Misfire Under Load

Engine Cranks But Won't Run Circuit:

Circuit Description:
The C3I ignition system uses a waste spark method of spark distribution. In this type of ignition system the ignition module triggers the #1/4 coil pair resulting in both #1 and #4 spark plugs firing at the same time. #1 cylinder is on the compression stroke at the same time #4 is on the exhaust stroke, resulting in a lower energy requirement to fire #4 spark plug. This leaves the remaining high voltage to fire #1 spark plug.

Chart C-4-2 Misfire Under Load Diagnostic Chart:

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

1. If the "misfire" complaint exists at idle, refer to Misfire At Idle Misfire at Idle the diagnostic CHART C-4-1 must be used.

2. A spark tester such as a ST-125 must be used because it is essential to verify adequate available secondary voltage at the spark plug (25,000 volts). Spark should jump the tester gap on all 6 leads. This simulates a "load" condition.

3. If ignition coils are carbon tracked, the coil tower spark plug wire boots may be damaged.

4. By installing a normally operating coil, a determination can be made as to the fault being the coil or ignition module.

A customer, under wide open throttle conditions, may experience fuel shut-off at high rpm's. Rpm fuel shut-off point is MEM-CAL controlled, based on axle ratio. This could be interpreted as misfire under load.