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Idle Problem - Yeah.. Yeah.. Same Common Problem... Or is it?

2914 Views 26 Replies 12 Participants Last post by  88351stang
I would reset the base idle using the usual methods. When coming to a stop, i depress the clutch and the RPM's jump to around 1800rpm. At a full stop, the RPM's will drop where I want it- 850rpms but not after hanging at high RPM. The Idle Bypass Valve is clean; Throttle Body is clean; the air-idle spacer thing has no effect. TPS is at 0.98volts and a recent dynotune says that all settings is where it should be.

I've done everything yet nothing seems to work.

I also cleaned the MAF element. One of the elements has a darker color than the other. I use almost a full can of throttle body cleaner on the darker color element but no change. I assume that it's clean despite the darker color.

The idle is irratic and it sounds as if it will die.. almost spurring. At 850rpms the idle should almost be rock solid but this is not the case.

I am suspicious my of alternator, a Power Master (the Black one). I think it puts out 130A. When i turn on my dual electric fan, the overall power drops slightly. Is there a way to test it?

At this point i'm believe that the problem has to be a bad part
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have you tried resetting your ecu?
Your sprayed your maf wires with throttle body cleaner?
Yes... I have sprayed the MAF wires. One appears to be slightly darker (brownish) than the other. I used a q-tip and lightly brushed it along with the other wire and gave both another dose of TB cleaner.

On a hunch, i bought a new Idle Bypass Valve and went through the reset procedure. I then started her up with the Idle Bypass Valve unplugged and it was aweful, at first. A few turns of the idle stop screw and it stablized. I then adjusted idle to where i want it (850rpms) and let it idle for a while.

During this time, the idle rests at 850rpm and then builds up to around 1500rpm then back down to 850, then back up at 1500... etc.

I do let it do this until it stablizes? or will it stablize when the Idle Bypass Valve is connected?
Your idle increases once you set it at 850? With the IAC off your idle should stay where you tell it to. The only time your idle will usually act goofy at idle is a vacuum leak, bad maf, or bad iac valve.

It definately should not be going up and down with your IAC disconnected.
I replaced IAC and I put in a brand new TPS, during the weekend. I don't think it helped.

When depressing the clutch to shift gears, the RPM's don't drop. They either stay at the current level or increase (surge). Then coming to a stop, I step on the clutch and the engine revs up instead of dropping to idle. I screw in the idle stop screw just up until the screw touches the blade. I then give it one full turn. I then set the TPS to 0.98 volts and start the car. Still nothing. Idle is still erratic. Yes... i've cleanerd


Can an exhaust leak at the header cause this strange behavior with the idle?
Treachery said:

Can an exhaust leak at the header cause this strange behavior with the idle?
I had both headers leaking near the H-pipe, (they had holes from rust) while I don't remember it making the idle really erratic, it did cause a slight hunt at times. What happens is, the exhaust stream pulls outside air into the exhaust, and past the O2s. The O2s smell this outside air with it's oxygen molecules intact, and reports to EEC that the motor must be running lean. EEC responds by injecting more fuel. So you'll have a car whose exhaust smells richer than Bill Gates, yet EEC tells you it's running lean.
This problem is analogous to a leak between the MAF sensor and the throttle body - they are both "false air" scenarios.
I went to a muffler shop yesterday, bad idle and all, and the mechanic found a header leak at the passenger side at the gasket & a leak on the header itself right above the collector.

I want to get this idle problem resolved before the new headers come in.
New headers will be JBA Titanium Coated 1 5/8 shorties back ordered until first week of November.

edit: i started up the car this morning. Cold starts has been very ugly as of late. I would have to hold the engine for about 20 seconds at 2000rpm or else it would just die out. The engine is warm and appears to be idling correctly. I take the car out and the high idle is back. When shifting, it sounds as if i do not let go of the gas pedal. RPM stays high. Coming up to a stop RPMs are at 1500-2000. Sometimes it will drop and idle somewhat correctly but then it will do the same thing as described.

