: This chart assumes engine cooling system is functioning properly (not overheating). Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
Code 14 indicates control module has seen low coolant sensor voltage signal (high temperature) at control module terminal for a precalibrated period of time. This checks if conditions for Code 14 still exist.
Chevrolet Caprice Classic 1991 - Test Number 2)
This tests for grounded sensor signal line between control module and coolant sensor.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
After engine is started, temperature should rise steadily to about 190?F (88?C) and then stabilize when thermostat opens. At normal operating temperature, signal voltage at control module terminal should be 1.5-2.0 volts. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table. When Code 14 is set, control module will turn on electric cooling fan(s) if equipped.
Chevrolet Caprice Classic 1991 CODE 14 TEMPERATURE-TO-RESISTANCE VALUES (1)
Temperature ?F (?C) Ohms 212 (100) 185 160 (70) 450 100 (38) 1800 70 (20) 3400 20 (-7) 13,500 0 (-18) 25,000 -40 (-40) 100,700
(1) Measure resistance across sensor terminals.
Chevrolet Caprice Classic 1991 - CODE 15, COOLANT TEMP SENSOR, SIGNAL VOLTAGE HIGH
Chevrolet Caprice Classic 1991 CODE 15 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color CTS Signal C10 Yellow CTS Ground A11 Black
NOTE: Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
Code 15 indicates control module has seen high resistance in coolant sensor circuit. This could be due to high resistance (cold temperature) or high voltage at coolant sensor terminal at control module for a precalibrated period of time. This checks if conditions for Code 15 still exist.
Chevrolet Caprice Classic 1991 - Test Number 2)
This test simulates a low voltage condition. If control module recognizes low voltage signal, "Scan" tester will display greater than 130?C. This indicates control module and wiring are not at fault.
Chevrolet Caprice Classic 1991 - Test Number 3)
This test determines if coolant sensor ground or signal circuit is open.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
After engine is started, temperature should rise steadily to about 190?F (88?C) and then stabilize when thermostat opens. At normal operating temperature, voltage at control module sensor signal line should be 1.5-2.0 volts. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table. When Code 14 is set, control module will turn on electric cooling fan(s) if equipped.
Chevrolet Caprice Classic 1991 CODE 15 TEMPERATURE-TO-RESISTANCE VALUES (1)
Temperature - ?F (?C) Ohms 212 (100) 185 160 (70) 450 100 (38) 1800 70 (20) 3400 20 (-7) 13,500 0 (-18) 25,000 -40 (-40) 100,700
(1) Measure resistance across sensor terminals.
Chevrolet Caprice Classic 1991 - CODE 21, THROTTLE POSITION SENSOR, SIGNAL VOLTAGE HIGH
Chevrolet Caprice Classic 1991 CODE 21 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color TPS Signal C13 Dark Blue TPS Ground A11 Black TPS Reference C14 Gray
NOTE: Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
This test checks if code is result of a hard failure or an intermittent condition.
Chevrolet Caprice Classic 1991 - Test Number 2)
This test simulates a low-voltage condition. If control module recognizes change of state, control module and wiring are okay.
Chevrolet Caprice Classic 1991 - Test Number 3)
This step isolates a faulty sensor, control module or open sensor ground circuit. If sensor ground is shared by another sensor, an accompanying code related to that sensor may exist.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
A "Scan" tester displays throttle position in volts. Closed throttle voltage should be low. Voltage should increase gradually to about 4.5 volts at a steady rate as throttle angle is increased. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.
Chevrolet Caprice Classic 1991 - CODE 22, THROTTLE POSITION SENSOR, SIGNAL VOLTAGE LOW
Chevrolet Caprice Classic 1991 CODE 22 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color TPS Signal C13 Dark Blue TPS Ground A11 Black TPS Reference C14 Gray
NOTE: Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
This test checks if code is result of a hard failure or an intermittent condition.
Chevrolet Caprice Classic 1991 - Test Number 2)
This test simulates conditions for a Code 21. If control module recognizes change of state, control module and wiring are okay.
Chevrolet Caprice Classic 1991 - Test Number 3)
This simulates a high signal voltage to check for an open in TPS signal line to control module. "Scan" tester should recognize this signal and display high TPS voltage.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
A "Scan" tester displays throttle position in volts. Closed throttle voltage should be low. Voltage should increase gradually to about 4.5 volts at a steady rate as throttle angle is increased. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.
