I have a 90 n/t with a full RB exhaust system and a cone filter. I am having a lot of trouble getting it to idle smoothly. It has never idled perfect, but has gotten very bad lately, fluctuates between 550 or so and 800 w/ no accessories on. Replaced spark plugs and wires a few days ago with no improvement. Should I check out the BAC vavle like everyone else is doing? Should I expect a smooth idle with the RB system?? What about the EGR?? Could that be causing the rough idle?? I recently used the NOS octane booster with no results. Thanks for any advice, I have done all I know to do.

 

First thing I would do is check the TPS (Throttle Position Sensor) to make sure that it is set correctly. If that is okay, next thing I would check for is vacuum leaks.

 

I have a full MAZDATRIX dual system from the RB headers to the tip of the mufflers, and I can guarantee you that the idle is smoother than smooth. Go look at the tps, bac, cut in vaccum hose, timing etc.EDIT....I do remember an article, might have been by Mazdatrix or Racing Beat, that mentioned a EGR valve with a broken diaphram causing some idle difficulties. I think you could just get on the vaccum line going to the egr and see if when you suck on it, if it is holding a vacum, or if it is'nt. Trying to say you should be able to tell if its diaphram is any good.