98 GMC Sierra 4.3 unstable idle


Old 08-10-05, 04:23 PM
reyes45's Avatar
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Join Date: Mar 2003
Location: Michigan, USA
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Angry 98 GMC Sierra 4.3 unstable idle

I have been experiencing high idle problems with my truck.
Often, when I first start my truck in the morning the rpm`s go as much as
1800 to 2000 rpm`s for 10 or 15 secondsthen it goes back down to normal but when I put it in gear(reverse or drive) the idle goes back up.
If I let the brake go, it will stay high and reach up to 35mph. without touchin the accelerator and if I hold the brake for 10 sec. or so it will come back down. Sometimes if I press the gas lightly just to get out of a parking spot or something it will idle high to where I have to press on the brake so hard to try to stop it. Also sometimes when i`m at a red light or stop sign, all of a sudden it will accelerate and catches me by suprise. problem more common after a long drive. New air temp sensor, new maf sensor,new IAC, tps, plugs wires cap and rotor. Checked for vacume leaks, none that I could detect.
My brain is fried please help me. Thank You.
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Old 08-11-05, 05:07 AM
Join Date: Dec 2001
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sounds like you have replaced alot of parts you should see a shop and have the vehicle diagnosed if the check engine light is ever on it may have a code stored, if not it will still require a scanner so you can monitor iac count and tps voltage along with outher sensor readings such as coolant sensor that could possibly effect the idle speed.
Old 08-19-05, 05:54 AM
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The first place I'd look is the egr valve, take it out and free it up then reinstall with a new gasket. Don't use a spray cleaner on an electonic egr as it will cause early failure. Most times there is some carbon build up that needs to come out cause its holding the valve open when it shouldn't be open. good luck
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