'88 GMC Idle problem....

[Will]

I’m having a problem with my ’88 GMC 1500. First off the problem occurs simultaneously with the service engine soon coming on. The idle on the truck is very erratic to say the least. While sitting at a stop light I have to keep my foot firmly planted on the brake pedal to keep it in place. If I put it in neutral it will rev way up and then down and then way up again. While driving it will almost stall and then pick back up. I’ve had this problem in an Oldsmobile, fuel injected, it was the idle control valve. However, I am not sure if my truck has an idle control valve?

 

Need any and all help, working on a tight budget.

 

Thanks.

[Shawn T]

I used to own an 88 with the 5.7L. Did the exact same thing, it was the idle control module. Before you change it, I would check all of the vacuum lines for leaks first though. I can't remember how much it cost but it was not expensive.

[citydriver]

While you're at it, check the TBI unit base gasket. They have been known to fail and create a vacuum leak.It was a common problem with the Throttle Body Injection units.

[Will]

While you're at it, check the TBI unit base gasket. They have been known to fail and create a vacuum leak.It was a common problem with the Throttle Body Injection units.

 

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Okay forgive my ignorance, but is thjat the gasket at the base of the carberator? Because it is cracked.

[citydriver]

While you're at it, check the TBI unit base gasket. They have been known to fail and create a vacuum leak.It was a common problem with the Throttle Body Injection units.

 

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Okay forgive my ignorance, but is thjat the gasket at the base of the carberator? Because it is cracked.

 

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YES, technically its a Throttle Body Injector unit, not a carb. but it mounts pretty much the same way. There should be 3 bolts that hold it to the intake manifold, and over time and engine vibrations, sometimes the bolts loosen slightly and cause the gasket to fail(crack), which cause a vacuum leak thus the irratic idle due to the air that is getting sucked thru gap in the gasket.