Misfire Detection

WARNING: This page is about a different car, the 2011 Jaguar XK and 2010 Jaguar XK. However, it is still accessible from the selected car via links, so may be relevant.

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 legislated emissions limit being exceeded by a given amount. The second level is a misfire that may cause catalytic converter 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, it 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 catalytic converters. If the number of misfires exceeds a given value, the engine control module (ECM) stores catalytic converter damage fault codes, along with the engine speed, engine load and engine coolant temperature.

The signal from the crankshaft position (CKP) sensor indicates how fast the poles on the drive plate 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 drive plate 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 (CKP) sensor and assign a roughness value to it. This roughness value can be viewed within the real time monitoring feature using Jaguar approved diagnostic equipment. The engine control module (ECM) will evaluate the signal against a number of factors and will decide whether to record the occurrence or ignore it. The engine control module (ECM) can assign a roughness and misfire signal for each cylinder.