I've owned my 91 NA for just about a year now and I've been experiencing some strange idle issues. Sometimes my car idles extremely low, around one or two hundred rpm, other times it idles fine. Typically it idles fine just after its been warmed up, but after around 20 minutes or so of driving it starts to idle low. Sometimes, oddly enough it'll go back to normal after a while. The idle usually kicks back up to normal when I turn the lights on, but sometimes it'll eventually makes its way back down to one or two hundred rpm. occasionally it seems to help when I shut off the fog lights (daytime running lights), other times it doesn't. I've adjusted my tps to spec, I've checked and tested my bac valve and looked for vaccume leaks. I've also tried to adjust my idle air mixture screw on the top of the throttle body, but if i back it out too far itll catch at 1200 rpm if i tap the throttle. I know that's a lot of info to take in but if anyone has any suggestions as to what could be causing this issue I'd be very grateful!

 

Is it a steady idle at 200rpm or barely staying running?

 

It's a suprisingly steady idle at 200 rpm.

I also forgot to mention that the rpms drop when I press the brake pedal too

 

check valve in brake booster line? cant remember if our cars have those...

 

I havent seen one on the brake vaccume line, i guess if there's supposed to be one its not there haha. I think it has more to do with the electrical load of turning on the brake lights because it seems to happend when the lights turn on. I've seen in the shop manual that there's an electrical load control unit tucked behind the ecu, maybe that went bad? I'm not sure how often those fry though.

 

If the car is idling low then press upwards on the wires feeding the BAC as one of the two wires may be pulled back within the plug connecting to the BAC which will cause the BAC to not operate. If you do this w/the engine idling low then if the plug connection is faulty the idle should increase when upward pressure is applied to the wires running to the BAC.