- Intermittent Malfunctions

Intermittent malfunctions may occur which may or may not cause the "Check Engine" or "Service Engine Soon" lamp to come on, or set trouble codes in the ECM memory. These intermittent conditions usually result in driveability complaints and cause trouble codes to be set in the ECM memory without turning on the indicator lamp, or are "read" as intermittent trouble codes when testing the system with the SCAN type tester. However, trouble code diagnosis charts cannot be used to diagnose this type of malfunction as a fault must be present in order to be located using the charts, and use of the charts to diagnose an intermittent condition may result in unnecessary replacement of components.

Most intermittent malfunction conditions are caused by poor electrical or vacuum hose connections, or defective or improperly routed wiring or hoses. The cause for most intermittent malfunctions can usually be located by performing a thorough visual inspection of the system vacuum and wiring harnesses and/or by road testing the vehicle while monitoring the suspected circuit with suitable test equipment. After performing the "Diagnostic Circuit Check" and ensuring that the indicator lamp is operating and that no "hard" codes are set in the ECM memory, use the following procedure to locate intermittent malfunctions:

1. Inspect vacuum hoses for splits, kinks and proper connection, and ensure that hoses are routed properly.
2. Inspect fuel system components for proper mounting, fuel, air or vacuum leaks, and correct as needed.
3. Inspect secondary ignition wiring and replace any wires that are burned, cracked or broken.
4. Inspect engine compartment and emission control system wiring and repair or replace wiring that is broken, chaffed or damaged.
5. Inspect electrical connectors noting the following:

a. Ensure that terminal is making proper contact with wire by removing terminal from connector and inspecting for proper crimping and broken wire strands.
b. Ensure that all terminals are fully seated in connector.
c. Ensure connector terminals are not damaged or deformed, and carefully reform terminals in connectors of suspected circuits to increase spring tension.
d. Ensure connector halves are properly mated and fully seated.

6. Inspect indicator lamp and ALDL connector wiring for shorts to ground and repair as needed.
7. Check continuity between proper terminals of ECM connector and engine ground, and repair wiring if no continuity exists.
8. Check for electrical interference caused by defective relays, ECM driven solenoids or switches, and secondary ignition system components, as they may cause voltage surges.
9. Ensure that EST wiring is properly routed and insulated from distributor wiring, distributor, ignition coil and alternator.
10. Inspect added accessories such as lights, two-way radios etc. for proper installation, ensuring that no accessories are patched into engine control system wiring.
11. Ensure diodes and resistors installed across accessories such as the A/C compressor clutch are not open.
12. Connect SCAN tool following manufacturer's instructions, or connect suitable voltmeter to suspected circuit and road test vehicle, noting any abnormal readings and the conditions under which they occur. Abnormal readings indicate that the tested circuit may be cause for malfunction.