I've followed the more detailed instructions for IAC adjustment that I found here on the TPI board. Problems:

After theoretically closing the IAC (by jumping terminals A and B while ignition on, engine off, for 30 seconds)and disconnecting it, I start the engine, which tears up to 1500 RPM, 1250 or so in drive. After REMOVING the idle screw (I'm not kidding, it's sitting next to the keyboard here), I got the idle down to 1000 or so, 800 in drive.

This seems strange enough to me, but it gets better.

After doing this, I plug the IAC back in, disgusted that I MUST have a vacuum leak somewhere. When I start the engine again, it idles at 600, 500 in drive. This is, of course, warmed up. I'm scared it won't start again if it gets cold now.

Any suggestions?

-Reno

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"We are the middle children of history. Our Great War is a spiritual war, our Great Depression is our lives" - T.D.

 

Most likely the IAC passages are dirty. When the pintle on the IAC tries to close the passage, it's not very successfull at it since it's probably covered in black soot. I'd take the TB off and clean the IAC passages really well. If that doesnt work, then you must have a huge vacuum leak somewhere. I've learned that spray carb cleaner around and waiting for the idle to go up doesnt work. I just use a piece of hose that I hold close to my ear and snoop around all the joints and vaccum hoses. You'll definetly hear the "hissing" if you have a leak.

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1992 Z28 5.7 TPI

ZZ4 longblock
Harland Sharp roller rockers
Accel base and SLP siamesed runners,
52MM SLP Throttle Body
Hooker Headers & Flowmaster catback
MSD6A ignition
aluminum driveshaft
World Class 5-speed

 

no this is the same problem I have to the T and its not the IAC passages being dirty. I dont think my IAC is closing all the way when I jump the terminal or maybe you have to unplug it with the ignition still on and then start the car - I dont know but I will replace it soon anyway.

 

hey 88 305 let us know if a new IAC fixes the problem, I've been fighting mine for a couple of months as well. some days it idle's great and some it won't even stay running. I have taken the IAC apart twice and cleaned it but it only helped for a little while. One thing I found it the hose coming from the pass. side valve cover was feeding alot of oil (probably tired motor) into the TB, so I removed the fitting and installed a breather, but that diden't seem to help either.

 

When you jump the A-B terminal, wait 30-60 seconds for the IAC to seat. Then, with the A-B terminal still jumped & ignition still on, unplug the IAC. Now remove the jumper & start the car.

Min. idle & TPSv specs. go as according to this chevrolet F.I. repair book..

85.. 5.0.. 500±50rpm TPS o.54±.o75v

86.. 5.0.. 400...... o.54±.o75v

87-88.. 5.0.. 400...... o.54±.o8
.. 5.7.. 450...... o.54±.o8

89-90.. 5.0.. 425±25...... o.54±.o8
.. 5.7.. 425±25...... o.54±.o8

book doesn't give squat for 91-92.

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I took the IAC passages off the TB totally and turned the ignition on and jumped the A+B terminals and it only took a couple of seconds for the IAC motor to fully seat the pintle. HOWEVER! then I disconnected it and the damn thing just snapped back to where it was?? isnt this how you are supposed to FIX the IAC closed so you can set the minimum air?? I dont get it.

I will let yall know what happens when I get the 30 bucks for another one.