1987 5.7 Liter Chevrolet Truck Idle Problem

What is, " idle speed motor wires"? That's a new one on me.

Maybe a large vacuum leak?

I have to make it as complicated as I can (it's a fault of my education) and it probably is a vacuum line disconnected, thank you very much.

And I always want to put my 'scope on my car, but not when it running properly.

As I understand it,there is a little air bypass motor screwed into the side of the throttle body air passage.

Although it has been a few years since I looked at the theory of TBI air/fuel systems, what I think I recall is that the ECM - read onboard computer (Electronic Control Module, or whatever they called it in this model) takes the signal from the throttle position potentiometer, the vacuum sensor, and maybe something else, and opens or closes the air bypass by pulsing this little stepper motor. Hence air for idle conditions.

The throttle body butterflies are closed when you take your foot off the gas, and without air bypass the engine stalls. In the old days we could crack the butterflies open with the various adjustments on most carburetors or there linkage, but that went out about the same time distributor points disappeared.

Referring to my first statements, and your thought, I maybe had better rethink my diagnosis.

Thanks, but I still might go out there with my 'scope. Sort of macho thing. It might impress the dog (see this weeks cover of Time magazine).

If you normally 'scope things out, by all means, go for it.

Take out The Idle Air Control Valve/Motor and clean it could be a vacum leak and or fuel filter.

Does your model use a mass air flow sensor (located just before the throttle body) I don't know if they were used that far back. A bad or dirty MAF sensor can cause just this type of problem, especially stalling when you let off the accelerator.

That is exactly the symptom, but I don't recall seeing anything that resembles a mass air flow sensor. I think MAF sensors came along later with the next generation of engine controls.

I think there is a very good possibility there is a vacuum leak as Lynlynch suggested; that too will cause stalling. Maybe I can get out there tonight for a few minutes.

It is interesting how much engine controls have changed in the past twenty years, considering that there was basically no change at all for almost 60 years!

If my memory is correct, the 1987 Chev. 5.7 liter fuel system is primarily based on speed-densisty (rpm - manifold pressure/temperature). There is month mass airflow meter -just a big acoustic plenum for sound attenuation. Faulty idle air control (IAC) can generally be traced to the following possibilities: 1). Faulty isle air bypass motor / plunger (DC motor / position sensor) 2) Throttle angle posiot sesnsor Vaccum leaks raise the airflow / manifold preeur and usual causes high idle / unwanted check engine light. Look for the idle air codes /fauts. Clean or replace IAC parts as necessary. Good luck.

Sounds like a typical TBI vacuum leak to me. Check the gaskets between the TBI and the manifold, possibly the manifold to the heads. Must be a good sized leak to cause a stall.