Thread: surging engine

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my 91 Chevy 5.7 throttle it body has developed a surging I'm not sure what's going on I start it and it will run from 600 to maybe 2000 rpm just up and down. I haven't looked at it yet I just took a different ride to store. It acts like a vacuum leak, but that would stay all the time. You can rev it up and it runs but at idle its all over the place. Any ideas before I start this venture?

Sounds like your Idle Air Controller (IAC) is bad to me, or perhaps it's unhooked. I guess it might be the Throttle Position Sensor too, but I'd start by looking at the IAC.

get a can of tb cleaner and rinse the iac good. they get nasty from crankcase fumes.

Thanks guys I changed the TPS as I had one. While doing that I found the EGR valve was rusted away the stem was all that was connected no spring Changed that. Idle is a bit high but no more surging. Bought this truck about 10 years ago and use it for plowing the drive way and hauling wood. I changed the front pad and the clutch. I figured if it runs why worry about it . Guess I'll treat it to a tune up now so it's good for this winter.

You may have to adjust the TPS for low idle. Need a volt/ohm meter. And the service manual to tell you the value it needs to be set at, if the minimum setting is a bit high, it'll hold the low idle at a higher speed than needed.