Engine Controls - Tests W/o Codes - Gasoline: Other Chevrolet Pickup C2500

Use this portion of article only AFTER performing these checks

1. Ensure on-vehicle diagnostics work (if equipped). Perform DIAGNOSTIC CIRCUIT CHECK in «BASIC TESTING - GASOLINE»(ref-19753) article in the ENGINE PERFORMANCE section.
2. Ensure PCM and MIL function properly.
3. Ensure no trouble codes, except intermittent ones, are stored.
4. Ensure fuel control system operates properly. Perform FIELD SERVICE MODE CHECK in «BASIC TESTING - GASOLINE»(ref-19753) article in the ENGINE PERFORMANCE section.
5. Perform fuel system pressure test.
6. Visually inspect all systems.

After performing these checks, verify customer complaint, and locate correct symptom from among those listed in this article. Not all items listed under each symptom apply to all models and systems. These procedures normally recommend testing of a system or component on vehicle, such as EGR, EST, TCC, etc. See appropriate SYSTEM/COMPONENT TESTS - GASOLINE article in the ENGINE PERFORMANCE section for specific testing procedures.

Note. If PCM displays data but engine fails to start, see NO-START DIAGNOSIS in BASIC TESTING - GASOLINE article in the ENGINE PERFORMANCE section.

Misfire Isolation

1. Start engine. Disconnect IAC motor. Using insulated pliers, remove one spark plug wire from a spark plug, and ground it against engine.
2. Note engine RPM as wire is grounded. Reconnect spark plug wire. Repeat procedure for all cylinders. Stop engine, and reconnect IAC motor.
3. If engine speed drops equally (within 50 RPM) on all cylinders, refer to ROUGH, UNSTABLE OR INCORRECT IDLE symptom. If there is no engine RPM drop or no excessive variation on one or more cylinders, check spark on respective cylinder(s). Possible Causes & Corrections: Check for restricted exhaust system. See EXHAUST SYSTEM BACKPRESSURE under MECHANICAL INSPECTION in «BASIC TESTING - GASOLINE»(ref-19753) article in the ENGINE PERFORMANCE section. Check for bent push rods, broken valve springs or worn camshaft lobes. Check ignition wires for short or faulty insulation. Check distributor cap (if equipped) for moisture, dust or cracks. Finely mist spark plug wires with water to check for shorts. Connect Spark Tester (ST-125) to spark plug, and check for adequate spark. Check ignition system for faulty grounds. Ensure EST wiring harness is not routed too close to wiring. EST wiring harness routed too close to wiring may cause induced voltage signals. Check ignition coil connections. Remove spark plugs, and check for incorrect heat range, wear, cracks, wetness, improper gap or heavy deposits. Check for proper pick-up coil (HEI distributor) resistance. Check for poor or contaminated fuel. Check for improper fuel pressure. Ensure EGR valve does not stick open. Check PCM for proper ground circuits. Ensure TP sensor does not stick or bind. TP sensor voltage should be less than 1.25 volts at idle.

Intermittent Simulation

Use following methods to reproduce conditions causing intermittent.

1. Apply light vibration to components.
2. Heat component.
3. Wiggle or bend wiring harness.
4. Spray component with water.
5. Remove/apply vacuum supply source.

Monitor circuit/component voltage or resistance while attempting to simulate intermittent. If vehicle is running, monitor for self-diagnostic codes. Use test results to identify faulty component or circuit.

Possible Causes & Corrections

Use following procedures to determine possible causes of intermittent Malfunction Indicator Light (MIL) operation.

1. Check for poor mating of one connector to another. Terminals may not be fully seated. Check for improperly formed or damaged terminals. Check wire-to-terminal connections.
2. Check for poor connection from ignition coil to ground or arcing at spark plug wires or plugs.
3. Check wire from Malfunction Indicator Light (MIL) to PCM for short to ground.
4. Check wire from Data Link Connector (DLC) test terminal for intermittent short to ground.
5. Check for poor connections in PCM ground terminals.
6. Check for loss of trouble code memory. To check code, disconnect TP sensor, and run engine at idle until Malfunction Indicator Light (MIL) comes on. Code 22 should be stored and retained in memory when ignition is turned off. If not, PCM is faulty.
7. Check for electrical system interference caused by defective relay, PCM-driven solenoid or switch, which may cause sharp electrical surge. This type of problem will normally occur when faulty component is operated.
8. Check for aftermarket parts which may not have been produced to manufacturers' specifications. Solenoids without original-equipment diodes for circuit protection and HEI-EST module or voltage regulator using transistors instead of silicone-chip circuitry may cause voltage surges (up to 300 volts) in PCM wiring, causing temporary PCM shutdown. PCM shutdown is a normal response to system overvoltage (over 16 volts on most models). PCM repowers when condition ceases to exist. A rapid shutdown and repower could cause a flickering Malfunction Indicator Light (MIL) with no codes set in memory.
9. Check for improper installation of electrical accessories such as auxiliary lights or 2-way radios.
10. Ensure EST wires are kept away from spark plug wires, distributor wires, distributor housing, ignition coil and generator. Ensure ground wire from PCM to distributor or ignition module is connected to a good ground.
11. Check for intermittent short to ground on terminal "B" (diagnostic enable) of Data Link Connector (DLC) or in Malfunction Indicator Light (MIL) circuit.
12. On vehicles not equipped with a driver information center, use scan tester to check for intermittent wiring problem. See SCAN TESTER USAGE in appropriate G - TESTS W/ CODES article in the ENGINE PERFORMANCE section below.
13. «TESTS W/CODES - 4.3L»(ref-19732)
14. «TESTS W/CODES - 5.0L»(ref-19750)
15. «TESTS W/CODES - 5.7L»(ref-19751)
16. «TESTS W/CODES - 7.4L»(ref-19752)