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

[MattC1996]

I’ve been having an odd problem for months now, whenever I’m driving at low speeds (5-10 mph) around a parking lot feather the gas pedal, it occasionally gets stuck at or around 1500 RPMs with no gas pedal input and won’t coast to a stop unless I tap the brake pedal. It’s a 2014 with a Roush S/C and JDM 700 kit to be specific. I noticed this after installing the full kit and prior to doing the plugs, pulley and tune I did have the twin 65mm TB on for a few months prior without issue. Some of the common issues associated with this could be a vacuum leak. To ensure I don’t have a vacuum leak I have thrown a smoke tester on it 3 different times to no avail. I’ve had it professionally checked twice and even checking the fuel trims after a 45 minute drive, stayed well within spec. Has anyone else ran into a problem like this? I can’t find any information on it anywhere.

 

[thatJeepguy]

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

 

[MFNG]

I have found this issue as well. 2014 all stock. She just needs more juice. If I idle/coast it feels like it’s hanging up. More gas solves the problem it seems.

Let her eat!

 

[Guardrail]

I have a 2013 with a 5star tune. it does it with the 5star tune, doesn't do it with the stock tune. I have both tunes saved with hp tuners, I was going to compare files when I had time, maybe data log the fuel trims when it happens. But if @MFNG is seeing it stock. I wont even bother. I'm used to it now, doesn't really bother me that much.

 

[MTF]

Keep in mind that the PCM has predictive learning calibration software in the PCM.
So, if you drive hard for a while, it will learn that drive cycle and visa versa.
A full drive cycle can take up to 300 miles to complete for the tranny and 50 miles for fuel trims.
Having said all that, there will be hiccups from time to time when driving pattern and weather changes,
as the PCM will have to write new predictive values.

And remember that there is a pressure release valve set to 170 degrees in the transmission.
So full transmission pressure won't happen till the tranny reaches 170.

 

[FordTechOne]

I’ve been having an odd problem for months now, whenever I’m driving at low speeds (5-10 mph) around a parking lot feather the gas pedal, it occasionally gets stuck at or around 1500 RPMs with no gas pedal input and won’t coast to a stop unless I tap the brake pedal. It’s a 2014 with a Roush S/C and JDM 700 kit to be specific. I noticed this after installing the full kit and prior to doing the plugs, pulley and tune I did have the twin 65mm TB on for a few months prior without issue. Some of the common issues associated with this could be a vacuum leak. To ensure I don’t have a vacuum leak I have thrown a smoke tester on it 3 different times to no avail. I’ve had it professionally checked twice and even checking the fuel trims after a 45 minute drive, stayed well within spec. Has anyone else ran into a problem like this? I can’t find any information on it anywhere.

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.

 

[MTF]

I’ve been having an odd problem for months now, whenever I’m driving at low speeds (5-10 mph) around a parking lot feather the gas pedal, it occasionally gets stuck at or around 1500 RPMs with no gas pedal input and won’t coast to a stop unless I tap the brake pedal. It’s a 2014 with a Roush S/C and JDM 700 kit to be specific. I noticed this after installing the full kit and prior to doing the plugs, pulley and tune I did have the twin 65mm TB on for a few months prior without issue. Some of the common issues associated with this could be a vacuum leak. To ensure I don’t have a vacuum leak I have thrown a smoke tester on it 3 different times to no avail. I’ve had it professionally checked twice and even checking the fuel trims after a 45 minute drive, stayed well within spec. Has anyone else ran into a problem like this? I can’t find any information on it anywhere.

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

 

[FordTechOne]

Keep in mind that the PCM has predictive learning calibration software in the PCM.
So, if you drive hard for a while, it will learn that drive cycle and visa versa.
A full drive cycle can take up to 300 miles to complete for the tranny and 50 miles for fuel trims.
Having said all that, there will be hiccups from time to time when driving pattern and weather changes,
as the PCM will have to write new predictive values.

The factory PCM strategy will never idle the engine at 1500RPM in gear. That would result in an unintended acceleration event.

And remember that there is a pressure release valve set to 170 degrees in the transmission.
So full transmission pressure won't happen till the tranny reaches 170.

There is no pressure relief valve for temperature. The thermal bypass valve opens at 185 degrees F to allow fluid to flow through the radiator and auxiliary cooler; that has no effect on driveability.

 

[MTF]

I didn't see that the OP stated no pedal input, sounds like Tow/Haul might be enabled, maybe.
JDM's tune wouldn't have it idle at 1,500 either. Their Tune is pretty solid using the SCT file format.
So, something else is going on.

When I went to get the transmission fluid exchange the tech showed me the procedure, I read 170 for the valve to open.
And there is restriction to the fluid until temp is reached.
But that was 7 years ago, I'm going to do it again soon I'll ask to read it again.

 

[EricM]

Can you run the stock T/B with that kit? Easy to swap out and see how it runs.