Engine Misfires

The PCM receives signals from the crankshaft position sensor to determine the position of the engine at various degrees of rotation. The PCM can then base misfire diagnostics on the principle that crankshaft velocity will increase as each cylinder contributes its power input. If a cylinder fires and the PCM does not see an increase in velocity, it will store misfire counts in 200 revolution segments. If the misfire on the cylinder(s) is 20-100% for 10 combustion events in a 200 crankshaft revolution test, the MIL (SERVICE ENGINE SOON) light will flash and continue to flash until the PCM no longer detects a catalyst damaging misfire. The PCM will turn the lamp On solid when that cylinder(s) has passed 16 succeeding 200 revolution tests.

If a misfire on a cylinder(s) is 2-19% for three combustion events in a 200 revolution test, the test will have failed. If a cylinder or combined cylinder(s) fail five out of 16 tests the MIL light will stay on solid for an emission related misfire. The PCM will set any active misfire counts in memory to zero when the key is turned Off and will test for a misfire under certain conditions. If the MIL light is illuminated it will require three "passes" under the same criteria the misfire was set at or 80 passed "trips" if the criteria was not met.