High idle goes away when revving

So basically to start off I have a 2004 Ford mustang gt (manual) with a high idle. And I’m not sure if the symptoms I have are all from one thing or multiple. But at idle my car vibrates a lot. If I rev the car at idle when I reach around 2k rpm the steering wheel vibrates a lot really fast up and down but goes away when I pass that point and hen the rpm’s fall back down to 2k it shakes again and goes away when it passes that. The car now has a issue where when drive at low rpm’s the car sputters and just basically goes back and forth with no acceleration power and goes away if driving in higher rpm this is a new problem though. Now also the car could be driving fine and when I reach a stop the car goes from regular idle like 600-700 to any rpm between 1k-2.5k, most commonly either 1.5 k or 2-2.5k and gets stuck there. The weird thing is when I rev the car every single time (might take multiple revs) but the high idle goes away and goes back to normal. I could also shut the car off and when I turn it on it’s back to normal. Sometimes when I’m driving from 1st to 2nd I could go to shift at 2k rpm and the rpm’s will just go to 2.5k with the clutch in causing me to shift horribly. I’ve had the car in 1st gear at 4 mph and with no pedal pressed down the car accelerates slowly as I see the rpms raise and as you can imagine make it a little hard to stop the car when this happens. The only things i can think of that I haven’t done are ignition coils, fuel injectors/pump/filter. Any ideas will help thanks!

It sounds like the idle air control valve is hunting for the proper idle speed.

The vibration off idle sounds like a different problem.


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I haven’t replaced that, but I did get a new gasket for it even got a restrictor plate that definitely helped with hanging idle and I did clean it once already. When testing for it when the high idle was there I unplugged the iac valve and idle went to 300. When I unplugged it with normal idle the car died which I’m assuming means the iac is okay?

Start the engine when it’s warmed up.

As the engine is idling, take the handle of a screwdriver and rap on the IAC valve.

If the engine idle speed changes, or the engine stalls, replace the IAC valve.


No luck with that, idle stayed the same.

I concur w/Tester above. The symptoms are consistent with having more than one malfunction. One of the problems seems like a misfire. Is the check engine light on? If it is, and there are misfire codes, suggest to ignore the high idle rpm, solve the misfire problem first.

Not particularly related to your car’s design, but I recently had a misfire problem on my Ford truck. Symptoms seemed consistent with ignition system problem, so I replaced a bunch of that stuff. No improvement. The only thing left was a fuel system problem, so I worked on that next. Totally solved. The moral of this story: Sometimes you have to make an educated guess, try that. If it doesn’t work, repeat the process until something does work.

Yeah I wish a code would pop up but so far no codes. No lights at all on either.

I’d hook up an OBD scanner and watch the fuel trims and battery voltage.

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I do remember occasionally I’d get a low voltage code. But both battery and alternator are less than a year old.

If no codes I would be looking hard at the Harmonic balancer and possible broken spring(s) in the clutch disc or something in the pressure plate… But a very bad vibration at 2k in neutral is not a good thing… (most likely)

Now does it vibrate (@2K) with the clutch pedal pressed to the floor and shifter in neutral??
or clutch pedal pressed (@2K) to the floor and in gear??
or clutch pedal not pressed (@2K) in neutral??

And it seems like you have multiple issues… Have you checked for pending codes??
Also check motor mounts for excessive movement or broken…

The vibration happens whether or not the clutch is in or in gear. New clutch is in with a resurfaced flywheel less than a year and a half. It was broken in properly as well. No pending codes as well. I’ll triple check the mounts but they seem okay.

Don’t forget the Harmonic balancer… And yes you can get parts that fail prematurely… lol

I’ll definitely give that a check and I also understand parts can fail prematurely except for the fact that the symptoms I described happened before and after parts were replaced. And I don’t think the parts came defective since the car didn’t get worse.

Can you take some video with your phone and post it?

I would but it says I need to convert the video

I had a higher than normal idle rpm problem on my Corolla one time, engine design is closer to your Mustang than my truck. That turned out to be a faulty idle air control.

Vibrations with the engine running are usually caused either by engine misfires or something that rotates when engine is running isn’t properly balanced. You can usually rule out anything out of balance in the transmission or further back if vibration continues unabated when you press on the clutch pedal, especially if trans is in neutral.

Unless there’s a reason to think a rotating engine part is out of balance, engine misfires seem much more likely as the cause. The crankshaft is powered along in its hopefully vibration free rotation by the pistons repeatedly pushing down on its arms, with the pushing forces repeating at a constant rate. If one piston doesn’t push, or not in the sequence it should, that will cause a vibration. It’s usually possible to confirm a misfire by feeling the exhaust gasses coming out of the exhaust pipe. They should come out in repeating bursts. Sometimes it is helpful to hold a dollar bill there. It should never move towards the exhaust pipe, only movement should be away.

Is there any chance the spark plug wires (or triggering coil signals) are not connected to the correct spark plugs? That will definitely cause a vibration.

I’m definitely going to give that dollar bill trick a shot since there isn’t any misfire codes. I’m able to record some live data from my obd and this is what I found for the fuel trims this isn’t normal correct? At idle

The long term fuel trims are just within the 10% limit, but indicates the computer is needing to inject more gasoline than it thinks it should be required to satisfy the O2 sensor, based on the measured airflow into the engine. Most likely either the measured airflow measurement is incorrect (b/c of an unmetered air path into the engine has developed for example), or the amount of gasoline it thinks it is injecting is incorrect (b/c the fuel pressure is lower than it should be for example).

It appears this problem is about the same on both banks, so whatever the cause, not exhaust bank related. The high idle symptom could be a clue there’s an unmetered air leak into the intake manifold. I wouldn’t expect a +9% fuel trim to cause the severe engine vibrations symptoms you are having, computer should be able to compensate, so maybe there is more than one problem. Compression problem for example.

With that fuel trim data in hand, you now have something to go on. It seems like your engine is just barely within the limits that turn on the CEL, producing an active diagnostic code. Is your scan tool also able to reading pending diagnostic codes?

The throttle stop screw kept moving up causing the really high idle. Still weird that it fixed itself when I revved it. I just put a nice amount of thread locker on it and it’s been fine so far. I still have the car bogging when I accelerate but atleast it’s just that now.

Glad you found the problem with your Mustang and engine is idling at proper rpm again. I’m surprised a 2004 model engine even has an idle-stop screw. You’d think there would be no adjustment possible for that b/c of federal emission regulations.