P0300

MISFIRING, RANDOMLY IN SEVERAL CYLINDERS





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

Conditions.
An engine load and speed dependent matrix in the engine control module's memory specifies 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.

If the number of misfires exceeds the matrix value, a diagnostic trouble code will be generated. Misfiring occurs in several cylinders.

Diagnostic Help.
The cause of misfiring could be:

^ low fuel level in the tank. If this is the case it will be indicated by diagnostic trouble code P1416 also being generated.
^ a fault in the fuel system
- low fuel pressure
- purge valve open continuously
- incorrect voltage from front heated oxygen sensor
- defective injectors

^ a fault in the ignition system
defective spark plugs
defective ignition discharge module
^ a fault in the basic engine
poor compression
^ faults in common components
pressure sensor
main relay
coolant temperature sensor.

The cause of incorrectly detected misfiring, diagnostic trouble code P0300 without misfiring, could be:

Engine control module pin 17 or 18 shorted to ground.

Momentary misfiring can be ascertained using an ISAT scan tool.

- Select "READ FUNCTIONS".
- Select "MISFIRING CYLINDER 1 ".

Other cylinders can also be selected.





Diagnostic Procedure.
1. Check additional Trionic diagnostic trouble codes

^ Read out all the diagnostic trouble codes with the ISAT scan tool.
^ P1416
^ P0105-P0135
^ P0170-P0172
^ P0335
^ P0441-P0445
^ P0505-P0507
^ P1170-P1172

Are any of the above diagnostic trouble codes present?
YES: Continue fault diagnosis for the relevant diagnostic trouble code.
NO: Continue with point 2.

NOTE: P1171 can be caused by persistent misfiring.





2. Check the basic causes of the fault

- Inspect the vacuum hose between the pressure sensor and intake manifold.
- Check the pressure sensor's electrical connections for moisture and corrosion. Also check that the seal in the female connector is present and undamaged.
- Inspect the vacuum hose between the fuel pressure regulator and intake manifold.
- Using an ISAT scan tool, check that the coolant temperature is possible.
- Using an ISAT scan tool, check that the operation of the pressure sensor is plausible:

With the ignition switch in the ON position, the reading obtained on the ISAT scan tool should be about 102 kPa

NOTE: The reading is dependent on air pressure.

With the engine idling, the reading obtained on the ISAT scan tool should be 40-60 kPa.

- Check The Wiring connected to the front heated oxygen sensor for shorting, continuity and chafing of the insulation.
- Check the front heated oxygen sensor's connector for moisture and corrosion.

Have any of the basic causes of the fault been found?
YES: Rectify the fault and proceed to point 8.
NO: Continue with point 3.





3. Check the condition of the ignition discharge module

- Remove the ignition discharge module from the camshaft cover.
- Check visually:

^ that the four rubber seals for the spark plugs are in good condition.
^ that no contact spring for the spark plugs is missing or that spring travel is inadequate.
^ the connector for moisture and corrosion.

Is the ignition discharge module OK?
YES: Continue with point 4.
NO: Rectify the fault and proceed to point 4.

4. Check the spark plugs

- Remove the spark plugs and check their condition visually.

Are the plugs OK?
YES: Continue with point 5.
NO: The electrode gap is wider than 1.4 mm or is otherwise obviously incorrect: Change the spark plugs and proceed to point8.





5. Check the operation of the ignition discharge module

- Fittest spark plugs (part No. 86 11 386) in the ignition discharge module.
- Ground the spark plugs using test cable 86 10 867.
- Activate each separate ignition coil, using the ISAT scan tool.
- Ignition switch in ON position.
- Select "ACTIVATE".
- Select "IGNITION COILS"
- Select 'IGNITION CYL 1", "CYL 2", "CYL 3" and "CYL 4".

Activation of an ignition coil takes place for 10 seconds at 200 Hz. It a longer activation time is required, the command must be repeated.

- Check that each activated spark plug produces a spark.

Is a spark produced?
YES: Continue with point 6.
NO: Change the ignition discharge module and proceed to point 8.

WARNING: The electronic ignition system generates up to 40,000 Volts. Such voltages can prove fatal to people with weak hearts or those who are fitted with a pacemaker. All due caution should therefore be observed when carrying out any work involving the ignition system.

IMPORTANT: The ignition discharge module must be positioned with the ignition coils facing down during the test so that the transformer oil will provide good insulation in the high-voltage part of the ignition coils.





6. Check the compression
- Check the compression of each cylinder.

Is compression OK?
YES: Change all spark plugs and continue with point 7.
NO: Continue fault diagnosis in the Basic Engine.

7. Check the fuel system
- Carry out fuel pressure and fuel flow measurement on the fuel pump and all the injectors.

Is the fuel system OK?
YES: Continue with point 8.
NO: Rectify the fault and proceed to point 8.





8. Final check:

- Clear the diagnostic trouble code.
- Implementation of driving cycle:
Drive the car at different engine rpm and varying loads, especially at high revs, while the engine is warming up.
- Evaluation of driving cycle:
Check whether the diagnostic trouble code has recurred.

Has the diagnostic trouble code recurred?
YES: Proceed at "Before Replacing A Control Module". Component Tests and General Diagnostics
NO: The remedial measure taken was correct.