Misfire Detection

Legislation requires that the engine control module (ECM) must be able to detect the presence of an engine misfire. It must be able to detect misfires at two separate levels. The first level is a misfire that could lead to the vehicle emissions exceeding 1.5 times the Federal Test Procedure (FTP) requirements for the engine. The second level is a misfire that may cause catalyst damage.

The engine control module (ECM) monitors the number of misfire occurrences within two engine speed ranges. If the engine control module (ECM) detects more than a predetermined number of misfire occurrences within either of these two ranges, over two consecutive journeys, the engine control module (ECM) will record a fault code and details of the engine speed, engine load and engine coolant temperature. In addition, the engine control module (ECM) monitors the number of misfire occurrences that happen in a 'window' of 200 engine revolutions. The misfire occurrences are assigned a weighting according to their likely impact on the catalysts. If the number of misfires exceeds a certain value, the engine control module (ECM) stores catalyst-damaging fault codes, along with the engine speed, engine load and engine coolant temperature.

The signal from the crankshaft position sensor indicates how fast the poles on the flywheel are passing the sensor tip. A sine wave is generated each time a pole passes the sensor tip. The engine control module (ECM) can detect variations in flywheel speed by monitoring the sine wave signal supplied by the crankshaft position sensor.

By assessing this signal, the engine control module (ECM) can detect the presence of an engine misfire. At this time, the engine control module (ECM) will assess the amount of variation in the signal received from the crankshaft position sensor and assigns a roughness value to it. This roughness value can be viewed within the real time monitoring feature, using T4. The engine control module (ECM) will evaluate the signal against a number of factors and will decide whether to count the occurrence or ignore it. The engine control module (ECM) can assign a roughness and misfire signal for each cylinder, (i.e. identify which cylinder is misfiring).