Chevrolet Caprice Classic 1991 - CODE 23, MAT SENSOR SIGNAL VOLTAGE HIGH
Chevrolet Caprice Classic 1991 CODE 23 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color MAT Signal C12 Tan MAT Ground A11 Black
NOTE: Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
This checks if code is result of a hard failure or an intermittent condition. Code 23 will set if engine has been running for a precalibrated period of time, has reached operating temperature and signal voltage indicates a MAT temperature less than -22?F (-30?C).
Chevrolet Caprice Classic 1991 - Test Number 2)
This simulates conditions for a Code 25. If "Scan" tester displays a high temperature, control module and wiring are not at fault.
Chevrolet Caprice Classic 1991 - Test Number 3)
This checks for continuity of sensor signal and ground circuits. If ground circuit is shared by other sensors and ground circuit is open, accompanying codes related to those sensors may be present.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
If engine is allowed to cool overnight, coolant and MAT sensor values should be close to each other when measured by "Scan" tester. Code 23 will result if signal and ground circuits become open. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table.
Chevrolet Caprice Classic 1991 CODE 23 TEMPERATURE-TO-RESISTANCE VALUES (1)
Temperature ?F (?C) Ohms 212 (100) 185 160 (70) 450 100 (38) 1800 70 (20) 3400 20 (-7) 13,500 0 (-18) 25,000 -40 (-40) 100,700
(1) Measure resistance across sensor terminals.
Chevrolet Caprice Classic 1991 - CODE 24, VEHICLE SPEED SENSOR (VSS)
NOTE: Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
Code 24 will set if VSS signal circuit voltage is constant, engine speed is 1200-4400 RPM, TPS indicates throttle near idle position, transmission not in Park or Neutral and all conditions met for at least 4 seconds.
Chevrolet Caprice Classic 1991 - Test Number 2)
A voltage of less than one volt at instrument panel connector indicates VSS signal circuit wire may be shorted to ground. Disconnect signal wire at VSS buffer. If voltage remains less than 10 volts, circuit is shorted to ground or open. If circuit is not open or shorted to ground, check for poor control module connection and faulty control module.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
A faulty or misadjusted park/neutral switch may set a false Code 24. Use "Scan" tester to check for proper signal in Drive while wiggling shifter.
NOTE: Disregard Code 24 that sets while wheels are not turning.
Chevrolet Caprice Classic 1991 - CODE 25, MAT SENSOR SIGNAL VOLTAGE LOW
Chevrolet Caprice Classic 1991 CODE 25 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color MAT Signal C12 Tan MAT Ground A11 Black
NOTE: Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
This checks if code is hard failure or intermittent condition. Code 25 will set if a MAT temperature greater than 266?F (130?C) is sensed for more than a precalibrated period.
Chevrolet Caprice Classic 1991 - Test Number 2)
This simulates condition for Code 23. If control module recognizes open circuit and "Scan" tester displays temperature of less than -30?C, control module and wiring are okay.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
If engine is allowed to cool overnight, coolant temperature sensor and MAT sensor values should be close to each other when measured by a "Scan" tester. A Code 25 will result if sensor signal circuit is shorted to ground. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table.
Chevrolet Caprice Classic 1991 CODE 25 TEMPERATURE-TO-RESISTANCE VALUES (1)
Temperature ?F (?C) Ohms 212 (100) 185 160 (70) 450 100 (38) 1800 70 (20) 3400 20 (-7) 13,500 0 (-18) 25,000 -40 (-40) 100,700
(1) Measure resistance across sensor terminals.
Chevrolet Caprice Classic 1991 - CODE 32, EGR SYSTEM ERROR (1 OF 3) Test Number 1)
Plugged Intake Passage - Shut off engine, and remove EGR valve. Plug exhaust side port using a shop rag or suitable plug. Attempt to start engine. If engine runs at a very high idle (up to 3000 RPM is possible) or if engine starts and stalls, EGR intake passage is not plugged. If vehicle starts and idles normally, intake side passage is plugged.
Plugged Exhaust Passage - With EGR valve removed, plug intake manifold side passage using a suitable plug. Start engine and listen for exhaust noise. If no exhaust gas escapes from open EGR port, exhaust passage is plugged.
Chevrolet Caprice Classic 1991 - Test Number 2)
By grounding ALDL test terminal "B", EGR solenoid should energize and allow vacuum to be applied to gauge. Vacuum at gauge may or may not slowly bleed off; however, gauge is able to read amount of vacuum being applied.
