[clintbaum]

When I first start my car up it idles just fine, but when I start driving and come up to a stop sign I put it in neutral and coast for awhile, and while I'm coasting the RPM's stay around 1600 until I come to a stop, and then they go back down to normal. I've also had the RPM's fluctuate up and down while coasting but it always goes back down to normal when the cars not moving. I just put on a new throttle body and plenum and I have a tuner with a bama tune, but I'm pretty sure I don't have a vacuum leak. Could this be part of the tune?

 

[wmburns]

Did you set the idle stop set screw per the manf recommendations? Likely there is just a little too much air entering at closed throttle.

It is NORMAL for the idle to be slightly higher when the car is moving. This is by design. The PCM will not enter slow idle state until the car's speed is zero. So don't expect a sub 1k idle while the car is moving.

Please review:

http://www.allfordmustangs.com/forums/4-6l-tech/49661-your-mustang-stumbling-dieing-idle-idling-rough-14.html#post2260649

 

[fordman777]

wmburns is right my 98 gt 5spd does the same thing if i push the clutch in or put it in nuetral while coasting the idle wont drop till im almost completly stopped i thought it was odd the first time i noticed it but i have drove other mustangs like mine that does it so i took it to being normal!

 

[clintbaum]

Well I've finally had some time to drive my car and mess with the idle set screw on the throttle body and it's still messed up. Sometimes it will do what I mentioned before but today while I was driving and put in the clutch the idle surged and hung at 3000 rpm's. Does this sound like a vacuum leak?

 

[clintbaum]

I was just thinking and I might have another theory about this. I went out to check for vacuum leaks and couldn't find any with carb cleaner, and I unplugged the IAC connector while the motor was running and it immediately died, so there really cant be a leak that would cause that kind of rpm surge without the IAC giving it away. What I think is happening it my idle set screw on my new throttle body is moving while I'm on the gas driving and its moving to give it a higher idle so when I let off the gas the computer is trying to figure out the new idle. Could this possibly be the case? I put loctite on the screw so hopefully tomorrow there is no problems.

 

[wmburns]

A bad TPS can cause a hanging idle as well. The PCM uses the TPS to determine if the throttle is closed. The absolute value of the TPS is not important. What is important is the TPS gives the SAME value each time.

What happens with a bad TPS is the PCM does not know that the throttle is actually closed. Therefore, it does not enter the idle subroutine.

The easiest way to trouble shoot this is with an ODB2 scanner. Monitor the TP-MODE PID. Should be "C/T" whenever the throttle is closed.

Also, any binding in the throttle linkage or a bad return spring can cause this.

Remember the opening post stated that the idle was normal when the car was stopped.