I have a 1991 4 cylinder EFI Mustang. When I first start it up, it idles rough, and if you don't hit the gas a little, it will almost stall out, or sometimes it will stall out. (This car is not currently on the road, so I haven't been able to drive it and see how it reacts; however, when it was last driven, there was no problem at all while it was driving, just when idling or stopped.) Recently though, after about 15 or 20 minutes of running the car, the problem seems to go away. However I don't know if it really goes away, or if a vacuum cable moved and blocked a vacuum leak, or what? Also, it doesn't really completely go away... even after the rough idle dissipates after that time period, the RPMs change every once in a while while it is idling. There is no problem at all above idling speed. I am not sure this is a vacuum problem though, because the RPMs don't pulsate or sway up and down, but rather they just slow down until the engine almost cuts, and then they go back up a bit and start over, or it dies, one of the two. Also, I disconnected the intake tube from the throttle body and inside the throttle body tube, before the valve, on the bottom of the tube was what appeared to be greenish sludge. It looked like it was dried on there though. What could this be, and what would have caused it? I'm kind of new to the vacuum system... I don't know how to find the vacuum lines. Which ones are they and where would they go to and come from so I can trace them to see if it's a vacuum leak? Also, I was wondering where to get a code reader that will read codes in 1991 year Mustangs? The ones that I found online at sears.com say they're only compatible with 1996 and up cars. Thanks for any help you guys can give me!