I've just recently noticed that my idle rpms will drop down to about 400-500rpms, the engine will get really rough, and then it will come back up to the stock idle and smooth out. This happens continually when the car is just running at idle. It almost feels like how you get an rpm drop when the AC compressor kicks on, but it does it w/o running the AC. The intake was recently changed to a NISMO, so the filter is fine. I've just finished upgrading the battery ground to 1/0, and running all new 1/0 grounds (per activetuning's kit instructions). The car drives fine, and doesn't seem to have lost power, but the idle is just not normal. The CEL hasn't popped on, but when the RPM's drop, it feels like it's going to stall out, then the RPM's will come back up and everything will smooth out. I've had the NISMO installed for about a month now, and I've just noticed the rough idle over the last few days (since installing the additional grounds). Any ideas? I don't want to take it to the stealership unless I have no other options. It's under warranty, but I know they will give me grief about the stereo/intake/ground upgrades. Also, Shell premium is the only fuel I run in the car.

 

ttt

 

Recheck the grounds. My assumption is that you might have an open GND somewhere....Reason for this is because it just started when you did the grounds, and that is a common problem when grounding is not at its best.

 

I went back over the grounds, the only factory ground that I touched was the negative battery to frame-rail. I actually removed it, sanded the frame to bare metal, and re-attached it. It was screwed to the frame-rail right over the paint. I didn't remove any other grounds. The other grounds I added just went to screws/bolts that were holding down brackets.