The Stovebolt Site Forums The Shop Area Engine Shop High Idle question

OK I am fed up with this 2.8 v6. The temp sensor fixed the hard start, now it idles to high to drive offroad. Any Ideas other than discarding the 2.8? it is a 1986 motor and is TBI. The TBI in not adjustable. Thanks in advance.

Do you have access to a computer scanner? There is a passageway in the throttle body where air bypasses the throttle plates, which is controlled by the Idle air control. (IAC) A signal from the computer steps the IAC in or out to control the idle speed, but it has a limited amount of travel. The base idle speed is set just like a carburetor idle adjustment, but the adjusting screw is under a thin sheet metal plug that can be pried out with an awl, or something similar. This adjustment is called "minimum air rate", and is adjusted by blocking the air passage to the IAC, and setting base idle with the adjusting screw. The passage is odd-shaped, and requires a specially-shaped plug to block it off. You could try a piece of rag with a punch or screwdriver blade wedged behind it, but you'll probably get an air leak that will affect the adjustment.

If things don't improve after adjusting the min air rate, you've probably got a stuck IAC, or a bad signal from the computer. Try removing the IAC unit and cleaning it up with spray carb cleaner and compressed air, or replace it. a scanner is necessary to monitor the signal to the IAC stepper motor.

Just had another thought! Any vacuum leak, a hole in a hose, PCV valve stuck open, or other source of air beyond the throttle plates will cause a too-high idle speed, that the IAC can't control.
Jerry

No access to scanner. I did replace the IAC attempting to solve the problem. It got worse (the instructions with the IAC said it might idle abnormally high) so i ran it to allow the computer to reset the plunger. There was no change. So I removed the IAC and reset the plunger manually by best guess as well as disconnecting the battery to try to reset. The idle came down but was higher than with the old IAC. I have checked vac lines and replaced or repaired all that are in bad shape. I will recheck the crank case vent. Thanks again.

I'd pull the plug over the throttle plate screw, and back it out about half a turn. Just be sure you don't let the plate close completely, or it will try to stick. Most of them have a "Torx" head on the screw. I've fixed a few of them that way.
Jerry

Before I started messing with a idle screw on a TBI or a TPI, I would want to find a book that covers that setup. The reason I say that is because any time you change the throttle position setting, you are also changing the TPS voltage. On a lot of these the TPS is not adjustable. Most of the time the IAC is disconnected when the idle screw is adj and the timing connecter plug may even be opened. I would for sure want to know what the idle voltage should be for the TPS. As you know, it is nothing more than a variable resister like the volume control on a old radio and they do wear out. Misadjusted or worn sends the wrong voltage signal to the computer and the wrong control signal comes back to the IAC.

Thanks Hotrod Lincoln you were right on the money. I was afraid to punch out the plate on the TBI since everything including the fan shroud said that the tbi was "non adjustable" they lied. Only took a small turn and it was fixed in a couple of minutes.