Misfire Diagnostics

The PCM receives A/C voltage pulses from the crankshaft position sensor to determine the position of the engine at various degrees of rotation. The PCM monitors the acceleration and deceleration rate of the crankshaft when each cylinder is in its firing event. The PCM bases misfire diagnostics on the principle that the crankshaft velocity will increase as each cylinder contributes its power input. A misfire on a cylinder occurs when the acceleration rate of the crankshaft falls below what the minimum acceleration rate should be on a normal firing cylinder.

The PCM begins monitoring the engine for misfires once misfire enable criteria, which can be found under DTC P0300, has been met. The PCM will perform 200 revolution segment tests. During this time, if any misfires are occurring, the Scan tool will display the number of misfires under parameter MISFIRE CURRENT CYLs. #1 - 4. Once a 200 revolution test is completed, the current misfire counters will be moved to the MISFIRE HISTORY CYLs. #1 - 4 and the current counters will be reset to 0. The history counters will be reset to 0 when the ignition is cycled, unless DTC P0300 is active. The history counters will not be cleared until DTCs are cleared using the Scan tool.

There are two types of misfires: an emission related misfire and a catalyst damaging misfire. An emission related misfire indicates that within a 200 revolution test (400 combustion events), 2 - 19% (8 - 76 combustion events) of the combustion events failed. If the PCM fails 10 out of 16, 200 revolution tests, DTC P0300 will set. If after the second ignition cycle, the PCM fails another 10 out of 16, 200 revolution tests, it will send a message to the I/P cluster over the Class II link to turn On the Malfunction Indicator Lamp (MIL).

A catalyst damaging misfire indicates that within a 200 revolution test (400 combustion events), 20% or greater (77 combustion events or greater) of the combustion events failed. If the PCM fails four out of 16, 200 revolution tests, DTC P0300 will set. The PCM will send a message to the I/P cluster over the Class II link to flash the MIL until it no longer detects a catalyst damaging misfire.

The MIL will be turned Off after three passed trips under the same criteria that set DTC P0300 or 80 passed trips if the criteria could not be met. DTC P0300 will be the only DTC stored in memory for a misfire condition regardless

if the misfire was specific or random. The misfire history counters under MISFIRE - DATA DISPLAY will have to he reviewed as to which cylinder(s) was misfiring.

CRANKSHAFT RELEARN - In order to correctly detect misfires, the PCM must learn the variation between the 7 crankshaft notches. This procedure is performed only once. However, if the crankshaft is replaced, the crankshaft variation must be performed again. This can be accomplished using the Scan tool or Service Stall System (SSS). The following is a description of what happens when this procedure is performed.

When the procedure is initiated, the Scan tool or SSS will send a message to the PCM to reset the CRANK LEARNED adaptive to NO. After this is accomplished, a delay counter in the PCM will remain constant until all misfire enable criteria has been met or MISFIRE ENABLED is set to YES. If vehicle speed is present or if TP angle is greater than 2.3% when CRANK LEARNED adaptive = NO, the delay counter will increment and the ignition will have be cycled Off for 10 seconds to reset the counter. The procedure would then have to be repeated.

As soon as MISFIRE ENABLED YES when CRANKSHAFT LEARNED = NO, no misfires can exist. The PCM will then start to decrement the delay counter down to zero, which will take two engine revolutions. Once the delay counter is at zero, a 400 revolution test at idle is performed to determine if the engine idle is stable enough to perform the relearn procedure. At this time the PCM determines if any misfires are present.

If one misfire occurs during the 2 + 400 revolution test, the PCM will not learn the crankshaft for that ignition cycle. The ignition will have to be turned Off for 10 seconds and the entire procedure repeated.

After the PCM has run the 2 + 400 revolution test without any misfires, the SERVICE telltale will flash. The PCM will then wait for the engine RPM to be raised between 3000 and 4000 rpm. After the RPM is raised, the PCM will then take two crankshaft revolutions to learn the crankshaft variation. The entire learning process is done only with these two revolutions.