Some times when im sitting at a red light, the engine drops to about 500 rpm and starts vibrating. What would cause this? I do have a cold air intake. Would that be responsible?
I doubt your cold air intake would be the root of this, unless you have a mass-air car and the intake leaks. Tops on the list of things to check would be the idle air controller and ignition module. The ignition modules on the 91-93 8-plug DIS cars are known for issues and if your car is a DIS car, this could be the source. Your idle air controller is right near your throttle body and controls idle. Remove it and clean it out real well with carb cleaner. If it doesn't help, it may have died or your ignition module could be to blame. If you can run codes on the computer, that may point you in a good direction.