GM Service Manual Online
For 1990-2009 cars only

Important Preliminary Checks

Important Preliminary Checks

Action

Before performing Symptom diagnosis

Before performing Symptom based diagnosis you must perform the Diagnostic System Check - Alternative Fuels (Engine Controls) .

The System Check will determine the following:

  1. The control modules - Alternative Fuels ECM and the gasoline VCM / PCM or ECM - and the MIL are operating correctly.
  2. There are no DTCs stored.
  3. Scan tool data is within normal operating range.

Several of the Symptom diagnostic tables require that a Visual and Physical Check be performed. Always perform a Visual and Physical Check before using a Symptom diagnostic table.

Visual and Physical Checks

    • Inspect the control modules (AF ECM and VCM / ECM or PCM) ground connections for being clean, tight and in the correct location.
    • Inspect the gasoline control module (VCM/PCM or ECM) for the correct Bi-fuel calibration. Program the gasoline control module if necessary.
    • Inspect the vacuum hoses for leaks, restrictions and proper connection as shown on the vehicle emission control information (VECI) label.
    • Inspect for any type of fuel leak or fuel line restriction.
    • Inspect for air leaks at all areas of the intake manifold sealing surfaces.
    • Inspect for proper installation of the bonnet and mixer.
    • Inspect for air leaks at the mixer.
    • Inspect the ignition wires for the following conditions:
       - Cracking
       - Hardness
       - Proper routing
       - Carbon tracking
    • Inspect vehicle wiring for the following:
       - Proper connection
       - Pinches
       - Cuts

Symptoms

  1. Verify the customer concern.
  2. Locate the Symptom table that indicates the customers concern.
  3. Test or Inspect for the items indicated for that symptom.

The following Symptom tables are located in this section:

    •  Hard Start
    •  Surges/Chuggles
    •  Cuts Out, Misses
    •  Poor Fuel Economy
    •  Poor Fuel Fill Quality
    •  Dieseling, Run-On
    •  Backfire