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1988 GMC Suburban idle problem

I have a idle problem with my suburban it has a fuel injected 350,

when started it barely keeps an idle sometime it will shut off 30 seconds to a minute after starting it, if I shift into drive or reverse it shuts off. If I turn the steering wheel (I have power steering) it shuts off.

I replaced the fuel filter,

I clean and changed out the IAC value.

I don’t know what to do, HELP!

motor has about 114,000 miles
transition has about 99,000 miles on it
whole truck 241,000 miles on it

You mention the IAC but the truth is that a 1988 GMC had different or at least less vacuum requirements than later models.
Your follow-up post tells us a different story of a very old truck with a newer but used motor and a newer but used transmission. It doesn’t take a detective to guess that these used replacement components are not exactly from your truck’s original year and kind. You can’t expect that older parts will hook right up with newer systems and that everything is going to work just fine.

Check fuel pressure. Specs are 9-13 psi. Unfortunately on TBI units you’ll have to test fuel pressure either at the TBI or at the fuel filter. There is no test port. I always like to pick on fuel pressure first on these trucks because the short piece of hose between the fuel pump and pipe is notorious for splitting.

How old are the spark plugs & wires? Air filter?

Remove the air filter housing and stand on a ladder or stool to look at the throttle body. On the base that the filter attaches to, toward the rear drivers side is a somewhat polygonal shaped hole which is the inlet to the IAC valve. With the engine idling spray carburetor cleaner into that hole in quick bursts, paying attention to the engines stumbling and efforts to recover. With some patience and diligence you can shorten the delays between bursts of cleaner and eventually get to a continuous spray with the engine struggling to keep itself running. The problem is often blockage in the IAC port and this will solve that problem. If the IAC is bad or there is problem with the ECM or wiring this won’t help. This method has been successful more than 50$ of the time here.

For the past few months I have tinkered around with it
Double checking EVERYTHING
Checked all the vacuum hosts
new fuel filter
New air cleaner
The fuel pumper when out so I replaced that with and a new tank As well
Checked the fuel lines
Made sure that every thing was gunk free
But finally I think the problem was starring me in the face
I checked the wiring under the hood and found a broken ground and fix that
And under a mass of electrical tape just be hind the motor I
Found three or four green wires all badly put together
And also to the left I found a red wire.
I got some wire and touched the red wire to the battery and found
That it was to the fuel pump but I can’t find where it goes and I can’t find what sequence the green wires go in.
I think this maybe the source of my problems because everything
Else seem mechanically sound.
Do you know where I can get a good wire diagram of the engine compartment for free?

Ps spark plugs are new and plug wires are fine

After changing the fuel pump it keeps an idle for longer
But still idles bad and occasionally shuts off just sitting or turning the wheel