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My stalling Grandson

I’m helping my grandson keep his 93 GMC K1500 w/5.7L auto running till he gets out of college. The problem is the engine stalls (sometimes) when put into gear (D or R) and more often when the lights and heater blower are turned on. There were two codes in the pcm (42 & 43) both referring to ignition problems. We did a tune up and erased the codes. After driving the truck for a week the pcm has no codes but the engine still stalls. We cleaned the IAC pintel and seat thinking there might be an air control problem, no help. The repair manual mentioned ignition advance as a problem particularly the ESC. The manual says this part is on the side of the throttle body but we can not find it. Anybody have any ideas about the stalling or where to find the ESC would be helpful.

The problem might be that the IAC valve is worn where it doesn’t go to the proper position for the idle condition.

A quick way to determine if it’s the IAC valve is, get in the habit of using your left foot on the brake pedal while slightly tipping into the gas pedal with your right foot.

If doing this prevents the engine from stalling when shifting into gear, or when coming to a stop, the problem is most likely with the IAC valve.


Thanks for such a quick response Tester! My grandson has been driving in this fashion for sometime now anticipating the stalling. I did not want him to through 60 or 70 bucks into the truck without a second or third or fourth opinion. Thanks.

The throttle body should have two sensors on it, a throttle position sensor (TPS) and an Idle position switch (IPS). With your foot off the gas, if the IPS does not make, then the computer thinks you are still controlling the throttle and it will not take over and control the idle speed via the IAC.

Some reasons for the switch not making, dirty throttle body keeping the throttle from fully closing, someone adjusted the throttle stop screw thinking that is was an idle speed screw or the switch itself is bad.

Hello Keith! We removed and cleaned the throttle body and only noted the IAC and the TPS, no throttle stop screw. We will look again. Thanks for the suggestion.

What’s the engine idle rpm measure in neutral and in drive with foot on brake? On some pre-OBD II vehicles like yours the computer will increase the advance the ignition timing to bump the idle rpm a little in certain engine loading conditions to prevent stalling. As to where the ESC is, ESC I guess means electronic spark control so it is a small electronic module likely near, possibly inside, the distributor. Before messing with it though, suggest to use a timing light to measure the appx ignition timing at idle in conditions where stalling has been a problem.

Hmmm… Not sure how many miles but when was the last time the timing was set? This engine surely is stretching its timing chain by now which will result in retarded timing and slower idle. Simply adjust the distributor to set the timing and properly set the idle if it hasn’t already increased its idle rpm’s from the timing adjustment.

Timing needs to be adjusted and ensured along the life span of these engines its rather important actually.


Thanks for the info. We did check timing when we did the tune up and the mark was steady on the balance wheel. My grandson call from school today to tell me that code 42 is back. The repair manual reads check wiring between dist ign module and PCM, (what a job that can be!) It also suggested checking the ign module, (which I thought either works or does not), and replacing the PCM(sounds expensive). After more reading I found code 42 maybe set by a problem with the electronic spark timing(EST) system and diagnosis should be left to a dealer. Is that really true?

Sounds like code 42 is saying there’s some kind of problem the ECM is detecting with the ignition system. The way it works most likely, just a guess mind you, there’s a crank shaft position sensor that the ECM uses to detect where the crankshaft is, then there’s another sensor used to decide if the valves are in the intake or exhaust cycle; often that’s some kind of camshaft or distributor shaft sensor. When the two sensors say it is time to fire cylinder number 1 for example, then the ECM sends an electronic pulse to the igniter (possibly called the ignition module), and the igniter pulses the coil and generates a spark, which the distributor routes to the spark plug for that cylinder. The ECM then may verify via the crank shaft sensor the spark caused the crank shaft to accelerate as expected.

So what you’ve got is a bit of a Columbo investigation on your hands there. Something is wrong somewhere in that chain. GM specialty shops would have diagnostic tests and tools to figure it out. If you want to do this yourself, probably best to start replacing the easiest stuff first, like the spark plugs, spark plugs wires, distributor cap, & rotor. Then move on the crank position sensor. Often the cam sensors have gaps you can measure to verify before replacing those.

Edit: The code 43 I think is a ping sensor code. Knock sensor fault. One possible cause, the ignition system is firing at the wrong time, which could be due to a crank or cam sensor problem.

Thanks for the input George! The 93 does not have a crank position sensor per say, the crank position is sensed by the ign module and info sent to the ECM. The EMC checks the EST system and adjusts timing accordingly, so the repair manual says. What is confusing is that the truck preforms good except for the stalling. We may have the ign professionally diagnosed. Just thought somebody else may have had this same problem and would share their experience.

If it suggest the electronic ignition module…just replace it…you may be chasing the symptoms of a failing electronic ignition control module. Which has in the past been known to drive stronger men toward full blown insanity.


Cannot tell you how often in this year of truck it turned out to be a bad fuel injectors. Most recent success story, our mechanics went through all the iterations, sent it to the dealer for proper analysis, if you can afford the dealer do it!