Intermittent high idle

[GMC1994K1500SierraSL]

Hey guys, my truck is a 95 305 TBI manual trans, every few start ups its idles at 1000-1500 rpm, turn it off, fire it up goes to 1000 then falls back to about 700, then it'll idle normally for a few more start ups and goes back to doing the high idle, I replace my IAC valve already, my connector for the IAC looks rough though (tab broke and the piece that holds the wires in place cracked) I'm going to replace it to see if that helps

Any idea what else could cause it to be so intermittent? I have a TBI rebuild kit, just waiting

Should I be able to see any lower TBI gasket looking down through the butterflies?


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[sntrym]

I'd check around the vacuum line connectors to see if one is loose or cracked. Otherwise I'd go for something electrical, like IAC or TPS.

 

[GMC1994K1500SierraSL]

I'd check around the vacuum line connectors to see if one is loose or cracked. Otherwise I'd go for something electrical, like IAC or TPS.

I read about a test where you pull the plug on the iac when it's running, no change in idle was at about 1000, turned it off, with it unplugged, started it up and it was still idling at 1000, I'm just not sure what that proves

Could my new IAC be bad out of the box? Is there a way I can test my plug?


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[Colonel Sanders]

What brand ICV did you get? Being DOA wouldn't be out of the question. As far as the plug I would try wiggling the wire around with the truck at idle and the ICV plugged in and see if the engine speed changes. I had a similar problem on my BMW with an intermittent drop out from the TPS, wiggled the wires right by the sensor and turned out one of the wires in the pigtail was loose.

 

[GMC1994K1500SierraSL]

AC Delco one, I'll try wiggling it and seeing what happens, I also have a Delphi TPS waiting to be put in if I need it, my TPS is extremely dirty and covered in crud, could be that


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[GMC1994K1500SierraSL]

I sprayed air intake cleaner in the hole right above the iac where it gets air, so far no high idle issues, must have cleared all the gunk out


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[leakers]

If you can see gasket when looking down the throttle bore I would suspect that the gasket is broken/leaking air, and/or that the bolts holding the throttle body to the manifold are loose. Ether way it is a simple job to check ..in fact I would replace the gasket as a matter of course as soon as I had an intermittent idle problem. Also if the truck has high mileage check the throttle shaft for wear...while the IAC can cover some air leaks it cannot compensate for the TPS on a worn shaft.

 

[Richard Jones]

I have same year vehicle, same engine and transmission and same problem. Is this what fixed yours?

I sprayed air intake cleaner in the hole right above the iac where it gets air, so far no high idle issues, must have cleared all the gunk out


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[DerekTheGreat]

Yep, if the IAC can't seat properly you'll have issues. Also, for anyone else that reads this and is trying to fix an idle surge, be sure that your base idle is set correctly. I had that problem with our '88 454 TBI truck. Fire it up it would go high, then low and repeat until it warmed up. Turns out some dingus tried messing with the base idle screw. I bumped that up and the surge went away. The official way to adjust the idle is out there somewhere, it involves grounding the ALDL connector pins or something like that which causes the IAC to completely close. Then you unplug the IAC, start the truck and **** around with the idle until its ~450 or so, at least that's what the GM service manual said to do. Engine hot of course.