P0300
RANDOM MISFIRING IN SEVERAL CYLINDERS
Fault Symptoms.
CHECK ENGINE lamp (MIL) on.
Lumpy running while 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.
How large a proportion of misfires is required for exhaust emissions to reach illegal levels or for the catalytic converter to be damaged is specified in a load and rpm dependent matrix in the control module's memory.
A diagnostic trouble code will be generated if the number of misfires exceeds the matrix figure.
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 P1585 also being generated.
^ fuel system malfunction.
- low fuel level
- continuously open purge valve
- air leakage after the mass air flow sensor
- defective/clogged injectors
^ ignition system malfunction ,
- defective or incorrect spark plugs
- defective ignition coil module, HT leads
^ basic engine malfunction
- poor compression
^ malfunction of common components.
- mass air flow sensor
- main relay/pump relay
- engine coolant temperature sensor
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 additional Motronic diagnostic trouble codes
- Use an ISAT scan tool to obtain readouts of all diagnostic trouble codes.
Does the ISAT scan tool show any other diagnostic trouble code?
YES: Continue fault diagnosis according to the diagnostic procedure for the relevant diagnostic trouble code.
NO: Continue with point 2.
2. Check basic causes of the fault
- Use the ISAT scan tool to check that engine coolant temperature is plausible.
- Use the ISAT scan tool to check that the performance of the mass air flow sensor is plausible. The ISAT scan tool should show the following air mass as a function of engine rpm:
^ idling speed: about 0.5 V
^ at 2500 rpm (no load): about 1.2 V.
Has a basic cause of the fault been found?
YES: Rectify the fault and continue with point 9.
NO: Continue with point 3.
3. Check the spark plugs
- Remove the spark plugs and check their condition visually.
Are the spark plugs OK?
YES: Continue with point 4.
NO: The electrode gaps are wider than 1.4 mm or otherwise obviously wrong. Change the spark plugs and continue with point 9.
4. Check the HT leads
- Inspect the HT leads visually for damage that could cause flashover and the like.
- Check the resistance of all HT leads:
^ HT leads 1-3-5: 7-9 kohm
^ HT leads 2-4-6: 2-6 kohm.
Are all resistance readings OK?
YES: Continue with point 5.
NO: Change the defective HT lead or leads and continue with point 9.
WARNING: The electronic ignition system generates charges of up to 40,000 volts. Such voltages may prove fatal to persons with a weak heart or those who are fitted with a pacemaker. Therefore, treat the ignition system with great respect and take all necessary precautions.
5. Check the performance of the ignition coil module
- Install a test spark plug, part No. 86 11 386, in cylinders 1 and 4.
- Connect the spark plugs to ground using test cable assembly 86 10 867.
- Turn the ignition switch to the ON position.
- Select "ACTIVATE".
- Select "IGNITION CYL 1-4".
- Check that a spark is obtained on each activated spark plug.
- Repeat with "IGNITION CYL 2-5" and "IGNITION CYL 3-6".
The command activates each ignition coil for 30 seconds.
Is a spark produced?
YES: Continue with point 7.
NO: Go to point 6.
6. Check the control module outputs to the ignition coil module
- Unplug the ignition coil module's 4-pin connector.
- Connect a test lamp to pins 4 and 3 (cyl. 1-4).
- Turn the ignition switch to the ON position.
- Select "ACTIVATE".
- Select "IGNITION CYL 1-4".
- Repeat the test procedure for the other cylinders with the test lamp connected to:
^ pins 4 and 1 (cyl. 2-5)
^ pins 4 and 2 (cyl. 3-6).
Select "IGNITION CYL 2-5" and "IGNITION CYL 3-6".
The command activates the relevant ignition coil at a frequency of 0.5 Hz for 30 seconds. The test lamp should flash.
Test lamp flashing?
YES: Change the ignition coil module and continue with point 9.
NO: Rectify the lead in question and proceed to point 9.
7. Check the compression
- Carry out a compression test on each cylinder.
Compression test OK?
YES: Change all spark plugs and continue with point 8.
NO: Continue fault diagnosis in the Basic Engine.
8. Check the fuel system
- Take fuel pressure and fuel flow readings on the fuel pump and all injectors.
Is the fuel system OK?
YES: Continue with point 9.
NO: Rectify the fault and continue with point 9.
9. Final check
IMPORTANT: If the control module's power supply has been disconnected, the control module must adapt the crankshaft position sensor.
^ Drive the car in 2nd gear up to 6000 rpm.
^ Release the accelerator completely so that fuel shut-off takes place and allow engine speed to drop to 2000 rpm.
^ Repeat five times.
- 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.
- Evaluation of driving cycle:
^ Check whether the diagnostic trouble code has recurred.
Has the diagnostic trouble code recurred?
YES: Continue at "Before Replacing a Control Module". Component Tests and General Diagnostics
NO: The steps taken to rectify the fault were correct.