tgates

i have 91 z28 5.7L

i just put the stock chip back in and put base timing at 12*. Runs great for a little bit then it starts to idle too high when at a stop and driving. at a stop it idles about 1400RPMs in drive and when i put it neutral it goes up to about 1700RPMs. it will drive like that for a while and go back and forth to driving normal.

Im lost and dont know where to go from here, any ideas???

I cleaned the TB already.

dman79z28

It sounds like your problem could be the TPS switch that controls the idle.
I would check this first. have had to replace several in the past!

CradleofFilth08

I've the same problem. Most of the time now, I have to turn it off when it won't drop below 2200 in P. It used to sit at 900 for idle in D but now I'm lucky if it's lower than 1400.

quickchicken

you may need to reset your IAC. give that a shot and see what happens.

snoopaloop1088

I actually had this same issue about 4 days ago

Mine ended being to be the TPS was shot and just needed to be replace... but before you go replace it, give the TB a nice cleaning with some carb cleaner see if that works, also you could try to adjust/calibrate the TPS... i had to do this still even after i replaced it (google how to adjust it)

I would also check if there is any air leaks because that might cause it, i wouldn't touch the IAC as a first option... clean the TB and maybe calibrate the TPS first

quickchicken

i was always told to use the 3M TB cleaner and not to use carb cleaner.
air leaks, thats good one!! i didnt even think of that which would cause issues. resetting the IAC and TPS can be found over on TGO in the tech data.

BadmeanIROCZ

Well, use a meter to check your TPS before replacing it. These things last along time even if they're off abit. Stock TPS voltage is at 05.4 or it maybe 0.54 volts. You want it there. Over time it can get off of target too. The IAC you should never really try to adjust yourself unless you need to and know what you're doing. Automatic equipped 3rd Gen's are to idle at 750 RPM's. If you're at 800 RPM's, not a big deal. 1,000 and above is too high. It'll also kill your transmission if it's idling real high while in drive and will eat your brakes up too.

Also, clean the t-body out as stated already. One thing that could also be messed up is your throttle body itself. Over time these things just get abit worn out and even when your TPS is good and your IAC is correct, there are some mechanisms within the throttle body that become worn out and will cause a higher idle. Sometimes if you give it abit of a rev or 2, it'll drop back down to the desired idle.

Chad97z

right. pull out the IAC and clean off the little plunger. It gets carbon built up on it from age. Then it doesnt close and seal off properly when the engine has warmer up. So the idle goes too high. At a cold start up, extra air is needed. The IAC is an electronic Choke. When the engine warms, you back off the choke. My grandfather used to have an old truck with a manual choke. I used to watch him drive it as a boy. I watched everything he did.

thatdavidkid

Just a thought but low coolant level can cause the ect sensor to read wrong, causing the car to run rich, causing the idle to go high to try to compensate for the rich condition.