Misfire Monitor

The input to the Misfire Monitor is the signal from the crankshaft position sensor and timing wheel. The signal is acquired and processed by the PCM and provided to the Misfire Monitor as individual tooth period measurements. The Monitor uses the tooth period measurements to calculate crankshaft acceleration signals for misfire detection. All misfire processing is performed in software (separate chips are no longer used except for some vehicle lines that may still be using older style PCMs.)

Two different technologies are used for misfire monitoring. They are the Low Data Rate (LDR), and High Data Rate (HDR) systems. The LDR system is capable of meeting the FTP monitoring requirements on most engines and is capable of meeting "full-range" misfire monitoring requirements on 3 and 4-cylinder engines. It is also used on 6 cylinder engines with rear mounted crank sensors. The HDR system is capable of meeting "full-range" misfire monitoring requirements on 8 and 10 cylinder engines. All software allows for detection of any misfires that occur 6 engine revolutions after initially cranking the engine. This meets the OBD-II requirement to identify misfires within 2 engine revolutions after exceeding the warm drive, idle rpm.

The Monitor includes a diagnostic check on the crank sensor input. The Monitor checks the number of tooth period measurements received on each cylinder event. A P1336 will be set if the Monitor receives an invalid number of tooth period measurements. A P1336 points to noise present on the crank sensor input or a lack of synchronization between the cam and crank sensors.

Low Data Rate System 

The LDR Misfire Monitor uses a low-data-rate crankshaft position signal, (i.e. one time measurement signal for each cylinder event). The PCM calculates crankshaft rotational velocity for each cylinder from this crankshaft position signal. The acceleration for each cylinder can then be calculated using successive velocity values. The changes in overall engine rpm are removed by subtracting the median engine acceleration over a complete engine cycle. The crankshaft acceleration is then processed by three algorithms. The first algorithm, called pattern cancellation, is optimized for detection of sporadic patterns of misfire. The algorithm learns the normal pattern of cylinder accelerations from the mostly good firing events and is then able to accurately detect deviations from that pattern. The second algorithm, called pattern cancellation by opposing engine revolution or "pc-rev", is optimized for single cylinder patterns. The algorithm compares the acceleration of a cylinder to its opposite cylinder on the opposing engine revolution. The algorithm learns the normal patterns that repeat every engine revolution and is then able to accurately detect deviations between the paired cylinders. The third algorithm is a non-filtered acceleration signal that is a general purpose signal for all patterns including multi-cylinder patterns. The resulting deviant cylinder acceleration values are used in evaluating misfire in the "General Misfire Algorithm Processing" section below.

High Data Rate System 

The High Data Rate (HDR) Misfire Monitor uses a high data rate crankshaft position signal, (i.e. one time measurement signal per each 2 teeth for a total of 36 measurements for one engine cycle on a 36-1 tooth wheel). This high-resolution signal is processed with a digital low pass filter. The low pass filter filters the high-resolution crankshaft velocity signal to remove some of the crankshaft torsional vibrations that degrade signal to noise. Two low pass filters are used to enhance detection capability - a "base" filter and a more aggressive filter to enhance single-cylinder capability at higher rpm. This significantly improves detection capability for continuous misfires on single cylinders up to redline. The high-resolution acceleration can then be calculated using successive velocity values. The changes in overall engine rpm are removed by subtracting the median engine acceleration over a complete engine cycle. The crankshaft acceleration is then processed by three algorithms similar to the LDR system. The final stage is to decimate the high resolution signals by selecting the peak acceleration values from within a window location for each cylinder. The resulting deviant cylinder acceleration values are used in evaluating misfire in the "General Misfire Algorithm Processing" section below.