The car (1996 Mustang GT)overall runs fine, and cranks/starts up fine, but wheneverI start the car, it shoots up to 2500 (I dont go anywhere near the gas pedal while starting it, and its happened too many times to be fuel pooling). What is even weirder, is that it doesnt come right back down, but slowly backs down to the normal idle of 700-900. It still drives fine, acts normal except whenever I give it a quick rev in neutral, it does the same slow backdown, not a quick rev like every other car I have owned. Is this normal for a 1996 Mustang?

 

Check all your vacuum lines. Have you done anything recent to it, work wise?

 

Just recently bought it (I've been slacking as well, with work I havent had the time to take some pics to post here []) and I'm going over everything in the car to make sure its alright. Ill check the vacuum lines, I was thinking it was something along those lines or a sensor. The engine is completely stock, right down to the air silencer on the intake (How this thing lasted 10 years without that being torn off is beyond me)

 

Does it stilldo it after it has warmed up for a little while? I know on my 96, when i first start it, it does the same thing. My mech. told me its going thru some type of test mode when it warming up. But after it has been run for a few minutes and the engine has warmed up, it dont do it anymore.

 

Sounds like a dirty PCV valve to me. Buy some Seafoam (it's an engine cleaner) and use it on your car. Should solve the problem right away.

 

Thanks for all the advice, ill def use the Seafoam to clean the engine just in general since I recently got the car. However, I also noticed another thing that could be related. After the car is warm, and idling for more than about 3 mins (I was stopped at a train) the idle will start to go sour. about every 30 seconds the idle increased from 800 to about 1100-1200, and then came back down in the same way it does on startup. This leads me more along the lines of a bad IAC sensor, or maybe an O2 sensor.