Diagnostic Notes

NOTE: Before beginning any diagnosis or repair of IGNITION SYSTEMS, always check TECHNICAL SERVICE BULLETINS. Many problems associated with these systems are covered by the manufacturer's TSB's.

^ Before using this section, perform ON-BOARD DIAGNOSTICS (extracting fault codes). Verify that control units and "CHECK ENGINE" light are operational, and that no trouble codes are stored.
^ See Flow of Diagnosis for proper testing sequence.
^ Several following symptom procedures call for a careful visual/physical check. This step is very important - it can lead to correcting a problem without further checks, thus saving valuable time.
^ Certain basic faults can be undetectable by the self-diagnostic system and in some cases can actually interfere with self-checking and fault memory operation. Low battery voltage, for example, can cause erroneous faults to set or can put a system into "limp home" mode without setting a fault in memory. Because system fault codes are cleared whenever ECU or battery are disconnected, codes should be read prior to any vehicle power interruption or troubleshooting.

CHECK THE FOLLOWING:

NOTE: To ensure accurate test results, vehicles must be warmed up to operating temperature, except when attempting to diagnose a cold driveability problem. Catalytic converters and oxygen sensors will not function as designed if this is not achieved.

COMPLAINT VERIFICATION

Whenever possible, the repairing technician should personally verify complaints. Experiencing the malfunction enables technicians to focus troubleshooting efforts.

MALFUNCTION VERIFICATION

Sophisticated automotive systems are easily misunderstood, potentially leading to attempts to force a particular system to perform as it was never intended to operate. The troubleshooting technician should compare actual system operation to nominal system operation as described in DESCRIPTION AND OPERATION. The technician is also encouraged to compare problem vehicle system operation with a known good vehicle.

PREVIOUS REPAIRS

Vehicle repair history can provide explanations to unusual complaints which seem to elude normal troubleshooting attempts. Incorrect components or unapproved repairs can have subtle influences on seemingly unrelated systems.

BATTERY STATE OF CHARGE

Batteries in a state of partial discharge can have a dramatic effect on control units and related components.

POSITIVE BATTERY CABLE INTEGRITY

All positive battery connections must be in perfect condition for trouble-free electronic system operation.

NEGATIVE GROUND CONNECTIONS

Negative battery connections must also be checked and verified to be in perfect condition. A poor "common" ground point will cause seemingly unrelated systems to influence one another. High current systems which encounter a poor "common" ground can back feed through other electrical systems, causing unusual operation and perhaps inexplicable component failure. As with the positive side of the electrical system, the ground side should be checked dynamically using the voltage drop technique.

POWER TRAIN CABLE ATTACHMENTS

Metal cables attached to engine or transmission which appear overheated and/or discolored indicate the need to thoroughly test all ground connections.

POWER SUPPLY RELAYS

The system (main) relay, as well as it's plug connection, can be a source of intermittent operation which will not set a code in ECU fault memory.

CRANKSHAFT SENSOR

Sensors should be checked for proper operation. Physical mounting and condition of the sensor must be assured.

HIGH TENSION COIL AND SECONDARY COMPONENTS WIRING

The secondary ignition system should be checked visibly as well as with a suitable engine analyzer and scope. All components should be examined for tight connections and freedom from carbon tracking, moisture, and corrosion.

FUEL DELIVERY

Fuel delivery must begin at once when cranking and fuel system pressure must be within specifications. Fuel pressure must be maintained between pump outlet and pressure regulator after engine shutdown.

VALVE CLEARANCE AND SPARK PLUGS

If out of specification, these two service items can significantly impair engine performance. Valve clearance adjustment procedures and specifications should be strictly observed. Spark plug type, condition, and gap must be verified according to specification.

AIR/FUEL RATIO

When troubleshooting idle quality or driveability complaints, consider the following:

^ Injector spray pattern quality
^ Presence of unmetered air leaks
^ Evaporative purge system
^ Excessive engine oil dilution
^ Substandard fuel or unapproved additives
^ Carbon build-up
^ EGR system

^ The following symptom charts contain groups of possible causes for each symptom. These procedures are not meant to be done in consecutive order. Proceed in a logical order, easiest to check or most likely cause, first.