P0301

MISFIRING CYLINDERS 1, 2, 3, 4





Fault Symptom.
The CHECK ENGINE lamp (MIL) is on or flashing.
Uneven running when misfiring.
Any other diagnostic trouble codes.

Conditions.
During the course of each cylinder's power stroke the rotational speed of the crankshaft increases temporarily. This gives rise to momentary increases in pulse frequency from the crankshaft position sensor. These frequency changes are evaluated by the control module and any misfiring is recorded.

An engine load and speed dependent matrix in the electronic control module's memory determines how large a proportion of misfires is required for exhaust emissions to reach prohibited limits or for the three way catalytic converter to be damaged.

A diagnostic trouble code will be generated if the number of misfires exceeds the matrix figure.

Diagnostic Help.
The cause of misfiring could be:

^ Defective HT cables.
^ Defective distributor.
^ Defective rotor.
^ Defective or incorrect spark plugs.
^ Defective injectors.

Check The Wiring.
Intermittent faults may occur as a result of temporary short circuits and breaks in the wiring. Jiggle the wiring at several points and in different directions to ascertain whether the wiring harness, including any connectors, is causing the trouble. Observe the multimeter, ISAT scan tool or test lamp while doing this.





Diagnostic Procedure.
1 Check other Motronic 4.1 diagnostic trouble codes

- Obtain readouts of all diagnostic trouble codes using the ISAT scan tool.

Does the ISAT scan tool show diagnostic trouble codes P1213-P1216, P1225-P1228?

YES:
^ Continue fault diagnosis for the relevant diagnostic trouble code.

NO:
^ Continue with point 2.





2 Check the spark plugs

- Remove the spark plug from the cylinder concerned and check its condition visually.
- Check that the electrode gap is not wider than 1.0 mm and that the spark plug is not obviously damaged.

Is the plug OK?

YES:
^ Proceed to point 3.

NO:
^ Change all spark plugs. Continue with point 7.





3 Check the ignition system

- Unplug the injector connectors.
- Install a test spark plug (part No. 86 11 386) in the relevant HT cable.
- Run the starter motor.
- Check that the test spark plug produces a spark.

Is a spark produced?

YES:
^ Change all spark plugs. Continue with point 5.

NO:
^ Continue with point 4.





4 Check the HT cables

- Try changing the relevant HT cable for a new one.
- Install a test spark plug to the relevant HT lead.
- Run the starter motor.
- Check that a spark is produced.

Is a spark produced?

YES:
^ Changing the HT cable was the correct procedure to rectify the fault. Continue with point 7.

NO:
^ The fault is to be found in the distributor. Rectify the fault and proceed to point 7.





5 Check the compression

- Carry out a compression test on the relevant cylinder.

Is the compression OK?

YES:
^ Continue with point 6.

NO:
^ Continue fault diagnosis at the basic engine.





6 Check the fuel delivery flow of the relevant injector

- Check the fuel delivery flow of the relevant injector.

Is the delivery flow OK?

YES:
^ Continue with point 7.

NO:
^ Change the injector. Continue with point 7.





7 Final check

- Clear the diagnostic trouble code.
- Implementation of driving cycle:
^ While the engine is warming up, drive the car at varying engine loads and rpm. Make a point of driving with the engine at high revs.
^ While driving, read the following on the ISAT scan tool:
"MISFIRING CYL 1, 2, 3, 4

- Evaluation of driving cycle:
^ Check whether misfiring occurs.

Does misfiring occur?

YES:
^ Continue at "Before Replacing a Control Module". Component Tests and General Diagnostics

NO:
^ The steps taken to rectify the fault were correct.