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ASEMaster35yrs, GMC Technician
Category: GM
Satisfied Customers: 2095
Experience:  ASE Certified since 1972,Master Auto tech,Heavy Truck,4 years GMC Tech
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1987 gmc: warmed..highway speeds..the check engine light comes on

Customer Question

I have a 1987 gmc v1500 with 305 fuel injection. My current problem seems to occur after the vehicle is warmed up and going at highway speeds. It will run and drive fine then all of a sudden the check engine light comes on. After that the truck seems to fall on its face as performance goes. I ran a check on the engine codes. It comes up with a code 32 something to do with the EGR circuit and a code 42 having something to do with the EST circuit. I also know there is a ESC circuit. I would like to know the difference between the two and any ideas what to look into for fixing it.
Submitted: 5 years ago.
Category: GM
Expert:  ASEMaster35yrs replied 5 years ago.

Hi, glad to be of help. Code 42 has more to do with your spark advance timing and is often caused by failure of your spark control module that looks like this:


code 43 (ESC) which is also spark control related deals more with your knock sensor as timing needs to be retarded when a ping is detected. A failure of your EGR valve can cause your engine to ping thereby retarding your spark. You should repair the lowest code first before going on to the next code. Repair your EGR problem first.

Here are some good links that will help you.


Forum links



This article will explain the difference between EST & ESC as EST failure will result in code 42 and ESC failure will result in code 43. Read what is listed for these two codes, and read for code 32.


Your problem could be your ECM as 1987-1989 had a recall on ECM's.

Typical causes for code 32 are as follows:

1) Faulty EGR valve-to-ECM connection
2) Plugged EGR passages and/or sticking EGR valve
3) Defective EGR valve
4) Defective ECM


Expert:  ASEMaster35yrs replied 5 years ago.
To put it simply, EST is spark timing advance and ESC is spark timing retard.
Customer: replied 5 years ago.

Hi again I have found that my knock sensor was disconnected and the pigtail that clips to it is missing. I purchased the one wire connector for it from the local parts store. The question I have is I can't seem to locate anything under the hood that would hook to it. I followed the wires from the ESC processor and all I found is a two wire pigtail with another pigtail attached to it going nowhere. There is no connector for the knock sensor on this one. What I would like to know if there is any online wiring diagrams showing this two wire connector going down to a one wire connector and then attaching to the sensor itself


Expert:  ASEMaster35yrs replied 5 years ago.

First of all, this is not the cause of your problem. Concern your focus on code 32 & code 42.

On your vehicle it doesn't show the knock sensor in the diagram. Here is your wiring diagram:





Read this article about code 42

That Annoying GM Code 42

Testing EGR Valve

The manual says that there are two procedures to vacuum test the EGR on 1988-1995 TBI engines.

1. vehicles under GVW 8500
Turn ignition OFF. Disconnect vacuum hose on the intake manifold side of the EGR solenoid valve and hook up a hand vacuum pump. Apply vacuum with the pump. The valve should hold vacuum. If it doesn't, replace the valve.

2. vehicle over GVW 8500
Turn ignition ON, but do not start engine. Disconnect vacuum hose on the intake manifold side of the EGR solenoid and hook up a hand vacuum pump. Apply vacuum with the pump. The EGR valve should NOT move. If the EGR valve moved, disconnect the electrical connector and repeat the test. If the EGR valve moved, replace the solenoid valve.

The manual says that is the limit of what you can test for the EGR because the operation is interrelated to the operation of the ECM (engine control module)


On a "Back Pressure" Valve, the valve operates on exhaust back pressure combined with an electronic signal from the ECM to an electronic controlled valve which in turn activates the valve which then allows vacuum to go to the back pressure EGR valve at a given determined time. This is controlled automatically by the ECM.