Idle remains high while rolling. Settles down at stop, but sometimes dies

I've done some reading on idle hang, and high idle, but no real luck in finding an answer.

I'm looking at purchasing an 03 Cobra. When I'm rolling the idle stays around 1200-1500 RPM until I stop. Once I stop it drops down to around 750. One time it surged up to 2000 rpm while rolling and then dipped below 500rpm and stalled out.

I've experienced this before with turbo cars when they have a vacuum line issue, but I'm new to a supercharger and the vacuum is -2x when idling, so I'm not sure. Looking for any advice.

if modded it could just be a tune issue

Sounds like the tune. Mine stays around 1100rpm until I'm stopped. I don't mind it so I haven't bothered to adjust it.

Agree with everyone else, tune is most likely the cause. My car did that prior to having a new tune as well.


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Thanks guys. The car was really solid and the exterior is in excellent condition. Got to drive it for a few hours this afternoon. Gonna pull the trigger. We have a very reputable mustang shop here, so I'll take it and get it tuned just to be safe anyway.

Yep, tune all day. Common issues on these cars. Tuner never reset it.

The hanging idle is often desirable in the tune. Mine is really hungry for dashpot/idle air and you have to really let it down easy.

However the fact it hangs but then stumbles low sometimes would make me also look closely at the IAC valve itself. I had one that would do the same kind of thing.

Of course you'd want to verify that the AFR is under control first. If it is, then look at the IAC.

your car should be able to just barely idle with the IAC valve unplugged. That will get you a baseline of where your TB blade needs to be prior to adding air in through the IAC valve. Check this item first! This will also help ensure your IAC duty cycle is in an acceptable range as I think max is 50%. For reference I had to open my VMP big TB blade quite a bit over how it came set from VMP. I have cams and a 3.4L Whipple, but the car idles really good.

Once that's done maybe your tuner can speed up the dashpot decay and it won't stall on you if it comes down faster.