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.
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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.
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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.)
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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.
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Treatments used so far, in this sequence:
1) Fuel filter2) New TPS
3) New Air-Idle Control valve4) New Mass-Air sensor (the actual sensor unit in the mass-air meter)
5) New A9L ProcessorEach treatment was used independently, but nothing has helped/worked.
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Besides a gun, does anyone know of a cure?
Patrick