FUEL PUMP DRIVEABILITY ISSUE DIAGNOSTIC INFORMATION

SUBJECT: FUEL PUMP RELATED DRIVEABILITY ISSUE (DIAGNOSTIC INFORMATION)

MODELS: 1987-92 C/K, S/T TRUCKS AND M/L VANS

CONDITION

Customer may comment of fuel pump noise/whine and/or cold start stall; engine hesitation, sag, loss of power; or check engine light with codes 32 or 44. Condition(s) may be intermittent, but would typically be experienced in the first 5 minutes of engine/vehicle operation.

CAUSE

Possible fuel starvation.

The matrix in Figure 1 is provided to help during diagnosis activities.

Diagnostic Aids:

1. Oxygen sensor reading biased toward lean when the engine is under load.

2. Fuel pressure drops or does not increase by at least 5 psi when throttle moves from idle to part throttle.

3. Fuel integrator values steadily increasing under load.

4. Intermittent condition temporarily eased when accelerator pedal is released. Condition retums when accelerator pedal is reapplied.

5. Complete loss of fuel pressure.

CORRECTION

Use Section 6E3 of the appropriate service manual to diagnose possible fuel system related problems. The vehicle may need to be operated under a load while monitoring fuel pressure to detect the problem.

Important

MAKE CERTAIN THE FUEL SYSTEM IS PER- FORMING TO SPECIFICATIONS BEFORE OTHER ENGINE CONTROL SYSTEM COMPONENTS ARE SERVICED.

SERVICE PROCEDURE

Follow procedures identified in the appropriate service manual. For electrical intermittents, see GMC Truck Service Bulletins 92-8A-09 issued September 1991 and 88-T-76 issued February 1988, as well as Section 8A of the service manual and instruction in required tool J-38125 usage.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.