Chevrolet Caprice Classic 1991 - Test Number 3)
When ALDL test terminal is ungrounded, gauge vacuum should bleed off through a vent in solenoid. Pump gauge vacuum may or may not bleed off; this does not indicate a problem.
Chevrolet Caprice Classic 1991 - Test Number 4)
This test determines if electrical control part of system is at fault or if connector or solenoid is at fault.
Chevrolet Caprice Classic 1991 - Test Number 5)
EGR valves used with this engine are stamped "P" for positive backpressure or "N" for negative backpressure. F body uses negative backpressure EGR valve only. Proceed to appropriate chart for valve being tested.
Chevrolet Caprice Classic 1991 - CODE 32, EGR SYSTEM ERROR (2 OF 3)
ECM operates a normally closed solenoid to regulate vacuum to EGR valve. When ECM provides a path to ground for EGR solenoid winding, solenoid opens, allowing vacuum to pass through to EGR valve. ECM monitors EGR effectiveness by shutting off vacuum to EGR valve during a steady cruise in excess of 50 MPH and monitoring change in MAP sensor vacuum signal. If vacuum signal change is not within a precalibrated window, Code 32 will be set.
NOTE: Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 6)
Remaining tests check ability of EGR valve to interact with exhaust system. This system uses a positive backpressure EGR valve which will not hold vacuum until sufficient exhaust backpressure is at base of EGR valve.
Chevrolet Caprice Classic 1991 - Test Number 7)
EGR valve diaphragm should move when sufficient backpressure is at base of valve and when vacuum is being supplied to valve. Rapidly snapping throttle from idle should provide sufficient backpressure to close internal valve vacuum bleed. With valve bleed closed, jumpered vacuum supply can now lift valve off its seat.
Chevrolet Caprice Classic 1991 - Test Number 8)
Excessive exhaust backpressure from bent or restricted exhaust system components could provide enough backpressure at base of EGR valve to allow undesired EGR action at idle.
Chevrolet Caprice Classic 1991 - Test Number 9)
Plugged EGR exhaust passages can block exhaust backpressure at base of EGR valve. If this occurs, internal EGR valve bleed will remain open, preventing vacuum from operating valve.
Chevrolet Caprice Classic 1991 - CODE 32, EGR SYSTEM ERROR (3 OF 3)
NOTE: Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 6)
Remaining tests check ability of EGR valve to interact with exhaust system. This system uses a negative backpressure EGR valve which will hold vacuum with engine off.
Chevrolet Caprice Classic 1991 - Test Number 7)
When engine is started, exhaust backpressure at base of EGR valve should open valve's internal bleed. This will vent applied vacuum, allowing valve to seat.
NOTE: Negative backpressure valve can be identified by the presence of the letter "N" in part number.
Chevrolet Caprice Classic 1991 - CODE 33, MAP SENSOR, SIGNAL VOLTAGE HIGH
Chevrolet Caprice Classic 1991 CODE 33 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color MAP Signal C11 Light Green MAP Ground D2 Purple MAP Reference C14 Gray
NOTE: Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
This test confirms Code 33 and determines if it is result of a hard failure or an intermittent condition. Code 33 will set when voltage signal reading is too high for greater than a precalibrated period of time, TPS voltage indicates throttle is closed and neither Code 21 nor 22 is present.
Chevrolet Caprice Classic 1991 - Test Number 2)
This step simulates conditions for a Code 34. If control module recognizes and sets Code 34, low MAP signal, control module and 5-volt reference and MAP signal circuits are not at fault. If ground circuit is shared with other sensors and ground circuit becomes open, additional codes related to these sensors may be set.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
With ignition switch in ON position and engine stopped, manifold pressure is equal to atmospheric pressure and signal voltage will be high. Comparison of BARO readings from a known good vehicle using same sensor is a good way to check accuracy of suspect sensor. Readings should be same within +/- .4 volt. Code 33 will result if ground circuit is open or MAP signal circuit is shorted to voltage or to 5-volt reference circuit.
Chevrolet Caprice Classic 1991 - CODE 34, MAP SENSOR, SIGNAL VOLTAGE LOW
Chevrolet Caprice Classic 1991 CODE 34 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color MAP Signal C11 Light Green MAP Ground D2 Purple MAP Reference C14 Gray
NOTE: Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
This confirms Code 34 and determines if code was caused by a hard failure or an intermittent fault. Code 34 will set when ignition is on and MAP signal voltage is low. On some systems, engine must be running to set code.
