Solve The Idle Problem From Hell -- Win a Prize!

Patient:
1993 Cobra/125,000 miles -- stock except: shorty headers, advanced timing, 73mm C&L mass-air meter and an A9L processor.
This combo ran solid for a number of years and the symptoms suddenly -- no mods or changes were made -- just appeared.
The symptoms have gone on now for about 1 1/2 years.
The frequency of the symptoms has slowly increased from maybe once a week to almost every day. But despite the symptoms, the car performs well without any popping, missing, stalling, gassy smells, or smoking. -----
Symptoms:
Reved-up idle. Sometimes. Some days, the car will run fine all day long, or for a couple days. Other days, the reved-up idle will continue to happen.
Idle speed varies, but generally sticks -- doesn't roll -- somewhere between 1600-2200 rpms. Never goes above 2200.
Generally, I get the reved-up idle a few minutes -- 3-4 minutes -- after a cold start AND after I dip into the throttle a little bit.
But frequently I get the reved-up idle when/after the motor is fully warmed up.
Miles and miles of driving -- 20-30 -- even at highway speeds, often, won't alter the reved-up idle.
Running the car hard, even really hard, doesn't change the idle (if anything, it often makes it go higher up in the 1600-2200 range).
Normally, running the car hard will trigger the reved-up idle. -----
Complications:
Shutting the car off and restarting _always_ fixes the idle, at least for a while.
No codes have _ever_ popped up in the computer.
Sometimes the idle will just fix itself while driving.
And a few times _while reved up_ I've had it just fall to the normal idle speed and stay there. And a few times, after falling to a normal idle, I've had it return to the reved-up idle -- like someone is controlling it by remote.
A scan tool was run once while the idle was reved up and, for some unknown reason, the reved-up idle didn't return for nearly 4 months. Also, a few minutes _after_ the [scan] tool was _unplugged_, the idle dropped to normal and then a few minutes later it reved back up. Coincidence? My mechanic, and he's extremely good, was at a loss, but thought the processor might have some issues. (Note: My mechanic drive my car home that night and the car ran PERFECT for him. It's about a 40-mile loop for him to drive to and from work.) -----
Recent developments:
A few weeks ago, I noticed on two occasions, a missing while driving/ cruising at highway speeds. The missing has since gone away.
Also, after a cold start leaving work one day, a few weeks ago, the idle went below the normal idle speed and the car acted like it was going to stall. After a few minutes of driving the problem went away and hasn't returned since. -----
Treatments used so far, in this sequence:
1) Fuel filter
2) New TPS
3) New Air-Idle Control valve
4) New Mass-Air sensor (the actual sensor unit in the mass-air meter)
5) New A9L Processor
Each treatment was used independently, but nothing has helped/worked. -----
Besides a gun, does anyone know of a cure?
Patrick
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snipped-for-privacy@aol.com wrote:

Have you looked at the BAP/MAP sensors? They allow engine control adjustments based on the weather (i.e. barometric pressure etc.). Have you ever changed the O2 sensors? They may be feeding periodic errors to the ECU. Other things that come to mind are vacuum leaks at various places (although I don't think this is it) like the throttle body/ERG spacer. My money is on a sensor going bad very slowly/intermittently. I'm surprised a code hasn't been tripped though.
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wrote:

smoking.
normal
good,
Have

to

I'll go with a vacuum leak. Maybe a split in one that doesn't bleed all the time, thus the inconsistency and absence of codes.
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Joe wrote:

This is typical with a car that is 15+ years old. There are so many components that just wear out and are hard to identify when they do. The Fox cars have just enough electronics with poor diagnostics to make them a pain for us old school mechanics.
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Another option is putting back in the original 1993 computer, because that year computer spits out more codes than the older A9L.
Patrick
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wrote:

VLeaks - check the little rubber elbow at the gas fume canister, front engine bay, lower, passenger side, they expand and get too loose, almost fall off, no replacement available, got to roll your own.
Clean the injectors ? (long shot)
replace distributor cap, (shorts on inside of cap) replace rotor too.......
the O2 sensors get "lazy" after long time
you could have a wiring problem, somewhere (like that helps!)
Seems like something is intermittent in open loop.
Let us know what you find,
thanks
-87 5.0 -93 5.0 8# KB
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I will look at that. Thanks!

