GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC C Pickup - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Cuts Out, Misses

Checks

Action

DEFINITION: A surging or jerking that follows engine speed which is usually more pronounced as the engine load increases. This surging or jerking is not normally felt above 1,500 RPM or 48 km/h (30 mph). The exhaust has a steady spitting sound at idle, low speed or hard acceleration for the fuel starvation that can cause the engine to cut-out.

Preliminary Checks

Refer to Symptoms .

Ignition System Checks

Important:: CNG is a gaseous fuel and requires higher secondary ignition system voltages for the equivalent gasoline operating conditions. Do not discount the possibility of a CNG-only ignition system failure if a problem is reported on the CNG system and not on the gasoline system. Test the system accordingly.

Confirm that the distributor is adjusted so that the CMP Retard is -2° to 2° on 5.7L engines. Refer to the Distributor Adjustment in Engine Mechanical - 5.0, 5.7L in the C/K Truck Service Manual if the retard is not within specifications.

Inspect for engine misfire using the following procedure:

  1. Start the engine.
  2. Allow the engine to stabilize.
  3. Disconnect the Idle Air Control (IAC) motor.

    4. Important:: Do not perform this test for more than 2 minutes in order to prevent damaging the catalytic converter.

  4. Remove one spark plug at a time using insulated pliers.
  5. Refer to Rough, Unstable or Incorrect Idle, Stalling if there is no RPM drop on one or more of the cylinders.
  6. Reconnect the IAC motor with the ignition OFF.
  7. Inspect for spark on the suspected cylinders with the J 26792 Spark Tester or the equivalent if there is no RPM drop on one or more of the cylinders or if there is an excessive variation in the drop.
  8. Refer to Enhanced Ignition System in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual if there is no spark.
  9. Remove the spark plugs if there is spark. Inspect for the following conditions:
    10.  - Insulation cracks
     - Wear
     - Improper gap
     - Burned electrodes
     - Heavy deposits
     - Spark plug wire resistance (resistance should not exceed 30,000 ohms)
     - The distributor rotor and the distributor cap
  10. Visually inspect the following items if the previous inspection did not find the problem:
    11.  - The ignition system for moisture, dust, cracks, burns, etc.
     - Spray the plug wires with a fine water mist with the engine running in order to inspect for shorts.

Engine Mechanical Checks
    • Perform a cylinder compression check.
    • Inspect the engine for the following conditions:
       - Improper valve timing
       - Bent pushrods
       - Worn rocker arms
       - Worn camshaft lobes
       - Broken or weak valve springs.
    • Inspect the intake and exhaust manifold passages for casting flash.

Fuel System Checks
    • Inspect the fuel system for a plugged fuel filter, low fuel pressure, etc. Refer to Fuel System Diagnosis .
    • Inspect the condition of the wiring to the high and low pressure lock-off valves.
    • Verify the correct bi-fuel VCM software is installed.

Additional Checks

Inspect for the following conditions which can indicate electromagnetic interference (EMI):

    • EMI on the reference circuit can cause a missing condition.
    • Monitoring the engine RPM with a scan tool can detect an EMI.
    • A sudden increase in the RPM with little change in the actual engine RPM indicates that EMI is present.
    • Verify the routing of the secondary wires and the ground circuit if the problem exists.