COLD START DRIVE. HESITATION NEW ACCELERATOR PUMP

1981-83 C15 MODELS WITH 5.0L V8 ENGINES WITH FEDERAL EMISSIONS

Some 1981-83 C15's with 5.0L V8 federal engines may experience hesitation and/or stumble upon acceleration after initial cold start and drive away, however, after full engine warm-up, the vehicle returns to an acceptable performance level. This driveability condition is affected by the relationship between the amount of accelerator pump fuel discharge and the period of time that the trapped spark system (temperature controlled vacuum spark advance) remains in operation. This condition can also be caused by a malfunctioning trapped spark system. Diagnosis information and recommended repair procedures to correct the cold start/drive away driveability condition are as follows:

DIAGNOSIS AND REPAIR PROCEDURE

1. Verify correct vehicle vacuum hose routings. Compare actual vehicle hose routings with routings shown on tune-up label schematic. Make any necessary routing adjustments.

2. Inspect vacuum hoses for cuts, splits, and burns. Remove any hoses exhibiting cuts, splits or burns, and replace with new hose.

3. Allow the engine to cool for eight (8) hours.

4. Disconnect vacuum hose from distributor vacuum advance. Install a 0-30" Hg vacuum gage into this hose.

5. Start engine and run at idle in park or set parking brake and idle in neutral.

6. Check vacuum reading. Reading should be 15" to 20" Hg. If this reading is not achieved, check for a restricted vacuum port or a bad hose. If hose is found to be bad, it should be replaced.

7. Depress the accelerator pedal to the floor and release rapidly while observing the vacuum gage. If there is no change in vacuum reading, the system is working correctly and Step 7A should be followed. If a rapid drop in vacuum reading occurs, follow procedure in Step 7B.

A. No change in vacuum reading:

Remove accelerator pump and thermal vacuum switch (TVS). Install a higher capacity accelerator pump, P/N 17051374, and a TVS switch which allows for extended control period for trapped spark operation, P/N 3055378.

B. Rapid drop in vacuum reading:

Remove vacuum delay valve. Install new vacuum delay valve, P/N 458169.

If malfunction persists, replace TVS with P/N 3041970.

Follow replacement procedures as outlined in the appropriate 10-30 Series Truck Shop Manual.

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.