Diagnostic Strategies

Flow Of Diagnosis:

Note: For catalytic converter and feedback system to function properly, it is essential that vehicle is at operating temperature, (except when attempting to diagnose cold start or cold driveability problem).

1) Verify Problem

Use the Vehicle Repair History And Driveability Worksheets In Technical/ Management Support to provide better communication between the customer and the mechanic.

- The 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.

Always verify the customer complaint. You must know the problem if you are to diagnose it.

Check any Technical Service Bulletins that may apply to the vehicle being serviced.

2) Extract And Clear Fault Codes

- Diagnostic Trouble Codes are cleared whenever the Engine Controller or the battery is disconnected. Therefore, all fault codes should be read prior to any vehicle power interruption or troubleshooting.

- Diagnose and repair any fault code related problems before proceeding with further diagnosis.

Note: This process also verifies PCM and control system operation.

3) Perform Visual Inspection:

- PCM grounds: clean, tight, and in their proper location.
- Vacuum hoses: splits, kinks and proper connections. Check thoroughly for any type of leak or restriction.
- Air leaks at throttle body mounting area and intake manifold sealing surfaces.
- Ignition wires for cracking, harness, proper routing, arcing and carbon tracking.
- Wiring for proper connections, pinches, and cuts.
- Harnesses (EST, ESC, etc...) for proper routing.

4) Check and Record Idle RPM

Warning: Be sure to apply parking brake and/or block wheels before performing any test with the engine running.

If vehicle is equipped with an automatic transaxle, test and record idle rpm when the vehicle is first brought into shop, to assist in diagnosing;

- Engine Stalling
- Creeping
- Hard Shifting

SEE Computers and Control Systems/ Testing And Inspection/ Procedures/ Component Tests/ Throttle Body Minimum Airflow Test.

Note: Prior to any teardown, repair or component replacement, the following steps should always be considered.

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 B+ connections must be in perfect condition for trouble-free electronic system operation. Refer to WIRING DIAGRAMS for B+ connections and splices.

Fuse Integrity

Fuses are employed to prevent possible damage to electrical components and wiring harnesses. The fuses 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 B- 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 B+ side of the electrical system, the ground side should be checked dynamically using the voltage drop technique.

Throttle and Speed Control Cables

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

Power Supply Relays

The system relay as well as its' plug connection can be a source of intermittent operation which will not set a fault in control unit fault memory.

Crankshaft Position/Distributor Pick-Up

Hall Effect switches should be checked statically and dynamically. Physical mounting and condition of the sensor must be assured.

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 the pressure in the fuel rails must be within specifications.

Fuel pressure retention must be maintained between the fuel pump outlet and the pressure regulator after engine shutdown.

Valve Clearance and Spark Plugs

These two service items, if out of specification, can significantly impair engine performance.

- Valve clearance adjustment procedures should be strictly observed.

- Spark plug type, condition and gap must verified 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 vacuum and/or air leaks
- Evaporative purge system
- Excessive engine oil dilution
- Contaminated fuel or unapproved additives
- Carbon build-up