Misfire Monitor Diagnostic Operation

The misfire monitor diagnostic is based on crankshaft rotational velocity, aka reference period, variations. The PCM determines crankshaft rotational velocity using the Crankshaft Position (CKP) Sensor and Camshaft Position (CMP) Sensor. When a cylinder misfires, the crankshaft slows down momentarily. By monitoring the crankshaft and CMP sensor signals, the PCM can calculate when a misfire occurs.

For a non-catalyst damaging misfire, the diagnostic is required to monitor a misfire present for between 1,000-3,200 engine revolutions.

For catalyst damage misfire, the diagnostic responds to the misfire within 200 engine revolutions

Rough roads may cause false misfire detection. A rough road applies sudden torque variations to the drive wheels and drivetrain. This torque can intermittently decrease the crankshaft rotational velocity. The Antilock Braking (ABS) System detects uneven speed between the vehicles wheels and sends data via the serial data bus to the PCM to disable the misfire monitor until the rough road is no longer detected.

On automatic transmission equipped vehicles, the Torque Converter Clutch (TCC) disables whenever a misfire is detected. Disabling the TCC isolates the engine from the rest of the drive line and minimizes the affect of the drive wheel inputs on crankshaft rotation.

When the TCC has disabled as a result of misfire detection, the TCC is re-enabled after approximately 3,200 engine revolutions if no misfire is detected. The TCC remains disabled whenever the misfire is detected, with or without a DTC set. This allows the misfire diagnostic to reevaluate the system.




Whenever a cylinder misfires, the misfire diagnostic counts the misfire and notes the crankshaft position at the time the misfire occurred.

A current and a history misfire counter is maintained for each cylinder. The misfire current counters, Misfire Cur #1-8, indicate the number of firing events out of the last 200 cylinder firing events which were misfires. The misfire current counters displays real time data without a misfire DTC stored. The misfire history counters, Misfire Hist #1-8, indicate the total number of cylinder firing events which were misfires. The misfire history counters display 0 until the misfire diagnostic has failed and a DTC P0300 is set. Once the misfire DTC sets, the misfire history counters will be updated every 200 cylinder firing events. The Misfire counters graphic illustrates how these misfire counters are maintained.

When crankshaft rotation is erratic, the PCM detects a misfire condition. Because of this erratic condition, the data that is collected by the diagnostic can sometimes incorrectly identify which cylinder is misfiring. The Misfire Counters graphic shows there are misfires counted from more than one cylinder. Cylinder #1 has the majority of counted misfires. In this case, the misfire counters would identify cylinder #1 as the misfiring cylinder. The misfires in the other counters were just background noise caused by the erratic rotation of the crankshaft. If the number of accumulated misfires is sufficient for the diagnostic to identify a true misfire, the diagnostic will set DTC P0300-Misfire Detected. The illustration depicts an accumulation in the history buffers.

If two cylinders in sequential tiring order are both misfiring, the first misfiring cylinder will accumulate misfires in the cylinder buffer, but the second misfiring cylinder will not. This is because the PCM compares a misfiring cylinder with the cylinder 90 degrees prior to that cylinder in the firing order. Therefore the PCM would be comparing crankshaft speed of the second misfiring cylinder to an already suspect cylinder. The PCM however, will be able to detect both misfiring cylinders after the engine exceeds 2,000 RPM. This is because the PCM then starts to compare misfires to the opposing cylinder rather than the previous cylinder in the firing order.

Use Techline equipment to monitor the misfire counter data on applicable vehicles. Knowing which specific cylinders misfire can lead to the root cause. Using the information in the misfire counters identifies which cylinders are misfiring. If the counters indicate cylinders number 1 and 4 misfired, look for a circuit or component common to both cylinders.

The misfire diagnostic may indicate a fault due to a temporary fault not necessarily caused by a vehicle emission system malfunction. Examples include the following items:
^ Contaminated fuel
^ Running out of fuel
^ Fuel fouled spark plugs
^ Basic engine fault