I always use Top Tier gas. And often the car runs soooooo good/ smooth.

I did that not too long ago. The idle problem remained.

The O2s are fairly new.

At times I can almost feel the problem through the gas pedal. Like click, and idle changes. Other times, it like someone is running it by remote control; the idle will just rev up and stay there, and sometimes it'll go back down.
I think the car is haunted.
Patrick
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I'm afraid at some point I may run out of things to try and just tear the whole top end off -- plenum, egr, t/b -- inspect everything and then put it all back together.
Patrick
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snipped-for-privacy@aol.com wrote:

Put a code reader in the car and carry it at all times. When the problem occurs connect it up and pull the codes. This might give you a clue.
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I have not.

One was changed a few months ago. It tripped a code. But the new sensor didn't affect/change the idle problem at all.

It is the weirdest thing. I just can't nail down when it does it. Some days the car runs flawless and others the idle keeps reving up.
Patrick
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snipped-for-privacy@aol.com wrote:

Does the car have air condtioning? Does working that control have any effect on the hi idle speed?
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wrote:

It does have A/C. But powering the A/C on or turning it off has no affect.
I too, quite a while back, thought the A/C might affect the idle, but as with everything other hunch I've tried, nothing has worked.
Patrick
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Had a similar problem with my 2001 GT but with a twist. Every few months I'd start the car and the idle would stay at about 2500. After the car warmed up a bit, the idle rpm when back down to normal. It happed for about the tenth time and this time the idle didn't return to normal. Took the car to the dealer, and they replaces the idle control valve. All seemed well for a while. A month later, I started the car and the idle dumped. I had to keep it running with my foot. After the car warmed up, it started to idle normally. It did this a few times over a three month period. Then, it happened and the rpm idle didn't come back up. It just dumped every time and I had to keep it going with my foot. Took it back to the dealer and it turned out that the replacement idle control valve was bad too. Replaced it again, and that cleared up the problem. That has been the only problem I've had with my 2001 since I purchased it brand new.
If the dealer cant find anything else and the problem continues, have a look again at the obvious things like the air idle valve. You could have gotten a "putz" replacement part like I did.
Michael
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Have also solved similar problem by replacing idle air control valve ( intermittent fast idle speeds without setting any trouble codes);
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Michael Ashby wrote:

Have seen the same in a Jeep, sticking idle air control valve... dirt/carbon buildup caused it to stick intermittently. Weird thing is, resetting the ECU always made it work ok for a short while. (disconnect battery cables). I guess the ECU re-read all sensors once and adjusted to the prevailing conditions (open valve)
Jan
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wrote:

Now four votes for an IAV. I think I'm going back to the Ford dealer.
Patrick
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wrote:

Two votes for IAC valve... hmmm.... I may try that, again.
Patrick
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Michael,
I might have to try another one because I'm out of ideas.
Patrick
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On Sun, 30 Dec 2007 14:19:45 -0800 (PST), snipped-for-privacy@aol.com wrote:

Even though you already replaced it, replace the IAV again, as one of the other poster mentioned. If you want to mess around before doing that you might also try readjusting the idle speed. I replaced the IAV on my 88 and it still had a lot of fluctuation until I adjusted it as follows...
Engine off, back out the idle stop screw till it no longer even touches the lever. Then screw it in to the point where is just barely touches the lever and then give it an additional quarter turn. Then try driving the car and see if things improve. If that doesn't work, give it another quarter turn in and try again. If that doesn't work, buy a new IACV.
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wrote:

If the idle stop screw was never touched in the first place, I'd stay away from it. Changing it might compound the problem, making diagnosis even more difficult.
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