Chevrolet Caprice Classic 1991 - Test Number 2)
Jumpering MAP signal to 5-volt reference at MAP harness connector will determine if sensor is at fault or if a problem exists with control module or wiring.
Chevrolet Caprice Classic 1991 - Test Number 3)
"Scan" tester may not display 12 volts. Control module recognizes voltage as greater than 4 volts (high MAP voltage signal), indicating control module and MAP signal circuit are not at fault.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
With ignition switch in ON position and engine stopped, manifold pressure is equal to atmospheric pressure and signal voltage will be high. Comparing BARO readings with a known good vehicle using same sensor is a good way to check accuracy of suspect sensor. Readings should be same within +/- .4 volt. A Code 34 will also result if 5-volt reference and MAP signal circuits are open or shorted to ground.
Chevrolet Caprice Classic 1991 - CODE 35, IAC IDLE SPEED ERROR, W/O TECH 1
Code 35 will set when closed throttle engine speed is 150 RPM greater or less than correct idle speed for 20 seconds.
NOTE: Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
IAC driver is used to extend and retract IAC valve. Movement is verified by an engine speed change. If no change in speed occurs, valve can be retested when removed from throttle body.
Chevrolet Caprice Classic 1991 - Test Number 2)
Checks IAC movement quality from step 1). Between 700-1500 RPM, engine speed should change smoothly with each flash of tester light in both extend and retract. If IAC valve is retracted beyond control range (about 1500 RPM), many flashes in extend position may occur before engine speed begins to drop. This is normal on certain engines. Fully extending IAC may cause engine to stall. This may be normal.
Chevrolet Caprice Classic 1991 - Test Number 3)
Steps 1) and 2) verified proper IAC valve operation while this step checks IAC circuits. Each light on node light should flash red and green while IAC valve is cycled. While sequence of color is not important, check circuits for faults beginning with poor terminal contacts if either light is off or does not flash red and green.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
A slow, unstable idle may be caused by a system problem which cannot be overcome by IAC. "Scan" counts will be greater than 60 if idle is too low and zero counts if idle is too high. If idle is too high, stop engine. Fully extend IAC using driver. Start engine. If idle speed is greater than 800 RPM, look for possible vacuum leaks.
Chevrolet Caprice Classic 1991 - System Too Lean
If air/fuel ratio is too lean, idle speed may be either too high (check for vacuum leaks) or too low. Engine speed may vary up and down; disconnecting IAC may not help. "Scan" and/or digital voltmeter (10-megohm) will read an oxygen (O2) sensor output less than 300 mv (.3 volt). Check for low fuel pressure or water in fuel. A contaminated O2 sensor (caused by silicone) will produce lean air/fuel mixtures with an O2 sensor output fixed greater than 800 mv (.8 volt). This may also set Code 45.
Chevrolet Caprice Classic 1991 - System Too Rich
If air/fuel ratio is too rich, idle speed will be too low and "Scan" tester counts will usually be greater than 80. System may be obviously rich, with black smoke from exhaust pipe. "Scan" tester and/or voltmeter will read an O2 sensor voltage signal fixed greater than 800 mv (.8 volt). Look for high fuel pressure and injectors leaking or sticking. Remove IAC, and inspect bore for foreign material and evidence of IAC valve dragging bore.
Chevrolet Caprice Classic 1991 - Throttle Body
Remove IAC, and inspect bore for evidence of IAC valve dragging.
Chevrolet Caprice Classic 1991 - IAC Valve Connections
Inspect carefully for loose or corroded connections.
Chevrolet Caprice Classic 1991 - PCV Valve
An incorrect PCV valve may cause incorrect idle speed.
Chevrolet Caprice Classic 1991 - CODE 35, IAC IDLE SPEED ERROR, USING TECH I
Code 35 will set when closed throttle engine speed is 150 RPM greater or less than correct idle speed for 20 seconds.
NOTE: Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
Tech 1 RPM control mode is used to extend and retract IAC valve. Movement is verified by an engine speed change. If no change in speed occurs, valve can be retested when removed from throttle body. If IAC valve is retracted beyond control range (about 1500 RPM), many flashes in extend position may occur before engine speed begins to drop. This is normal on certain engines. Fully extending IAC may cause engine to stall. This may be normal.
