GEN 1 RPMs stick at 1500 when coasting at low speeds.

[MattC1996]

Maybe something like throttle linkage or perhaps intake manifold sensor is dirty or failing?

I’m pretty good about cleaning my MAF sensor after every filter clean, is that what your referring to?

 

[MattC1996]

What throttle body is on now?
Why did you change the Ford Racing T65mm which JDM tunes for?

Yes it’s the twin 65 and it’s the correct tune from the kit as well.

 

[MattC1996]

Professionals rely on published Service Information for diagnosis; the variables of your aftermarket supercharger, TB, and tune cannot be accounted for. If it started after the mods, it’s likely at least part of the root cause.

The electronic throttle control has redundant position sensors and processors for safety purposes, and the commanded vs actual position of the throttle plate is closed scrutinized by the PCM. So if the throttle we’re hanging open, a wrench light should set immediately with a corresponding DTCs and a derate strategy.

Which means the issue is most likely either mechanical, such as a sticking accelerator pedal or vacuum leak, or software related, meaning your tune.

I have read issues on here with supercharger inlet elbows (?) cracking/leaking and causing similar symptoms, not sure if your particular system is susceptible to that.

Yeah it’s just an odd problem, even at operating temp it does it. I can coast around Forever stuck at 1500 til I tap the brake ones and it releases. I didn’t think it would be a issue with the TB since it’s electronically controlled, plus I didn’t have the issue prior to the upgrades. Maybe I can test the resistance of the gas pedal at some point. I do have a slight whistle now when I first start up but it seems to go away without 2-3 minutes. But no leaks visible behind the supercharger or anywhere in the intake. I’ve had they confirmed with a local shop as well. I paid careful attention to the elbow gasket when re installed because it was miserable to get off and on, I didn’t want to do that again!

 

[Nex]

The bigger question is, is this happening on your prius?

 

[1BAD454SSv2]

It does sound like a vacuum leak .You will have to get a smoke test if you cant find anything else . But his procedure might be worth a shot .

 

[Blu Bug]

My 2013 Roush with JDM tune does the same thing. I haven’t tried to run it down yet.

 

[StangGT5]

Yeah it’s just an odd problem, even at operating temp it does it. I can coast around Forever stuck at 1500 til I tap the brake ones and it releases. I didn’t think it would be a issue with the TB since it’s electronically controlled, plus I didn’t have the issue prior to the upgrades. Maybe I can test the resistance of the gas pedal at some point. I do have a slight whistle now when I first start up but it seems to go away without 2-3 minutes. But no leaks visible behind the supercharger or anywhere in the intake. I’ve had they confirmed with a local shop as well. I paid careful attention to the elbow gasket when re installed because it was miserable to get off and on, I didn’t want to do that again!

If it stops when you tap the brakes I'd think that means you either (1) have a software issue and the brake sensor is stopping it (maybe the tune is making the ECM try to engage cruise control??) or (2) there's a vacuum leak in the booster and you're moving the diaphragm. Usually a booster leak will cause high idle all the time or get worse when brakes are applied (press brake to go haha). I'd lean towards software.