Engine - Cold Start Driveability, Hesitation/Stumble
84chevy10Subject: COLD START DRIVEABILITY HESITATION AND/OR STUMBLE COMMENTS DURING WARM-UP ON 5.OL V8 ENGINE WITH FEDERAL EMISSIONS
Model and Year: 1981-83 C-10
Number 83-143
Section 6E (VIt)
Date: Oct, 1984
TO: ALL CHEVROLET DEALERS
Some 1981-83 C-10' with 5.OL V8 federal engines 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 malfunctioning 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" Hg vacuum gage into this hose.
5. Start engine and run at idle in neutral.
6. Check vacuum reading. Reading should be 15" to 20" Hg. If this readinq is not achieved, replace the entire vacuum hose harness assembly or check for restricted vacuum part or hose.
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 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 10-30 Series Truck Shop Manual.