High idle.

[limey]

I have a 1990 ZR-1 with a fresh built stock motor.

Car has headers - & exhaust, but all internals and such are stock.

Pulls to redline with no codes, excellent oil pressure & cool running with no engine noises.

Problem i have is the idle, if i start up cold in the am, it goes to 1800 rpm, if i blip the throttle, it can go to 2000 - 3000 RPM, i take it out on the street and dip the clutch pedal and it stays @ 2000 + rpm all day long.

Things i have checked:



Pulled plenum and checked for flatness, as well as injector housing, they are perfect flat.
Replaced known good IAC Motor
Checked TPS Volts .46
Checked TQ specs on injector housing to head bolts, accurate.
Checked TQ specs on plenum to injector housings, accurate.
Replaced PCV hoses and valves with new
Replaced PCV baffle hoses with new
Pulled injectors and installed all new o-rings
Checked gasket for TB to plenum & TQ specs of bolts, accurate

Exhaust has a very slight leak at the collector, not yet fixed. Not sure this would cause this high idle though, hence the delay in sorting this out.

Any suggestions?

Anyone have a bone stock 1990 PROM i can borrow to test the idle.

[Jarek5_7]

do you have stock chip? from `90? if yes I suggest buying a chip from Mark Haibeck. I bought it around two weeks ago. I have 1990Z and had very high idle. I checked all vacuum conections and it didnt help a lot, it wasnt leaking a lot. the `90 had a bad programing and commonly had high idle. someone here posted a service bulleting showing that gm replaced ecm eprom couple of years later in the 1990 Z.

when I put in the M. Haibeck chip the rpm are much lower. these cars have around 1250 rpm when decelerating and until stop, but before chip rpm were hanging in that area or above for a long couple of seconds, now after chip from Mark rpm imiediatly are falling to good low idle after car is stopped, and while decelerating in neutral they are much lower. (I would like them to be like idle at stop , but from what I read all the Z have a little higher rpm that stop idle when comming to a stop 0 kph. just the way the cars are I think)

also the fans are turning sooner and now in traffic I my Z didnt get hotter and half of scale.

mhm what else, after cold start rpm are dropping much quicker also then on the stock 1990 chip.

so if you have a stock 1990 chip, I would suggest replacing the chip.
I dont think you can buy an gm updated chip now so only solution is to buy from a tuner.

you can if you have a stock or updated chip. just unscrew the small "door" on ecm and read the symbols on the chip. auah something is for stock.

[tccrab]

Idle is too high even for first generation PROM.
Things (other than a vacuum leak) I'm thinking of are:
-DAG fubared in throttle body
-ECM
-ECM ground
I mention the ECM and ground because of a member here a couple of years ago had a similar problem and he spent months and months throwing money and time at it without success. Replaced the ECM, and bingo, good idle.

TomC
"Crabs"

[Aurora40]

Courtesy of Jeffvette, you can narrow it down to the TB, or not the TB, by blocking it off with a very sturdy piece of something while the engine is running. Cardboard is too flimsy. Something like a notebook/binder works well. If it keeps running, there is clearly and air leak somewhere else that is feeding the engine. If it dies, well the extra air is coming in the throttle body for one reason or another.

Also to see if the ECM is trying to control the idle, scanning it will help. You can see what the desired idle is, and also you can see how many steps the ECM thinks are left. If it is 0, then the ECM has closed the IAC as much as it thinks it can, and can't control the idle further. If there are steps, then the ECM is doing something wonky or something.

[Jeffvette]

Replaced known good IAC Motor
Checked TPS Volts .46




Did you reset the IAC? Do you have a IAC controller? And the voltage on the TPS should be around .54 volts, although the "acceptable" range is .46 to .62.

[Jarek5_7]

Jeff how to correcly reset iac? and what is iac controller?