Engine Cranks, But Will Not Run
***THIS ARTICLE HAD BEEN UPDATED BY TSB # 61-65-42 DATED 10/96***
Circuit Description
The Electronic Ignition system uses a waste spark method of spark distribution. In this type of ignition system, the ignition control module triggers the correct coil based on the sync signal received from the crankshaft position sensor, resulting in both spark plugs connected to the coil firing at the same time. The spark plug for the cylinder on the exhaust stroke requires a lower energy requirement to fire, leaving the remaining high voltage to fire the spark plug for the cylinder on the compression stroke.
During crank, the ignition control module monitors the Crankshaft Position (CKP) sync signal. The CKP sync signal is used to determine the correct cylinder pair to fire first. The CKP sync signal is used only by the ignition control module to initiate ignition coil firing sequence. It is not passed to the PCM. After the ignition control module processes the CKP sync signal, the ignition control module monitors the 18X CKP signal and sends 18X and 3X reference pulses to the PCM. When the PCM receives these pulses, the PCM will command all six injectors to allow a priming shot of fuel for all the cylinders. After the priming, the injectors are left OFF during the next six 3X reference pulses from the ignition control module. This allows each cylinder a chance to use the fuel from the priming shot. During this waiting period, the PCM receives a camshaft position signal pulse. The camshaft position signal allows the PCM to operate the injectors sequentially based on camshaft position.
It the camshaft position signal is not present at start-up, the PCM will begin sequential fuel delivery with a 1 in 6 chance that fuel delivery is correct.
Diagnostic Aids
An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Check for the following items:
Poor connection or damaged harness - Inspect the PCM harness and connectors for improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
Malfunctioning Engine Coolant Temperature Sensor - Using a scan tool, compare Engine Coolant Temperature with Intake Air Temperature on a completely cool engine or compare Engine Coolant Temperature with actual coolant temperature read with a thermometer on a warm engine. Engine Coolant Temperature should be within 100 C of Intake Air Temperature or actual coolant temperature. If not, replace the ECT sensor.
Stuck TP sensor - Ensure that TP Angle displays 00/0 with the throttle closed. A TP sensor that causes the PCM to command clear flood mode (TP angle over 80% during crank) will cause an engine cranks but will not run condition.
Skewed or unresponsive MAP sensor - The barometric pressure used to calculate the default Mass Air Flow value is based on the MAP sensor reading. A skewed MAP sensor at key ON will cause the BARO reading to be incorrectly calculated. Also, with the engine running, an unresponsive MAP sensor (due to poor vacuum connections, damaged vacuum source, or malfunctioning vacuum hose(s), unmetered air into the manifold) will cause inaccurate BARO reading updates during wide open throttle conditions. Both of these conditions result in a difference between the actual MAF sensor signal and the predicted MAF value (PCM calculated). If a large difference between these two values occurs DTC P0101 will set. This condition may also cause abnormal AC counts. With the lAO at an incorrect count value, a hard start / no start condition may occur.
Test Description
Number(s) below refer to the Step number(s) on the Diagnostic Table:
5. A blinking test light verifies that the PCM is monitoring the 3X and 18X reference signals and is capable of activating the injectors.
7. The crankshaft position sensor has been verified as functioning properly. A fuel pressure test will separate the diagnostic path into either a fuel related fault or an ignition system malfunction.
9. By using J 26792 (ST 125) or equivalent to test for spark on the plug wires, each ignition coil's ability to produce at least 25,000 volts is verified.
15. By testing the problem coil's control circuit with a test light, a determination can be made as to the problem coil being faulty or the ignition control module's internal driver circuit for the affected coil being the cause of the complaint.
37. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.