I´ve got a 91 GT, I just threw on an RX-7 VAF and a K&N FIPK intake kit.
I´ve been having this strange idle problem ever since I got the car, and when I got the car it was bone stock.
Here´s what happens:
1.The car idles normally, at around 850rpm when it´s all warmed up.
2.I rev the engine up to about 3500rpm
3.I then let off the throttle
4. The rpms drop to about 900rpm for about 1 second
5. then the rpms "dip down" ANOTHER 400 rpm (now it´s down to 500rpm) or so until the engine almost stalls
6. It stays at around 500rpm for about 1/4second and then it goes back up to the idle speed of around 900rpm.
The problem is, when it "dips down" like that, the engine almost stalls, and sometimes the headlights dim or whatever. It´s exceedingly annoying, because sometimes when I let off the throttle, the engine will cut out altogether.
It seems like it´s a problem with the idle speed control valve (ISC valve)... can anybody throw some ideas at me? I don´t know how the ISC valve works, but I was kinda hoping it was something as insignificant as a vacuum leak... because a new ISC valve is exceedingly expensive.