Diagnostic Notes
Before using this section you should have performed the SELF-DIAGNOSTIC procedures and made any necessary repairs, as directed by the self diagnostic system.
^ Always use the FLOW OF DIAGNOSIS for proper testing sequence.
^ Several of the 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 and can save valuable time.
^ Certain basic faults can be undetectable by the self-diagnostic system and in some cases can actually interfere with the self-checking and fault memory operation. Low battery voltage, for example, can cause erroneous faults to set in control unit fault memories or can cause a system to go into "limp home mode" or open loop operation without setting a fault in memory. On the other hand, system fault memories are cleared whenever the control unit or the battery is disconnected. Therefore, all fault memories should be read prior to any vehicle power interruption or troubleshooting.
CHECK THE FOLLOWING:
NOTE: It is essential that the engine has been properly warmed up and is at operating temperature, except when attempting to diagnose a cold driveability problem. The catalytic converter and the oxygen sensor will not function as designed if this is not done. Failure to do so will result in inaccurate test results.
COMPLAINT VERIFICATION
Whenever possible the repairing technician should personally verify the complaint. Having experienced the malfunction, the technician is less likely to try to repair non-existent faults.
MALFUNCTION VERIFICATION
Today's sophisticated automotive systems are easily misunderstood, which can lead to repairs that attempt to force a particular system to perform in a way that it was never intended to operate. Therefore, the troubleshooting technician should compare the system operation to the nominal system operation as described in the section DESCRIPTION AND OPERATION. Furthermore, the technician is also encouraged to compare the problem vehicle system operation with a known good vehicle.
PREVIOUS REPAIRS
The vehicle repair history can often 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 CABLE INTEGRITY
Battery cable connections must be clean and tight for trouble-free electronic system operation.
FUSIBLE LINK INTEGRITY
Fusible links are employed to prevent possible damage to electrical components and wiring harnesses. These links and their connections must be without dynamic resistance. Dynamic resistance can only be checked using the voltage drop method of testing.
NEGATIVE GROUND CONNECTIONS
As all electrical circuits are a circle, all negative connections must also be clean and tight. A poor "common" ground point can 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 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 the engine or transmission which appear overheated and/or discolored indicate the need to thoroughly test all ground connections. Be sure there is a good engine ground, otherwise high current devices may find ground through a component not designed to carry high current.
POWER SUPPLY RELAYS
Fuel pump and overvoltage relays as well as their plug connections can be a source of intermittent operation which may not set a code in the fault memory.
CRANKSHAFT POSITION SENSOR
Monolithic sensors should be checked statically and dynamically. Physical mounting and condition of the sensor and segment wheel 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 have clean, tight connections and be free of carbon tracking, moisture and corrosion.
FUEL DELIVERY AND PRESSURE
For easy starting, the pressure in the fuel system must be within specifications and fuel delivery must begin at once when cranking. To ensure this, a minimum fuel pressure must be maintained while the engine is not operating.
VALVE CLEARANCE AND SPARK PLUGS
These two service items, if out of specification, can significantly impair engine performance. Valve clearance adjustment procedures and specifications should be strictly observed. Spark plug type, condition and gap, must be correct according to specification.
AIR/FUEL RATIO
When troubleshooting idle quality or driveability complaints, it is necessary to consider the following:
^ Injector spray pattern quality
^ Presence of intake air leaks
^ Evaporative purge system
^ Excessive engine oil dilution
^ Fuel quality
^ Carbon build-up
The symptom charts contain groups of possible causes for each symptom. These procedures are not necessarily described in the order in which they should be performed. Proceed in a logical order, easiest to check or most likely cause first. To determine if a particular system or component is used on a specific vehicle, refer to DESCRIPTION AND OPERATION.