Hello, i own a 1998 Mustang GT and my RPM goes down quite slowly if i rev my car up, does anyone know what is wrong if anything at all? I could post a video on how slowly it goes down but no one else has noticed it besides me(talking about my family wise). Should i be concerned about it?
My 98 Mustang GT has a similar problem. I cleaned out my Throttle Body and replaced my PCV valve and replaced my air filter and it seemed to help it out some. It also caused mine to stall out in second gear because it wasn't getting the air that it needed. Give that a try.
The 96+ Mustang is programmed with two idles. Slow and fast idle. The slow idle is only activated while the car is stopped. This means that the car's speed plays a role in idle selection.
Sooooo..... if the idle drops to 1000 while the car is moving after a few seconds, this is normal. You may also observe the idle dropping in stages on it's way down to a slow idle. This is NORMAL. It is done for SMOG reasons.
The trick is to figure out if your Mustang is normal or not. It's NOT normal to idle above 1000 RPM's when the car is stopped. It's not normal if it takes minutes to return to idle.
What to do. Look for and repair any vacuum leaks. Even a tiny vacuum leak will cause problems. Replace the PCV value. Check the throttle return spring.
Here's some more information regarding idle and IAC trouble shooting.
It's also possible to add an IAC restricter between the IAC and the throttle body. There are TONS of write ups on how to make one or buy it on ebay. Warning. the restrictor works best on a car that is in good repair to begin with. If there are underling problems such as a vacuum leak, adding the restrictor will only trade one problem for another.