Engine - Overheat Diagnosis: Overview
91chevy06
Number: 91-167-6B
Section: 6B
Date: January 1991
Corporate Bulletin No.: 066204
Subject: ENGINE OVERHEAT
Model and Year: 1988-91 C/K & R/V TRUCKS WITH 5.7L AND 7.4L ENGINES
TO: ALL CHEVROLET DEALERS
The purpose of this bulletin is to provide assistance in diagnosing a possible engine overheat condition on 1988-91 C/K and R/V trucks equipped with 5.7L or 7.4L engines. The information contained herein has been taken from cases submitted to the Technical Assistance Center, and from engineering reviews on vehicles exhibiting engine overheat.
Included in this bulletin are copies of all previously published service bulletins which relate to 1988-90 C/K and RN cooling systems. A review of these is recommended before proceeding.
When starting your diagnosis, it is very important that you first confirm at what temperature the engine is operating. Start by connecting a scan tool to the vehicles ALDL Connector and observe the engine coolant temperature. Compare this reading to the temperature gauge (if equipped) in the instrument panel cluster. Do they agree?
If the gauge reading is significantly higher (more than 10 degrees), diagnose the gauge, sensor and related wiring first. If they agree, continue in your diagnosis of the engine cooling system. On vehicles equipped with a temperature indicator light (no gauges), verify the engine temperature when the temperature light turns on. The light is designed to turn on at 250 degrees F. If the light is turning on and the engine coolant temperature on the scan tool is displaying 245 degrees F or less, diagnose the temperature light, switch and related wiring first. If the engine coolant temperature is between 245 and 265 degrees F when the light comes on, continue with your diagnosis of the cooling system. On C/K models equipped with gauges, verify the engine temperature when the check gauges light turns on. The light is designed to turn on at 245 degrees F. If the light is turning on before 245 degrees F, diagnose the check gauges light first. See GMC Dealer Service Bulletin No. 88-177-6B for additional information.
When diagnosing a complaint of engine overheat it is very important to find out exactly when the overheat occurs. Most engine overheats fall into one of the following categories overheats when idling, or overheats when operating under a heavy load or towing.
If the overheat occurs when idling, the following information should be obtained before starting your diagnosis.
1. What is the coolant temperature when the engine first begins to idle?
2. How long (minutes) does it take from the time the engine starts to idle for the overheat to occur?
3. Is the transmission in drive/reverse, or is it in park/neutral during this period of time?
4. If equipped with air conditioning, is the A/C in use when the overheat occurs?
5. What is the approximate ambient temperature outside the vehicle at the time?
6. How was the vehicle being driven before idling: highway, mountains, city, offroad, etc.?
If the overheat occurs when under heavy loads or towing, the following information should be obtained before starting your diagnosis.
1. At what road speed is the vehicle being driven when the overheat occurs?
2. If equipped with air conditioning, is the A/C in use when the overheat occurs?
3. What is the approximate ambient temperature outside the vehicle at the time?
4. Where is the vehicle being driven (highway, mountains, off-road)?
5. What is the weight of the load or trailer?
6. What is the Gross Vehicle Weight Rating (GVWR) and the Gross Combination Weight Rating (GCWR) of the vehicle? This is needed to determine if the vehicle is being overloaded.
The following is a list of cooling system components as well as what you should be looking for when conducting your diagnosis.