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ChevyTech · 11-28-2018, 11:57 AM

The light is coming on because of the IAC trouble code.
The IAC position is at zero in your data and the engine is above the desired RPM.
Either the IAC or its wiring has failed so it will not close when sent pulses by the computer or the engine has a vacuum leak.

A common vacuum leak on these trucks is the gasket that is right under TBI unit.

You can look down the passenger rear corner of the TBI unit and see the IAC plunger to see if it looks like it has a the passage closed. If the IAC is truly fully extended and has the passage fully closed off and the engine is running at 700 RPM then there is a vacuum leak somewhere.

11-28-2018, 11:57 AM	#14
ChevyTech Registered User Join Date: May 2004 Location: Minnesota Posts: 4,621	Re: check engine lamp The light is coming on because of the IAC trouble code. The IAC position is at zero in your data and the engine is above the desired RPM. Either the IAC or its wiring has failed so it will not close when sent pulses by the computer or the engine has a vacuum leak. A common vacuum leak on these trucks is the gasket that is right under TBI unit. You can look down the passenger rear corner of the TBI unit and see the IAC plunger to see if it looks like it has a the passage closed. If the IAC is truly fully extended and has the passage fully closed off and the engine is running at 700 RPM then there is a vacuum leak somewhere. __________________ For those of you that are wondering why you are not getting replies to your thread: Did you give the model, year, engine, fuel system type, and transmission information? If it is modified from what came stock from the factory, let us know that too.