Brothers 91 Chevrolet CK1500 Weird Idle Issue

My brother has spent a lot of his time and money trying to fix his 1991 Chevrolet CK1500 V6. A month after he bought he went out to start it and it died almost immediately. However, he can keep it alive by giving it throttle. When the truck is warm it seems to idle for the most part, but occasionally it will just stall on him. He has changed the IAC, TPS, quite a few other sensors, took it to a shop and they changed the computer, he even recently purchased a new (used) throttle body.

Still no fix. However, when the shop changed the computer, it fixed the majority of the problem. The truck no only dies when cold, and when he is coming to a complete stop.

We have checked for vacuum leaks and haven’t found any.

Can someone please help us resolve his problem? Much appreciated,

Bump. Any mechanics out there want to chime in?

Has anyone run a smoke test? It sounds like a vacuum leak to me. A leak-down test may also be good to make sure the valves are sound.

A new (used) throttle body was purchased. Has it been installed? Regardless, while standing at the front of the truck looking at the throttle body, at the 2 o’clock position on the throttle body there is a peculiar shaped hole that is the intake for the IAC. With the engine running spray carburetor cleaner into that port in short bursts and the idle speed should be increased by the computer to overcome the rpm loss from the spray. You should be able to spray in ever increasing bursts for several minutes after which the idle hopefully will stabilize and the engine run without stalling.

The fact the idling problem is worse when the engine is cold is consistent with a lean mixgture condition. This would usually show up as a check engine light. Is that light on?

There’s either not enough gas or too much air in the mixture. I’d expect the latter, unmetered air getting into the engine. It sure sounds like some kind of air leak. Are you sure you’ve properly checked the vacuum system? Did you use a vacuum pump and check each device one by one?

Other ideas … hmmm … besides just bringing all the routine maintenance up to date … new spark plugs, new air filter, new filters … here’s some other possibilities …

Faulty O2 sensor, crank or cam sensor, map/maf sensor, fuel pressure regulator (assuming it is fuel injected), poor compression, ignition or valve timing problem.

There are usually certain conditions which make the engine idle speed go faster. That can be a clue. Like when the engine is cold, when you turn on the headlights, the rear window defroster, turn the steering wheel (if it has power steering). Do all those things make the engine idle noticeably faster?

This one may prove difficult to fix without the proper engine analysis tools. Is there a local inde mechanic who specializes in Chevy’s? Might be worth it to fork over some dough for him to hook it up to his Chevy engine analyzer tool at least.