98 Sierra Idle Problem

mrmre · March 17, 2011

I'm new here and I don't have a lot of experience with vehicles, but my truck is teaching me a lot. I own a 1998 Sierra SLE 1500 with 5.7l V8.

I will do my best to describe what is happening. This problem is comes and goes and when I take it to a garage it is hard for them to understand what is going on even after a test drive.

Currently, in my driveway at a cold start up, the engine will idle real low below 1k.. around 500 in park. I will put the truck in reverse and the truck will "buck" as in surge with power, and by the time I throw it in drive the rpms shoot up- the truck is in grear and without me applying any gas, the truck is already going 30 mph or higher if I let it. I resort to going back to neutral. Sometimes the rpms "calm down" to something normal before this, but as of lately it does not self correct.

It feels like something is sticking. I can rev my truck up while in park and I can watch the rpms climb up to 2k and expect the needle to drop back, but it just stays there. This problem comes and goes, and if I am not attentive with the rpms when I shut the engine off I cannot start the truck easily afterwards.

I took it to a Firestone total care place and they said the IAC and EGR, another place says its the TPS, and I am leaning to the IAC+EGR.. but like I said I dont know much.

We have put a lot money back in to this truck, and between three different garages, no one has been able to solve the idle, but they sure did replace OTHER things as well.

New distributor cap, plugs, all new cats, new O2 sensors, new fuel pump, new water pump..

If it comes down to something like the IAC and EGR, can I replace those myself as they are on the throttle body.. ? I appreciate your time.

GMstrong · March 17, 2011

IAC should be on the side of the TB.When you find it unplug it and see if the problem goes away same with the TPS.

I doubt its the egr valve since the egr is only opening upon acceleration

GMstrong · March 17, 2011

sounds like a vacuum leak.sorry for double post.

mrmre · March 19, 2011

unplugging the TPS causes more problems than good, and unplugging the IAC made my existing problems more evident and persistent! I cleaned out the IAC and the port and it I havent seen those problems come back! It idles in the right spot, but now I am seeing little hesitation when I accelerate in 1st and 2nd gears. Does it cost much to test the TPS?

GMstrong · March 19, 2011

better off buying a new one.if you never ran seafoam or anything like that then i would say go to a GM dealer ask them for the upper cylinder and injector cleaner its 3x beetter then seafoam and will clean off your piston and such like your cleaning them by hand.

gregrocks89 · March 20, 2011

i agree with keith, I'd run a good injector cleaner thru your truck see if that helps at all. Is your trucking throwing any codes at you?

mrmre · March 23, 2011

Thank you GMstrong and gregrocks89 I fixed it thanks to you guys.. VIDEO: http://www.youtube.com/watch?v=AYqSsC3ERRY

At the end, the EGR was throwing a code, but the mechanics and myself were confused by all the other things. I replaced the TPS, cleaning the IAC motor, but it wasnt till I replaced the EGR did the truck act right again.