Why is my 98 Mustang GT idling funny?


Asked by Jan 27, 2015 at 03:38 PM about the 1998 Ford Mustang GT Convertible

Question type: Maintenance & Repair

The car sits at about 800 to 1000 rpm when idling. I've noticed when I turn on the A/C,
heat or the defrost the car drops down to about 200 to 300 rpm and then jumps back
up to 1000 rpm then repeats almost stalling out the car. Recently I have had the intake
manifold replaced due to a crack and the fuel pump going out. Not sure if anything that
has been replaced could cause it. I've looked into the IAC valve and thinks thats what
I need but not entirely sure. There is a small humming noise coming out of the IAC

18 Answers


If the IAC is humming or hooting replace it. You may get by with cleaning it with carb cleaner, but I would replace it. I would recommend a genuine Ford Motorcraft IAC and not some of the junk the aftermarket sells.

2 of 2 people found this helpful.

The processor would still remember the problem after replacement. The relearn procedure would need to be done after power disconnect, then drive cycle and retest for codes, but what code is in it now?


Yeah Dannyl you lost me.


I think he is telling you to pull the battery cable after installing a new IAC valve. Since the IAC is mechanical in nature I think it is unnecessary. Is the check engine light on?


Talking about a rolling idle. If an EEC processor learns that condition, you extract codes, run adaptive strategy, take it for a drive cycle following the relearn. since you have the idle issue already, and plan to go thru this, you make sure the throttle plate is clean on both sides first. after driving you retest for codes and show it is clean with a pass code, if not, the customer thinks you are milking him. SOP for Ford technicians. everyday, all day. as per TSB, SSM, or whatever applies in my experience.


Dannyl, the ecu tries to maintain the idle speed at a preset number. It can not be programmed to perform or remember a rolling idle. Clean the throttle body and replace the IAC.

1 of 1 people found this helpful.

The Ford ECA has been able to learn and relearn, TSB's have been issued. The adaptive strategy and complete drive cycles required are evidence of this. You can find the engineering discussions about it in the professional Automotive databases. I received TSB home mailer after Ford training and read it at home for years. Then when they quit paying for it, we changed policy and ran an Oasis on every car that had a repair order written. Giving the Oasis a service inquiry and code is the fastest way to find the real truth, call a dealer. We had to fix and retune every screwed up car for miles around, take out modules made in france, "mighty" spark plugs, and $14 crap wires, do what engineering told us, and use Motorcraft parts and the TSB's that pertained. Processors suffering from "nightmares" were given correct procedure, not replaced like in the disneylube shops in town. We handed back bags of brightly colored malfunctioning garbage to customers when we returned their calmly idling powerplants , returned to specification. I am not surprised at all that you have not read these engineering bulletins, but I have had engineers explain their validity and help me help people free of charge over the phone after goofy, daffy, and foghorn were done taking all their money. We used to say "It may be your car, but it's still our baby". Ask for help, that's what they live to see, a mustang , with all its horses, on a mission.

1 of 1 people found this helpful.

The car should be idling at 750 RPM's. If it's at 800-1000 then slow the idle down, it only idles high, when it's cold 1100-1300.The fact, that's it's dropping down to 200-300, is a concern. Repeating back and forth, is common, when the compressor is kicking on and off, but it should not drop that low, just a 100 RPM'S OR NOT AT ALL, There's a drag on your compressor clutch, That's what I would replace, it's the bearings that are about to go, or the clutch on the compressor is about to freeze up!!!!! hope this helps


Excuse me ? "then slow the idle down" ? Does this not take place during the very procedure ? Thanks rst 429 ! You just gave a morsel of the giant crumbcake. You cannot adjust a computer controlled cars idle unless the computer is having the adaptive function accessed. Then it relearns. We had some specific tools we used, but you can do it if you read the service manuals and the TSB bulletins. It is a spec on the cafe regulation sticker. Putting the car in spec is the job of a technician if he thinks he can get engine performance certified to perform these procedures? Then by all means, if not ? he should not tamper, but instead ? Ask his Ford Technicians.


I a'm not a FORD TECHNICIAN,I'm a TINSMITH!!!!! I just go with the person's question,the IAC can be clean, and some one already answered that question, but if the idle is dropping 700-800 RPM's when the air compressor kicks on, then, thats what i would address first!!!!!!! some times a morsel, is bigger then the hole cake.......


I was thanking you for your input. They were saying they did not understand. They had not addressed the correction for the rise and fall of the RPM completely. I was trying to impress upon them that the processor needs to forget the rolling idle issue. They were saying the processor did not have that ability designed into it. I was sharing that the adjustments people attempt must be taken on by the processor during a zero voltage, restart, adaptive strategy procedure. Your mention was helpful to the thread in my opinion. since bob has a limited understanding of the processors abilities, and the procedure is required. it prevents the resulting confusion. (But after what you said). the things that are the cause must be found and corrected. The resulting processor reaction to these things that labor the idle or cause it to rise? this can cause a new driveability issue. That was the designed intent behind Ford issuing and reissuing these procedural changes. He is best advised to consult them. Thanks again because you offered helpful ideas.

1 of 1 people found this helpful.

Dannyl, I appreciate the Thanks, but I, myself and a lot of others on this site are only on it to help others, and I learn a lot in the same process.....hope this helps


I appreciate the opportunity to relay the info the way it is presented in the factory bulletin, These guys leave tracks and others with similar issues may read this thread. I think it can be very helpful to know we can find our answers free of charge, and the MFR has published the truth. Thanks again.

1 of 1 people found this helpful.

Hey guys sorry for the delay. The check engine light is not on. Also I was wrong about the RPM's. It's sitting around 750 RPM then falling to about 200 and the reving back up. I have bought the IAC valve and plan on putting it on its just what do I need to do about relearning of the codes?


Put the IAC on and disconnect the battery for about 5 min. and you should be all set.

1 of 1 people found this helpful.
Best Answer Mark helpful

Thanks for all the help. It definitely worked.

1 of 1 people found this helpful.

That is good news Bamaboy!


Glad to hear it. So that unnecessary procedure cleared up the hiccup... imagine the fluck outta that ! Great news. Thanks Ford engineering for giving us the truth. It works !

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