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1993 C1500 1/2 Ton Chevy Idle Problem

When I first start my truck it idles really rough, I have to give it gas to prevent it from stalling. Then, the longer I drive my truck the RPMs gradually begin to drop which leads my truck to stall constantly if I don’t give it a little gas. I’m at my wit’s end trying to figure out what’s causing this issue. The engine has been rebuilt and it runs like a dream, except for this idle issue. Any help would be great!!

Sounds like a possible vacuum leak to me but I have no idea what you’ve already done to try to correct the problem. What/how have you checked so far?

The problem might be with the Idle Air Control valve.

The IAC valve controls the engines idle speed under all conditions whenever the accelerator is released.

Here’s the IAC valve for your truck.


We had the same truck at work, similar problems, the mechanics were stumped so took it to the dealer, bad/clogged fuel injectors. Truck is still running today. About a year ago about the same truck as I recall, had been through everything and I suggested fuel injectors, he found one so clogged he was surprised it ran, Not saying this is your issue but certainly something to keep in mind.

I’m guessing a 1993 Chevy 1500 truck has tbi . . . ?

Just recalling events, TBI still uses injectors? It was GMC branded but not sure if that matters.

Huh . . . ?

TBI stands for throttle body injection

What else would they use, if not fuel injectors?

Sure, the injectors are different, versus efi injectors, but they’re still fuel injectors

Glad we are on the same page, so fuel injectors are a possibility?

Sure, they’re a possibility

But I’d take a closer look at the idle air control valve first

OP’s symptoms lead me to think that may be the culprit

If the injectors are throwing out an umbrella shaped fine mist they are likely clean. If not Techron or similar products can improve the situation eventually, i.e., after a couple of tanks of treated fuel. But after all these years even if the injectors need cleaning the trottle body and the IAC valve port need cleaning and possibly the IAC needs replacing.The IAC port is a keystone shaped hole in the throttle body at about the 2 o’clock position and the IAC valve is installed into the throttle body from behind.

Now might be a good time to replace the fuel filter and PCV valve just for the heck of it.

My first guess is the same as most of the others here, the idle air control gadget isn’t working or there’s gunk built up in the throttle valve area that is making the throttle stick closed too much. If OP is able to measure the idle rpm for both warm and cold engines, and compare it against the manufacturer’s specs that would be diagnostic towards the above theory.

If this engine sports one of those low-fuel-pressure tbi designs, clogged fuel injectors or other fuel system clogging also need to be considered. A fuel pressure test if such a thing is possible makes sense too.

All of the suggested routine engine maintenance is up to date, right OP? Engine is known to be running at the correct operating temperature?

The wiring that controls the IAC is hokey on these vehicles. Wiggle the wires at the IAC connector to see if that helps.