Misfire at Idle

Misfire At Idle:

Ignition Control Circuit:

Circuit Description
The Electronic Ignition (EI) system uses a waste spark method of distribution. In this type of system, the ignition module triggers the number 1-4 coil pair resulting in both number 1 and number 4 spark plugs firing at the same time. Number 1 cylinder is on the compression stroke at the same time number 4 is on the exhaust stroke, resulting in a lower energy requirement to fire number 4 spark plug. This leaves the remainder of the high voltage to be used to fire number 1 spark plug. The Crankshaft Position (CKP) sensor is remotely mounted beside the module/coil assembly and protrudes through the block to within approximately .050" of the crankshaft reluctor. Since the reluctor is a machined portion of the crankshaft and the CKP sensor is mounted in a fixed position on the block, timing adjustments are not possible or necessary.

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

1. If the "Misfire" complaint exists under load only, the "Misfire Under Load" chart must be used. Engine RPM should drop approximately equally on all plug leads.

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).

3. If the spark jumps the tester gap after grounding the opposite plug cable, it indicates excessive resistance in the plug which was bypassed. A faulty or poor connection at that plug could also result in a misfire condition. Also, check for carbon deposits inside the spark plug boot.

4. If carbon tracking is evident, replace coil and be sure plug cables relating to that coil are clean and tight. Excessive wire resistance or faulty connections could have caused the coil to be damaged.

5. If the no spark condition follows the suspected coil, that coil is faulty. Otherwise, the ignition module is the cause of no spark. This test could also be performed by substituting a known good coil for the one causing the no spark condition.