Getting Frustrated

Check the IAC. Various sources show that the no-load warm idle adjustment for SD cars requires IAC to function and flow properly (different than MAF).

How is the cold idle and loaded idle? Is your VSS hooked up?

Absent other ancillary info, I would clean the IAC for starters.

Good luck.

And if you choose to clean the IAC, here is some info about cleaning it up.

Good luck.

Not that it apply's to you but and don't laugh my dads 86 diesel jetta would't stop the engine when the key turned off. but when you touched the brakes when the key was off it would kill the engine. turns out a ground in the rear brake light assemble was loose and it was causing the problems. In your case i would say IAC. You can remove it and clean it up. Perhaps when you are comming off the gas the computer is not getting the Throttle postion correctly. You can check to make sure you have about .96 volts on that sensor. no more the 1.00 volts and no less then .8 i believe.

I cant say on the booster. If the old one was really leaking, it needed replacement for safety's sake. Normally the brake pedal will get hard or feel like you really have to push hard to slow the car (like how it feels if you coast to a stop with the car off).

If you are still SD (the assumption for now), the vacuum signal is critical for the engine management (one reason to replace a leaky booster). So before if there was a leak, the computer thinks you have the throttle pressed down when you dont (while at idle). And ironically, a vac leak would have raised your idle up, and allowed the idle to hang high upon coming off the throttle. So this might have masked the underlying issue which you are now dealing with.

If you want to check your vacuum, a stockish motor should read about 18-20" hg at idle.

We should also have noted that retrieving the trouble codes is always a good place to start. A SD stang does not have a functional CEL (even though there is an outline for one). A very wise man in here named Jrichker very often posts links and info on retrieving the trouble codes.

Good luck bud.