corvette1989bham

I went to set my idle for the first time on my show car 89 18,000 vette. The adjustment "tab" had to be punched out for the first time. I jumped a to b waited 30 sec. unplugged the IAC then unplugged the 5 volt spark brw.tan wire . and removed the jumper. Car STILL idled at around 80 rpm. I adjusted the idle screw all the way in to where it wasnt even touching the throttle and it still idled at around 800 rpm. I needed it around 450. Never could get the idle below 700 ish. Plugged back up the iac read .44...adjusted it to .54 . Idled at around 850 then came down to around 650 ...still high. plugged the ecm wire back up restarted car ..idles fine (was fine before),cleared the codes that were set during the adjustment, but still a little high 650ish-755 idle hot. Why couldnt I idle it down without the iac plugged in and the spark wire off...cant figure it out?

VetBoy89II

Are you sure you were working with the IAC and NOT the TPS? TPS setting is .54 +/- .08. Did you check your timing before you started changing you idle speed? Idle speed adjustments can be a major PITA on these cars; that's why that plug is there.

hippy

That sure sounds like what he did. His IAC kept compensating when the idle screw was being closed.

Your IAC is almost directly below your TPS, if I am not mistaken you have fuel lines that run right by it.

REDC4CORVETTE

You can only set the idle when the engine is cold,first start of the morning. Should be set at 450 to 500 rpm's.IAC disconected.Then set the .54.

TPI

I believe you first set the timeing, then idle rpm, then TPS. Try setting the IAC count to 28-30.

hippy

You sure about this?

corvette1989bham

I'll get back into tomorrow evening, I 'll post the finding. I am sure I made a mistake somewhere?

REDC4CORVETTE

More positative than any one could ever be about anything !
Not only that, but you may have to do it several times,in the am.
I have an 86 that I thought I could never get set right but the 5 th time I got it perferct.These engines should purr like a kitten if it is set right.
I think I have the set up for it.I 'll look to see where I put it.
Here is some info not the one I am looking for.
http://www.corvetteactioncenter.com/...ht=TPS+voltage

hippy

Your IAC is a square plug about 3/4". Your TPS is a 3 wire rectangle plug, 3/8"x7/8". These numbers are guesses as I haven't looked at my car for 2 damn months now.

VetBoy89II

The IAC is not adjustable, you can only reset it. The TPS on a stock '89 is adjustable. If the IAC seems to be an issue, the best thing to do is pull it and clean it with some throttle body cleaner, NOT CARB cleaner, CARB cleaner is tooooo harsh on sensors and leaves a residue that can be bad for the IAC and other sensors within your system.

You can follow the IAC reset procedure by doing a search here at the forum, I can't remember who has the procedure off the top of my head.

hippy

Maybe if you would have set it when the motor was at its operating temperature it would have only taken 1 time to get it right?
Just a guess.

VetBoy89II

You read my mind ...

Seriously, all these settings need to be done at normal operating temperature.

REDC4CORVETTE

You are just showing you nothing about these motors!
I like my Corvette's to run perferct not 14's and 13's in the 1/4 mile.

hippy

Me?
You maybe right, as it took me one time to get my idle set after I put a set of heads, cam, intake, headers and a chip in my car.

hippy

My idles just fine at 850 in D. Nice smooth steady idle. If I lift my foot off the accelerator pedal I guess I could slow it down to a 13 second qtr mile car.

REDC4CORVETTE

89 Minimum idle set

Midnight 85

I am having a hell of a time right now getting my car to idle right and stay running when I put it in gear. I've always done it at operating temp but I might just give a shot when it's cold, it makes sense that if it's right when it's cold then it should be fine when it's warmed up. It ran beautiful until I added aluminum heads, headers and 1.6 rockers. Now it won't start unless I open the throttle and will die when I put it in gear. To say that adjusting idle is a nightmare is an understatement. Also remember anytime your idle is different than factory, you have to have the new idle speed burned into the prom.

cv67

Maybe you just got lucky and started running low 12's? It could happen, ya know

hippy

Even the blind dog gets lucky and catches the squirrel once in a while.

SunCr

Wow - this is getting allmost as mystical as false knock - understand how it works or better yet, leave it alone. The IAC controls air flow into the Plenum with the throttle plate closed. It's a stepper motor that operates a plunger, moving it in and out to control air flow and that way, the engine can maintain whatever idle RPM is programmed into the ECM. It isn't always exact, but generally it hits the programmed number within +/- 25 rpm. In order for it to do it's job, there has to be a little bit of air flowing around the butterfly. That adjustment, minimum air, can only be made by taking the IAC out of the picture. You do this by extending the plunger or pintle so that there is no air flow around it and that only the air getting past the butterfly is keeping it running. You can achieve this by grounding the diagnostic terminal with the ignition on. Though this will turn on the fan, you should be able to hear the IAC grinding away as the pintle is fully extended into it's bore. Once the noise stops or after 15 seconds or so, disconnect the IAC harness. Start the engine, and with a TACH or scanner hooked up, note the RPM. Turn the adjustment screw to set it to specs (though allmost every injected GM specs out at about 450 rpms). Once you've got it set, reset TPS voltage (for those that are adjustable) as moving the throttle plate changes it. To finish up, plug eveything back in, depress the accelerator slightly and restart it. Let it run for 10 seconds, then off for 5 seconds, then restart it and if everything is up to snuf, it should be idling at +/- 50 rpms from the ECM target in Drive; +/- 100 rpms in Neutral or Park. For those with scanners, look for 15 to 20 counts on the IAC screen (a/c off). To see if the IAC is functioning correctly, note the RPM and turn it off and then start it back up. Idle rpm should be higher and then return to what it was. If you have a scanner, you can capture where the IAC parked itself when you shut it down - usually about 120 or so counts. That insures plenty of air for a quick restart.

I can't remember ever trying this on a cold engine. The CTS signal is going to fatten up injector pulse width and there may not be enough air to keep it running or the additional fuel is going to pump up the idle speed.

If it won't idle down after you've grounded the diagnostic terminal, you either didn't wait long enough or the IAC is shot or hung up in it's bore, or air is coming in from somewhere else. For the latter two, you'd have other problems.