View Full Version : Car jumps to high idle (~1700 RPM)

11-23-2002, 01:29 PM
Has anyone run into the problem with the engine jumping to high idle at random times? I've repalce the DIS control module and oxygen sensor. Any ideas?

Car: 1990 Super Coupe


11-23-2002, 03:30 PM
IAC valve maybe. or tps sensor.

11-23-2002, 03:44 PM
My money would be on the IAC Valve also. It is mounted on the side of the throttle body with 2 screws, and has some wires connected on the rear. Its purpose is to control the engines running while idling, and off throttle. When they go bad, they almost always are stuck open. (Its like a miniture throttle valve)

The part will be around $95 at Ford, and $60 at Auto Zone, etc. Same part.

11-23-2002, 03:46 PM
Yea totally, my broke on me wide open too.

11-23-2002, 04:30 PM
IAC is new also...Ford part replaced by Ford Dealer. Any other ideas? The random nature of this is what is baffling me.


11-23-2002, 04:34 PM
Best thing to do is scan the car and see what codes it throws at you. Note, if you unplug the connector on the iac and it still idles that high or doesnt change, the unit can be defective... i mean come on we are talking about ford :-p

as an SC ownder, the best investment you can buy is the 40$ scanner from your local autopart store.

11-23-2002, 07:06 PM
Thanks for all the feedback! Unfortunately there are no codes being thrown. I'm relatively certain that it's something elecrtrical since I can stop the engine, and the idle returns to normal when restarted...just don't know what else to replace.

Any other ideas?


11-23-2002, 07:11 PM
Well if it turns out to be your IAC valve do NOT go aftermarket. They do not work because they were made to mount on backwards....and I don't know why. So you have to stick with ford and get another new one of its defective just for an FYI.

11-24-2002, 04:17 PM
if you unplug the iac your car should stall. try the tps.

11-24-2002, 04:28 PM
Well, if you unplug the IAC and its working properly, the car should stall. If you unplug it and nothing changes, its bad. Fast idle is usually due to a bad IAC, could also be due to a vaccuum leak. But unless you've been doing some work on the car, I'd go with the IAC.

11-24-2002, 11:29 PM
Check it for air leaks at every air connection after the MAF. I had the same problem and found a leak at my spanner nut. Spray propane at every IC jount before you spend anything on parts. Good Luck

11-24-2002, 11:29 PM
Check it for air leaks at every air connection after the MAF. I had the same problem and found a leak at my spanner nut. Spray propane at every IC jount before you spend anything on parts. Good Luck

12-19-2002, 11:07 PM
They are called Air Bypass Valves.

12-20-2002, 01:36 AM
I don't get it. There is only 1 Air Bypass Valve on the SC engine and its located behind the SC. It works like the t-body because it has a butterfly inside and the only thing that goes wrong with it is that it can stick and cause bucking when going into boost. But that is usually fixed by WD40.

Les Borda
12-20-2002, 09:37 AM
What your talking about is also a bypass valve for the charger, it allows air to go through the intake during idle, and when your easy on the throttle like a naturally asperated engine, when you step on it the valve closes(vacuum operated) and closes off the uncharged air path and opens the supercharged air path into the intake.

There is also an idle bypass valve, IAC etc ect on the throttle body.

David Neibert
12-20-2002, 06:21 PM
IAC Valve= Idle Air Control Valve

12-23-2002, 12:35 AM
My car did the same thing I just unplugged the IAC and it idles just fine. Anyone know if it is it bad to drive around with an unplugged IAC?

12-23-2002, 12:58 AM
It won't hurt anything to leave it unpluged, but now the computer has no control of the idle. If the load changes the idle will drop.

The IAC is a solenoid valve. There is a port before the throttle plate and one after the throttle plate. The IAC closes off this port. When the engine is idling the the computer monitors the engine speed. When a load is applied to the engine like the AC compressor or the trans is shifted into drive (for you auto guys) load from lights or anything that causes the engine speed to stray from where the computer wants it. The IAC is opened more or closed to compensate. The computer sends pulses to the IAC the longer the pulse the more the valve opens. It's a spring return to the closed state.

Over time the oil vapor from the crank case gets sucked right through the IAC and gums it up. Most of the time this is the cause of a high idle.


11-21-2008, 09:32 AM
I've got a similar problem (see post "Extreme high idle?"), except that mine will go as high as 2500 RPM and just sit there. As you noted, I can unplug the IAC and the engine will idle back down. Mine won't go all the way to 550, only about 800-900, but it does go down. I'm assuming that means the computer is actually commanding the IAC to increase the engine speed, I just wish I knew why.

Like you, I'm getting no codes. I did replace the IAC. I'm not sure what folks are talking about with aftermarket ones not working. I've replaced it before when the "accordian" broke inside and have had the aftermarket replacement on for more than 10 years without a problem.

Based on some of the replies in this thread, I'm going to try checking for vacuum leaks around the IC connections tonight. I always use RTV to seal those, but maybe something broke loose. Only problem is, a vacuum leak wouldn't explain why the computer is commanding the IAC to bring the RPMs up. :(

Please let me know if you ever figured this out!


11-23-2008, 11:53 AM
It's the same problem the IAC, since it is a spring return to the closed position will get hung up on all the goop that builds up and that causes the plunger to be held back (Open), which causes the high idle. The IAC is pulsed several times a second, and every time it is pulsed the farther back it opens because of the gunk that builds up. I don't remember how often the computer does this (10 Hz comes to mind), but I seem to remember the pulse width was really short when not really needed and longer when needed. Basically the IAC is pulsed whether it's needed or not (if I remember correctly).

So do you have a new IAC or a new 10 years ago IAC. A little longer ago than that some of the IAC's people were buying from some stores were backwards meaning that they were normally open instead of normally closed. This caused the computer to shut the valve instead of opening it, so the longer the pulse width the more closed it would stay causing the engine to die or have a really low idle instead if idle up. The other problem I remember seeing was that some of the IAC's were positioned incorrectly, so that the connector would stick straight up in the air, so that you couldn't close the hood without breaking it.

If your idle stays about 550 when the IAC is unplugged then I would say that having a vacuum leak is not the problem.


11-23-2008, 04:58 PM
Just to reiterate, the car should idle but lower with a disconnected IAC. If it doesn't then the IAC by-pass and/or the actual throttle plate are not adjusted correctly :rolleyes:.


11-25-2008, 09:49 AM
If you haven't solved this yet, try driving around with the ECT (coolant temp) sensor unplugged. I've come to the conclusion that this is what was causing my problem. When you unplug the ECT, the fan will run continuously (good safety net) and you'll get a Check Engine Light. Apparently if the computer gets a bad temp reading some really wierd stuff can happen. With the ECT unplugged, my car has been running without any idle problems.

Just something to try!


12-01-2008, 11:48 AM
This thread is 6 years old just to let you know. Lets hope he finally fixed the problem by now!