Chevrolet Caprice Classic 1991 - Test Number 2)
This test uses Tech 1 to command IAC-controlled idle speed. Control module issues commands to obtain requested idle speed. Each light on node light should flash red and green while IAC valve is cycled. While sequence of color is not important, check circuits for faults beginning with poor terminal contacts if either light is off or does not flash red and green.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
A slow, unstable idle may be caused by a system problem which cannot be overcome by IAC. "Scan" counts will be greater than 60 if idle is too low and zero counts if idle is too high. If idle is too high, stop engine. Fully extend IAC with driver. Start engine. If idle speed is greater than 800 RPM, look for possible vacuum leaks.
Chevrolet Caprice Classic 1991 - System Too Lean
If air/fuel ratio is too lean, idle speed may be either too high (check for vacuum leaks) or too low. Engine speed may vary up and down; disconnecting IAC may not help. "Scan" and/or digital voltmeter (10-megohm) will read an oxygen (O2) sensor output less than 300 mv (.3 volt). Check for low fuel pressure or water in fuel. A contaminated O2 sensor (caused by silicone) will produce lean air/fuel mixtures with an O2 sensor output fixed greater than 800 mv (.8 volt). This may also set Code 45.
Chevrolet Caprice Classic 1991 - System Too Rich
If air/fuel ratio is too rich, idle speed will be too low and "Scan" tester counts will usually be greater than 80. System may be obviously rich, with black smoke from exhaust pipe. "Scan" tester and/or voltmeter will read an O2 sensor voltage signal fixed greater than 800 mv (.8 volt). Look for high fuel pressure and injectors leaking or sticking. Remove IAC, and inspect bore for foreign material and evidence of IAC valve dragging bore.
Chevrolet Caprice Classic 1991 - Throttle Body
Remove IAC, and inspect bore for evidence of IAC valve dragging.
Chevrolet Caprice Classic 1991 - IAC Valve Connections
Inspect carefully for loose or corroded connections.
Chevrolet Caprice Classic 1991 - PCV Valve
An incorrect PCV valve may cause incorrect idle speed.
Chevrolet Caprice Classic 1991 - CODE 42, EST CIRCUIT OPEN OR GROUNDED
Code 42 indicates ECM has seen an open or short to ground in HEI EST or by-pass circuit.
NOTE: Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
This test confirms Code 42 and determines if fault is a hard failure or intermittent condition.
Chevrolet Caprice Classic 1991 - Test Number 2)
This test checks for a normal EST ground path through ignition module. If EST circuit is shorted to ground, reading will be less than 500 ohms.
Chevrolet Caprice Classic 1991 - Test Number 3)
As test light voltage touches by-pass circuit, module should switch. This will cause ohmmeter to switch from hundreds of ohms to thousands of ohms. This test assures module "switched".
Chevrolet Caprice Classic 1991 - Test Number 4)
If module did not switch, this tests for a short in EST circuit, an open in by-pass circuit and a faulty ignition module connection or module.
Chevrolet Caprice Classic 1991 - Test Number 5)
This step confirms Code 42 is a faulty ECM and not an intermittent problem in EST or by-pass circuits.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
"Scan" tester does not have ability to help diagnose a Code 42 problem. See INTERMITTENTS in TESTS W/O CODES article in this section.
Chevrolet Caprice Classic 1991 - CODE 43, ESC ERROR
NOTE: Test numbers refer to numbers on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
If conditions for a Code 43 exist, "Scan" tester will indicate knock signal presence. Knock signal should not exist at idle unless an internal or system problem exists.
Chevrolet Caprice Classic 1991 - Test Number 2)
Determines if system is currently functioning. Usually, a knock signal can be generated by tapping on exhaust manifold. If knock signal is not generated, try tapping on engine block closer to sensor.
Chevrolet Caprice Classic 1991 - Test Number 3)
Because Code 43 sets when signal voltage on spark retard line remains low, this test should cause signal on that line to go high. Control module should see 12-volt signal as a "no knock" signal if control module and wiring are okay.
Chevrolet Caprice Classic 1991 - Test Number 4)
This test will determine if knock signal is being detected on sensor-to-controller line or if ESC module is at fault.
Chevrolet Caprice Classic 1991 - Test Number 5)
If sensor line is routed too close to secondary ignition wires, ESC module may see induced interference as a knock signal.
Chevrolet Caprice Classic 1991 - Test Number 6)
This checks ground circuit to module. An open ground will cause voltage on monitored line to constantly remain about 12 volts. This would cause Code 43 functional test to fail.
