With VCM

DEFINITION
Steady pulsation or jerking that follows engine speed, usually more pronounced as engine load increases. The exhaust has a steady spitting sound at idle or low speed.

PRELIMINARY CHECKS
^ Refer to Important Preliminary Checks as described at start of Diagnosis by Symptom. Testing and Inspection

IGNITION SYSTEM
Check for cylinder miss by:
1. Start engine, allow engine to stabilize then disconnect Idle Air Control (IAC) motor. Remove one spark plug wire at a time, using insulated pliers.

CAUTION: Do not perform this test for more than 2 minutes as this test may cause damage to the catalytic converter.

2. If there is an RPM drop, on all cylinders, (equal to within 50 RPM), go to Rough, Unstable Or Incorrect Idle, Stalling symptom. Reconnect IAC motor with ignition "OFF." Rough, Unstable, or Incorrect Idle, Stalling
3. If there is no RPM drop on one or more cylinders, or excessive variation in drop, check for spark, on the suspected cylinder(s) with J 26792 (ST 125) Spark Tester or equivalent. Check one spark plug cable at a time and connect the companion cylinder spark plug cable (plug end) to a good ground, away from any electronic components.
4. If no spark, refer to Ignition System. Ignition System
5. If there is spark, remove spark plug(s) in these cylinders and check for:
- Insulation cracks.
- Wear.
- Improper gap.
- Burned electrodes.
- Heavy deposits.

^ Spark plug wire resistance (should not exceed 30,000 ohms), also, check rotor and distributor cap.
^ If the previous checks did not find the problem:
- Visually inspect ignition system for moisture, dust, cracks, burns, etc. With engine running, spray plug wires with fine water mist to check for shorts.

ENGINE MECHANICAL
Check:
^ Perform a cylinder compression check. If compression is low, repair as necessary.
^ For proper valve timing, low compression, bent pushrods, worn rocker arms, broken or weak valve springs, worn camshaft lobes.
^ Intake and exhaust manifold passage for casting flash.

FUEL SYSTEM
Check:
^ Fuel system - plugged fuel filter, low pressure. Use Chart A-6 Fuel System Diagnosis. Related Tests, Information and Procedures
^ Contaminated fuel.
^ Injector driver circuit. Disconnect all injector harness connectors at the injectors. Connect J 34730-2B injector test light or equivalent 12 volts test light between the harness terminals, of each injector connector and note light while cranking. If test light fails to blink at any connector, it is a faulty injector drive circuit harness, connector or terminal.
^ Injector resistance. Measure injector resistance of each injector. Nominal resistance of each injector should be 1.16 to 1.36 ohms. Resistance will increase slightly at higher temperatures.

ADDITIONAL CHECKS
Check:
^ For Electromagnetic Interference (EMI). A missing condition can be caused by EMI on the reference circuit. EMI can usually be detected by monitoring engine RPM with a scan tool. A sudden increase in RPM with little change in actual engine RPM change, indicates EMI is present. If the problem exists, check routing of secondary wires, check ground circuit.