Misfire Monitor Diagnostic Tests

Misfire Counter:

DESCRIPTION
When the engine is running and a cylinder misfires the crankshaft slows down momentarily. The control module has the ability to detect this change in speed by monitoring the input signals from the following sensors:

^ The Crankshaft Position (CKP) sensor.
^ The Camshaft Position (CMP) sensor.

MISFIRE COUNTER STORES
When the control module is running the misfire on-board diagnostic tests and a misfire is detected the misfire counter stores (accumulates) the position and the number of misfires. Two counters (accumulators) are assigned to each of the cylinders. The counters are as follows:

^ The Current Misfire Counters.
- The current misfire counters store the number misfires that have occurred in the last 200 engine cycles. When the control module detects a unacceptable amount of misfires stored in a current misfire counter the misfire on-board diagnostic test tails and a misfire DTC will set.

^ The History Misfire Counters.
- When a misfire DTC is set, the history misfire counters store the total number of misfires that have occurred every 200 engine cycles.

DIAGNOSTIC EXECUTIVE
The Misfire Counters graphic illustrates how the misfire diagnosis and the Diagnostic Executive use the misfire counters (accumulators). When a misfire diagnostic test fails and reports to the Diagnostic Executive that a misfire condition exist, the Diagnostic Executive reviews all of the misfire counters in order to store the most current misfire information.

In the example illustrated by the Misfire Counters graphic, a misfire DTC has been set and the misfires are stored in more than one History Misfire counter (accumulator). The cylinder number 1 History Misfire and Current Misfire counters indicates a far greater number of misfires than the other cylinders. The misfires stored in the cylinder number 2 and the cylinder number 4 History Misfire counters (accumulators) indicate the erratic rotation of the crankshaft caused by the misfire of number 1 cylinder. In this example, viewing the Current Misfire and History Misfire counters indicate that a true misfire is present and the malfunction is located at cylinder number 1.

DIAGNOSIS
Use a scan tool in order to monitor the misfire counters data. Identify which cylinder or cylinders are misfiring. Inspect the systems and components common to the cylinders if multiple cylinders indicate a misfire is present.