Chevrolet Caprice Classic 1991 - Test Number 7)
Contacting sensor-to-controller wire using a test light connected to 12 volts will generate a knock signal to controller. This will determine if ESC controller is operating correctly.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
Code 43 can be caused by a faulty connection at knock sensor, ESC module or control module. Check controller-to-control module signal line for an open or short to ground. Check for poor connections and damaged harness. Inspect control module harness connectors for backed-out sensor signal and ECM input signal terminals, improper mating, broken locks, improperly formed or damaged terminals and damaged harness. If connections and harness are okay, monitor knock signal parameter using "Scan" tester while moving related connectors and wiring harness. If failure is induced, knock signal will abruptly change. This may help to isolate malfunction.
Chevrolet Caprice Classic 1991 - CODE 43, ESC ERROR, W/O ESC MODULE
NOTE: Test numbers refer to numbers on diagnostic chart. If an audible knock is heard from engine at idle, repair internal engine problems before proceeding.
Chevrolet Caprice Classic 1991 - Test Number 1)
If tapping on engine does not produce a knock signal on "Scan" tester, try tapping engine closer to sensor before proceeding.
Chevrolet Caprice Classic 1991 - Test Number 2)
ECM supplies a 5-volt reference signal on knock sensor line. With knock sensor connected, this signal is pulled low to about 2.5 volts. Knock signal, which is an AC signal, rides on this 2.5-volt DC signal.
Chevrolet Caprice Classic 1991 - Test Number 3)
This test determines if knock sensor is faulty or if ESC portion of MEM-CAL is faulty.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
Check knock sensor signal circuit for a potential open or short to ground. An open knock signal circuit would cause 5-volt reference to remain high at ECM knock signal terminal. A short to ground in knock signal circuit would cause 5-volt reference signal to pull low to near zero volts. Resistance to ground through knock sensor should be 3300-4500 ohms. Also check for proper installation of MEM-CAL.
Chevrolet Caprice Classic 1991 - CODE 44, LEAN EXHAUST INDICATION
Chevrolet Caprice Classic 1991 CODE 44 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color O2 Signal D7 Purple O2 Ground D6 Tan
O2 sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600?F (316?C). An open sensor circuit or cold sensor causes "open loop" operation.
NOTE: Test number refers to number on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
Checks to see if O2 sensor is registering a lean condition. Code 44 is set when O2 sensor voltage signal at control module is low (less than .3 volt) for a precalibrated period and system is operating in "closed loop".
Chevrolet Caprice Classic 1991 - Diagnostic Aids
Using "Scan" tester, observe Block Learn Memory (BLM) value at different RPMs. If conditions for a Code 44 exist, block learn value will be about 150.
Chevrolet Caprice Classic 1991 - O2 Sensor Wire
O2 sensor wire may be mispositioned and laying against exhaust manifold. Check for ground between sensor and wire connector.
Chevrolet Caprice Classic 1991 - Fuel Contamination
Water, even small amounts, near in-tank fuel pump inlet can be delivered to injector. Water may cause a lean exhaust and set Code 44.
Chevrolet Caprice Classic 1991 - Fuel Pressure
System will be lean if fuel pressure is low. If necessary, monitor fuel pressure while driving vehicle. For fuel pressure checking procedure, see BASIC TESTING article in this section.
Chevrolet Caprice Classic 1991 - Exhaust Leaks
If exhaust system has large leaks, exhaust system negative pressure pulses can cause outside air to be drawn into system and past O2 sensor. Vacuum or crankcase leaks can also cause a lean condition.
Chevrolet Caprice Classic 1991 - Misfire Or Stall
If engine misfires or stalls (including running out of fuel) while vehicle is moving, a Code 44 may set. If Code 44 is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.
Chevrolet Caprice Classic 1991 - CODE 45, RICH EXHAUST INDICATION
Chevrolet Caprice Classic 1991 CODE 45 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color O2 Signal D7 Purple O2 Ground D6 Tan
O2 sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600?F (316?C). An open sensor circuit or cold sensor causes "open loop" operation.
Code 45 indicates a rich exhaust. Diagnosis should begin with fuel pressure, leaking injector, HEI shielding (ground), vapor canister fuel saturation, coolant sensor, MAP sensor, O2 sensor contamination and TPS intermittent output.
