Misfires

Misfire is measured by the constant velocity method from input received by the crank sensor.

The sensor requires that the reluctor ring is in good condition and the gap between tabs be 5 mm. If these conditions are not met then the engine may develop real misfire or improperly flag a misfire fault without symptoms.





It is critical for proper misfire detection that the pick-up and reluctor conform with the diagram above. Check all teeth individually if you are trying to trace a problem.

After checking the reluctor ring and gap give attention to all aspects of the engine that can interrupt a cylinders ability to burn the air/fuel mixture. These include plug leads, spark plugs, coils, injectors, fuel pressure, fuel contamination, air/vacuum leaks, wiring, GEMS ECM, cylinder compression, valve stick, valve seats, mechanical cam/crank timing. Remember to check Freeze Frame Data to determine which fault set the MIL on.

The following symptoms have been observed on a variety of vehicles:

If the following codes - P0300, P030n, P1316 where "n" is a cylinder number that is believed to be misfiring (P1313, P1314 on occasions) are seen and a check engine light is on with no symptoms present, this could be a result of a false crank signal to the GEMS ECU from the crank sensor. This may be caused by one or more reluctor ring tabs being bent, the crank sensor tip being bent, or the crank sensor mounting being loose. The best way to diagnose this is to use a scope to analyze the signal.

New valve introduction March 96.

^ Higher mileage (typically 10,000 miles or more) misfire faults - P0300, PO30n, P1316, where cylinder "n" is the cylinder number that is believed to be misfiring.

Symptoms include rough idle or hesitation under load at low rpm (1500rpm) can be checked out with vacuum readings that fluctuate. This is an indication that cylinder "n" has its exhaust valve sticking, due to the use of poor fuel.

A very small number have been seen to date however, if this problem is evident please report this to the HelpLine so we can track its occurrence.

New valve introduction - March 96.

^ Low mileage misfire complaints and complaint of tapping/knocking from top of engine, misfire codes: P0300, P030n, P1316.

On cylinder "n" you will probably find a broken valve spring.

^ Misfire evident on one or more cylinder. Check for tightness of coil connections, plug lead security on the coil towers and spark plugs, to be sure that they are pushed fully home.

^ Take care when bolting up the rocker cover near the coil harness so that the coil connections will not be trapped by the rocker cover.