I just joined this forum and will be aboard for a while. i am in desperate need of advice of my 96 z71 why it is idling high. It has 260,000 miles on it, i have replaced the IAC 2 times now only to get a normal idle for about two days( that gets expensive ). The symptoms are as follows:1. I can hold the brake and it will go to normal idle after about 5 seconds but riding the brake she keeps pushing hard. 2, On the highway i can take my foot off the pedal and she will run at about 55-65 mph like cruise control. 3, recently the code showed a #6 cylinder misfire, the plug had a burnt white crust on the end ( i changed all 8 ) and she still has a rough hard miss until she warms up and then the miss clears out. 4, fuel comsumption has been higher than normal. I have changed the TPS, MAF, O2, and MAP sensors, and the IAC twice. Could the miss be a bad lifter, cam lobe, worn push rod and lifter or internal affairs???? But why the hard white crust on #6 ???? Would that cause the high idle???? or is it i have just got two IAC's that were no good??? OR is there anyother sensors that will effect the idle???? I have misted the intake with starting fluid and vacuum lines to see if it screams at me but there is no change in the high idle RPM thinking a vacuum or gasket leak. Any help will be greatly appreciated. I have been fighting this for over a year now.

Thanks for the advice to all.

 

So you've changed the iac twice with short term success. I'd say it's going to be in that area causing your problem, also your spark plug is "not normal" at least what I read on #6.
I'll start looking in my Haynes manual as I am on medical with nothing to do. Try looking at this from NGK to read your plug(s),
NGK Spark Plugs USA

It might be a valve.

 

When you installed the IAC, was the end of valve (pintle) to the mounting base less than 1 1/8"? Two things that are hmm, misfire, and deposit on the plug. The pcm controls the iac, and ignition. I am not sure exactly what engine you have, does it have individual ignition coil packs? If so, I'd replace #6. The deposit on the plug, you can look at the NGK to compare.

 

The pintle was about 1 1/8 in from the mounting base. No less I think that's as far as I could push in down and she doesnt have individual coil packs. Its the 5.7 liter ( 350 ci )

 

What about intake gskts leaking? Are you losing coolant?