1995 suburban erratic idle

chevrolet
suburban

#1

Hi everyone! I am a shadetree mechanic, and I am stumped. I have a 1995 Suburban, 2500, 2 wheel drive, with a 5.7 liter. I have a weird starting problem that I can not pin down.
If I walk out to my truck in the morning, and start it, 99.9% of the time it will start without a problem. At that point, I am batting 100%. So let’s say I drive to work, and turn off my truck. 20 minutes later I may have to go out and start it, and it will start easily, as always, as soon as I let go of the key and let it snap back to the run position, my truck will die. No funny sounds, just a dead truck. It will start back up easily, no problem. As soon as I let go of the key and let it snap back to the run position, it will die as before.
So normally I will push down the fuel, start, and when I let go of key, I will hold down the fuel a little for a few extra seconds, and then it will ‘stay running’; but it will do this surging thing until I put it in gear. It will idle up a little, and then drag down so low that I think it is about to die. Then it will idle up again. Once I put it in gear and push the fuel to go, the problem is non existent.
It doesn’t act up at stop lights or signs, only when I start it after having driven it shortly before hand.
Sometimes after having driven it, and then trying to start it, using my ‘technique’, as soon as I let off the fuel, it will idle up real high, like around 3 thousand or so, and then slowly back down to regular idle.
I have put new distributor, new coil, new sparkplugs, new injectors (throttle body), new fuel pump, new fuel filter. I can’t remember what all else I have done related to this problem.
Is my weird idle issue related to why it dies? What part or system is it that makes a car/truck idle up for a few seconds after you start it, so that it can distribute the oil and let it warm up for a second? I have noticed most cars and trucks idle slightly higher than normal for a few seconds when you first start them. Why? Thanks guys!!!


#2

I would clean the idle air control valve (IACV) and it’s port. It should be mounted to the side of the throttle body. It’s a pretty simple device. It is responsible for metering the air when your foot is not on the throttle. If it does stay closed (stuck or electrically) the truck won’t stay running when you first start it. You working the pedal allows the air and keeps it running. Surges and falls are the IACV hunting for the right spot. This is all my theory of you situation anyway. Why it only acts up under some circumstances and not others, I can’t say, but things like heat often matter when things go wonky.


#3

I wonder if the throttle position sensor is getting glitchy?


#4

It’s probably the IAC as @cigroller mentioned. I’d start with that. Could be the throttle sensor too, but less likely. Testing the TPS is a fairly easy thing to do on most cars, so it might be worth it to have it tested and eliminated as a possible problem source.

One other common thing that causes surging is when the computer gets confused on how to set the air/fuel mixture. Unlike older cars, modern engines are totally air-sealed systems. They have to be for the electronic fuel injection to work. All of the air that gets into the engine is supposed to be metered (measured), and just enough gas is then injected by the computer to use up the air. But if air is getting into the engine and somehow not getting measured, that can confuse the computer and cause this surging as it hunts for the proper amount of gas to inject.

That can be caused by vacuum leaks – a common one is the brake booster diaphragm has developed a leak. But any interface to the intake manifold could develop a leak. Mechanics have methods to test them one by one. One last thing … in cars of your vintage, if it has an idle speed adjustment screw, if that screw has been adjusted to compensate for an idle problem which shouldn’t be solved that way, then that can confuse the computer and cause this type of surging too.


#5

These are some good, answers, and some good places to start looking. Thanks a bunch. I will take any further comments also.
And as a side note, my driver’s door is sagging. The holes where the hinge pins go are ‘ovaled’. I bought new hinges, and read the directions on how to install. I am stumped again. The directions say I would see little indents where the original cold wells were; but I do not see them. Do I need to remove the paint first to see the indents in the hinges, which I then need to drill out? Thanks again y’all!!


#6

Good ideas already but there may be another culprit. You may have a bad ignition switch. You start in the start position and when you release the key…you go to the run or on position. I’ve seen this problem before and a new ignition switch fixed the problem.


#7

@‌charliemann

Clean those old hinges first, remove paint as needed

But trust me, those marks are there

We have tons of older GM trucks in our fleet, and they all have those marks