NOTE: Test number refers to number on diagnostic chart.
Chevrolet Caprice Classic 1991 - Test Number 1)
Test checks to see if O2 sensor is registering a rich condition. Code 45 is set when vehicle is at operating temperature (in "closed loop"), throttle angle is greater than idle, O2 sensor signal at control module is greater than .7 volt for a precalibrated period and time since engine start is one minute or more.
Chevrolet Caprice Classic 1991 - Diagnostic Aids
If other codes of lower number are set with Code 45, use those charts first. Malfunction in MAP or TPS sensor circuits can cause a Code 45 to set. If other codes are not set, Code 45, rich exhaust, is most likely caused by:
Chevrolet Caprice Classic 1991 - Fuel Pressure High
If fuel pressure is too high, air/fuel ratio will be rich. For fuel pressure checking procedure, see BASIC TESTING article in this section. Control module can compensate for slight increases, but a Code 45 will be set if air/fuel ratio becomes too rich.
Chevrolet Caprice Classic 1991 - Ignition Ground
If an open occurs on HEI ground circuit, HEI induced electrical "noise" may result, causing simulated reference pulses to be picked up by control module on reference line of EST harness. Additional pulses result in a higher than actual engine speed signal. Control module will increase injector pulse width ("on" time) to match increased RPM signal. "Scan" tester will show higher than actual RPM, which can help in diagnosing this problem.
Chevrolet Caprice Classic 1991 - Evaporative Fuel Canister
Fuel saturation of charcoal canister will cause a rich air/fuel ratio. If canister is full of fuel, check canister control valves and hoses.
Chevrolet Caprice Classic 1991 - MAP Sensor
An output causing control module to sense a higher than normal manifold pressure (low vacuum) can cause system to go rich. Disconnecting MAP sensor will allow control module to substitute a fixed value for MAP sensor. If condition disappears, substitute a different MAP sensor, and continue testing.
Chevrolet Caprice Classic 1991 - TPS
An intermittent TPS output will cause system to operate rich due to a false indication of engine acceleration.
Chevrolet Caprice Classic 1991 - O2 Sensor Contamination
O2 sensor contamination, caused by silicone in certain fuels or use of improper RTV sealant, may cause a white powdery coating to cover exterior of O2 sensor. False high signal voltage (low oxygen content sensed) produced is interpreted by control module as a rich mixture, causing control module to set Code 45.
Chevrolet Caprice Classic 1991 - EGR Problem
EGR valve sticking open at idle is usually accompanied by a rough idle and/or stalling.
Also check for shorted or leaking injector and fuel-contaminated oil. If Code 45 is intermittent, see INTERMITTENTS in the TESTS W/O CODES article in this section.
Chevrolet Caprice Classic 1991 - CODE 51, FAULTY PROM/MEM-CAL
Ensure all pins are fully inserted in socket. If pins are okay, replace PROM/MEM-CAL, clear memory and recheck. If Code 51 reappears, replace control module.
Chevrolet Caprice Classic 1991 - CODE 52, FAULTY CAL-PAK
Ensure all pins are fully inserted in socket. If pins are okay, replace CAL-PAK, clear memory and recheck. If Code 51 reappears, replace control module.
Chevrolet Caprice Classic 1991 - CODE 54, FUEL PUMP VOLTAGE LOW
ECM monitors fuel pump pressure switch circuit voltage for fuel system adjustments. Signal is also used to store a trouble code if fuel pump relay is defective or fuel pump voltage is lost while engine is running. About 12 volts should exist on ECM fuel pump voltage monitor circuit for 2 seconds after ignition is turned on or any time reference pulses are being received by ECM.
Code 54 will set if voltage on voltage monitor circuit is low for 2-4 seconds since last reference pulse is cycled off. However, if voltage detected is low with engine running, light will only remain on while condition exists. If no fault is found, refer to INTERMITTENTS in TESTS W/O CODES in this section.
Chevrolet Caprice Classic 1991 - CODE 55, ECM ERROR
Ensure correct MEM-CAL or PROM is being used and that it is properly installed. If so, replace control module. Clear codes, confirm closed loop operation and check operation of SERVICE ENGINE SOON light.
Chevrolet Caprice Classic 1991 - SUMMARY
If no hard fault codes are present, driveability symptoms exist or intermittent codes exist, proceed to TESTS W/O CODES article in this section for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.) or intermittent diagnostic procedures. PCM/BCM CODE CHARTS