Idle issue

Asked by May 16, 2017 at 07:22 PM about the 1994 GMC Sierra 1500 K1500 SLE 4WD Standard Cab LB

Question type: Maintenance & Repair

When at a stop truck goes from 2500 rpm to
500 rpm back and forth. Smooths out in
neutral. Runs great on the go. New
distributor, egr valve and sylanoid. Fuel
pump and filter 18 months ago. TB looks to
be fogging fine.

6 Answers


IAC valve or did you do that already? There is a vacuum leak somewhere. Good Luck!

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That is next. I'm also pretty sure a vacuum leak as well. Thank you.

Damn. New IAC but same issue. No more check engine light. Shifts smooth. Mechanic said he believes performance chip bad. Discontinued. Dealer wants 450 for new ecm (I think that's right).


I have one more idea that is concerning a vacuum leak possibly with the brake booster that I thought should be checked since you mentioned when you brake at a stop this issue happens. There is usually a check valve before the brake might be bad or the vacuum brake booster might have a pin hole in diaphragm. Thought I'd throw that in there before you shell out $ on the ECM. Also if it is the ECM is it possible to open the unit up? If it's just a bad solder joint you might be in luck and be able to DIY the thing. If not perhaps you can find a used one. Good Luck!

Makes sense. Breaks are always going soft. Adjusted rear drum, new fro

Any way to diagnose. 300$

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