Circuit Description

Crankshaft Position (CKP) sensor sends pulses to ECM as reluctor wheel teeth rotate past sensor. Besides using these pulses to synchronize ignition and fuel injector operation, ECM also times interval between each pulse and compares each new time interval with previous one to determine when an excessive change in crankshaft speed has occurred.

A misfire causes an unexpected change in crankshaft speed. A certain amount of acceleration/deceleration is expected between each firing stroke, but if crankshaft speed changes more than expected amount, ECM will interpret this as a misfire. ECM continuously calculates crankshaft position from CKP sensor signals.

This information is used to determine which cylinder is misfiring so ECM can increment appropriate misfire counter and set appropriate DTC, if necessary. If ECM is able to determine a specific cylinder is misfiring, DTC for that cylinder will set. If misfire is random and on multiple cylinders, DTC P0300 will set.

Conditions for setting DTC:

• Misfire greater than legislated limit (amount of misfire ranging from 1.0-12.5 percent.
• Condition must be met for at least 5 camshaft revolutions.