P0300

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

P0300

Fault symptoms

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.

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 being generated;

•	fuel system malfunction; - continuously EVAP canister purge valve - defective injectors - low fuel pressure;

•	ignition system malfunction: - defective or incorrect spark plugs - faulty HT cables - faulty ignition coil - faulty distributor - faulty rotor;

•	basic engine malfunction: -poor compression

•	malfunction of common components: - pump relay - engine coolant temperature sensor.

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

•	Malfunctioning of the rough road 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 other Motronic 4.1 diagnostic trouble codes

-	Use an ISAT scan tool to obtain readouts of all diagnostic trouble codes.

Any other diagnostic trouble codes?

yes

Continue fault diagnosis according to the diagnostic procedure for the relevant diagnostic trouble code.

no

Continue with point 2.

2. Check the basic causes of the fault

-	All electrical equipment should be turned off (A/C, radiator fan, etc.).

-	Use the ISAT scan tool to check that engine coolant temperature is plausible.

-	Check that the performance of the mass air flow sensor is plausible. The ISAT scan tool should show the following air mass (V) as a function of engine rpm:

Has a basic cause of the fault been found?

yes

Rectify the fault. Continue with point 11.

no

Continue with point 3.

3. Check the HT cables

-	Check visually:

Are the HT cables OK?

yes

Continue with point 4.

no

Rectify the fault. Continue with point 4.

4. Check the spark plugs

-	Remove the spark plugs and check their condition visually.

-	Check that the electrode gap is not wider than 1.0 mm and that the spark plugs are not obviously damaged.

Are the spark plugs OK?

yes

Continue with point 5.

no

Change the spark plugs. Continue with point 9.

5. Check the performance of the ignition coil

-	Unplug the injector connectors.

-	Move the selector lever to position N.

-	Fit a test spark plug (part No. 86 11 386) to the HT cable from the ignition coil.

-	Run the starter motor.

Is a spark produced?

yes

Continue with point 6.

no

Continue with point 8.

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.

6. Check the distributor

-	Check the distributor visually for insulation trouble and damage.

Is the distributor OK?

yes

Continue with point 7.

no

Change the defective component. Continue with point 11.

7. Check the HT leads between spark plugs and distributor

-	Move the selector lever to position N.

-	Fit a test spark plug (part No. 86 11 386) in the relevant HT cable.

-	Run the starter motor.

Are sparks produced by all spark plugs?

yes

Change all spark plugs. Continue with point 8.

no

Change the HT lead concerned. Continue with point 11.

8. Check the wiring between ignition coil and control module

-	Test for continuity and shorting between pin 25 of the control module and pin 1 of the ignition coil.

Is the wiring OK?

yes

Continue with point 9.

no

Repair or replace the wiring. Continue with point 11.

9. Check the compression

-	Carry out a compression test on each cylinder.

Is the compression OK?

yes

Change all spark plugs. Continue with point 10.

no

Continue fault diagnosis as described in Basic engine .

10. 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 11.

no

Rectify the fault. Continue with point 11.

11. Final check

-	Clear the diagnostic trouble code.

-	Implementation of driving cycle:

-	Select "MISFIRING CYLINDER 1".

-

2"

-

3"

-

4"

Does misfiring occur?

yes

Continue as described in .

no

The steps taken to rectify the fault were correct.