Fustration is mounting.
Have you hooked up a vacuum gauge to see how much vacuum are getting? Sounds like a vacuum leak to me.

You should have atleast 14" of vacuum. Have you tried manually pulling the throttle cable back to the closed position?

A while back I bought a throttle body and the shaft that goes turns the TPS broke in half and would not close all the way unless I pulled the butterfly closed myself. That TB is trash now.
Quick review, please. How to do a vacuum check... :rolleyes: No really... That's one thing i did not check and do not know how to do.

I finally got the car to idle somewhat steadily, this morning. The RPM's now behaves normally when shift gears and the idle surges only a few hundred RPMs.

Ready for the weird part? The idle rests at 850, right where i want it. But now, after a few minutes, the idle will drop to around 700 hundred or so, as if the car will stall. I would then have to press on the gas pedal to bring up the rev's. The engine will idle normally and then would drop again after a few minutes.

Any thoughts?
Scratch that whole thing about exhaust leaks and idle. I ran mine for a whole day on open headers and no problems, no check engine lights all day. Neighbors loved it.
Doesn't the EEC try to make the car idle at 650 when warm. I didn't think you could change the idle speed without a tuner or chip.
jerry_dawg said:
Scratch that whole thing about exhaust leaks and idle. I ran mine for a whole day on open headers and no problems, no check engine lights all day. Neighbors loved it.
True, no illumination of the check engine light, but, were you to have scanned EEC, you would have pulled some codes. Also, were your O2s disconnected when you did this? That would make a difference, although, I would think that you woulda gotten a light.
A leak at the head will only affect that cylinder's worth of exhaust. But a leak down by the collector, affects all four cylinders on that bank.
93BlackMustangGT, I would go ahead and fix the exhaust, do any other tuneup stuff you haven't done (use the checklist on this site as a guide), and then, lastly, throw in a new set of O2s and disconnect the battery for thirty minutes, reconnect and start the car and let it idle for a few minutes. If it still acts up, put your old O2s back in, and repost.
In response to what '95.0 stang said, if I understand Charles O. Probst correctly, the MAF foxes will let you set the speed - as long as it's between 650 and 700 RPM.
Yeah, but he was trying to make it idle at 850. And who is Charles O. Probst?
wow guys i have been paying close attention to this conversation mainly because i came to post the same problem. Might i add i have been having it for quite awhile and the exhaust thing is really an issue since in notice a nasty lil sag right at the x pipe. However my motor races or dies there is no rhyme or reason for it no particular time hot or cold it driving me crazy. Has any one bounced around the idea that this could be a grounding issue electrical not vacuum cause if you have a leakage issue you will have it all the time but electrical problems will throw you fits. you might want to do the improve grounding thing in the above post and then see where you are...whew that was long winded i apologize
Charles O. Probst wrote: How To Understand, Service and Modify Ford Fuel Injection & Electronic Engine Control. The idle setting procedure he gives (only for the Mass Air fox bodies) is complex, and I don't think that I really understand it. But I think that it's somewhat tailored to the first startup and run in of the engine. I bought that book knowing that it would be alot more help than the Haynes manual in that area. My car came with a removed "check engine" light bulb - or MIL.
Once I resolved all my issues, all I did, was adjust my throttle body so that the blade wouldn't bottom out, and went with it.
Another thing we're neglecting here, is battery cables. I've noticed my '92 will idle poorly and roll, when the battery terminals start to get cruddy.
I never thought of that, my idle got a little stronger after I regrounded my engine too.
Any Luck? i have the same problem with my automatic car. since it has a torque converter it loads it up while cruising around the neighborhood and accelerates the car to about 30. and thus stopping is harder since im fighting the car. when i thow the car in neutral it jumps to about 1500 and stays there. but when i come to a stop it decreases to about 1000. please help!!!
and what is the proper way to clean the IAC valve
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