Chart C-4B Misfire Check

Chart C-4B:

Distributor Ignition System Circuit:

CHART TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.

1. By performing Chart C-2A Injector Tests first all injectors must be operating OK at this point. This chart covers ignition system related misfire only. Chart C-2A Injector Coil Test Procedure

2. A thorough visual and physical check of the engine components which typically may cause a misfire may eliminate the need for further diagnostics. Use fuel pressure gage J34730-1. Check fuel pressure. Pressure after ignition "ON" should be 283-325 kPa (41-47 psi). If fuel pressure is not within specifications, perform Chart A-7 Fuel System Diagnosis. Related Tests, Information and Procedures

3. 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). This simulates a "load" condition. It is necessary to unscrew the top terminal on the ST 125 spark tester for a proper fit in the distributor end of the coil wire.

4. With engine running, try to induce the misfire by moving related harness connectors. This may help locate an intermittent connection. Using the Tech 1 scanner's snapshot feature may help locate an intermittent failure. Refer to the Tech 1 scanner operators manual. An intermittent low resolution signal can cause misfire. The Tech 1 scanner displays the time between low resolution pulses in milliseconds. The time between low resolution pulses is inversely proportional to engine RPM. As RPM increases, the low resolution time value decreases. With the Tech 1 scanner in snapshot mode, try to duplicate the conditions under which the misfire is present. When misfire occurs, manually trigger snapshot. When reviewing the captured data, compare Throttle Position (TP), Mass Air Flow (MAF), Manifold Absolute Pressure (MAP), Low Resolution Signal and RPM. Watch low resolution and RPM for fluctuations without corresponding changes in TP, MAF or MAP, which may correspond with missing or extra low resolution pulses.

5. If spark plugs check OK a mechanical engine problem may be causing misfire (i.e. worn camshaft, bent pushrod, low compression, etc.). Refer to Engine/Mechanical.

DIAGNOSTIC AIDS
The distributor vent system should also be checked. An inoperative system can cause premature distributor failure. A blocked air supply to the distributor will cause vacuum to build within the distributor causing misfire. Refer to Chart C-4C Distributor Vent System Check. Chart C-4C Distributor Vent System Check