COLD START DRIVE. HESITATION NEW ACCELERATOR PUMP

VEHICLES AFFECTED: 1981-83 C10/15 With 5.0L V-8 W/Federal Emissions

Some 1981-83 C-10/15's with 5.0L V-8 federal engine may experience hesitation and/or stumble upon throttle maneuvers after the 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 amount of accelerator pump fuel and the period of time that the trapped spark system (temperature controlled vacuum spark advance) remains in operation or by a malfuntioning trapped spark system. This bulletin provides diagnosis information and the recommended repair procedures to correct the cold start/drive away driveability condition.

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 in. Hg vacuum gauge into this hose.

5. Start engine and run at idle in neutral.

6. Check vacuum reading. Reading should be 15 in. to 20 in. Hg. If this reading is not achieved, replace the engine vacuum hose harness assembly and check for restricted vacuum part or hose.

7. Depress the accelerator pedal to the floor and release rapidly while observing the vacuum gauge. 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 outlined 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 Lt. Duty 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.