I've had a few issues that I think are connected so i'm going to go into detail explaning them. I've been trying to figure this out myself for weeks now.
Car is R32 GTS-T
One of the things I had to replace when I got the car was the air filter, put a K&N in. Car would go into a limp mode occasionally, I put this in a "the K&N filter oil is polluting the MAF" category, would use MAF cleaner and this would be fixed. Or is it? Later, after I went back to a paper style filter, I had this issue once again. It would happen during a drive then go away. It even happened on the way to an autocross and was fine the entire night.
I think this is a separate issue than the main issue that's been killing my drivability.
A while back to improve cooling I had a dual electric fan setup installed. I did not install this myself, paid a local shop to do it. He said I needed a new alternator and I was going to take care of that myself. I turned on AC and drove off the lot and immediately had issues. Car was dying, like the key was turned off. I checked this off on the "yup alternator" box and eventually got it home after an annoying drive. If I turned on all accessories I would get a significant drop in voltage, however the AC absolutely killed it. The AC was set up, when it was turned on, to turn on one of the fans. The other fan was set up to turn on by water temp. If both fans were on the car would die when driving. On the way home it also seemed to not want to rev above 3000rpm. However where the MAF issue above it would feet like it would just hit a rev limiter just at 3000rpm this would basically completely cut off all ability to use the throttle only occasionally accepting my input again or just outright dying. At first i was confused and thought issue above was related to this.
Ordered a RB25 alternator online, RB25 alternators are interchangeable with RB20.
After Alternator Install
Today I installed it and haven't had much of a chance to thoroughly test it out since I had to go to work not long after I finished. Turned on the AC and the idle RPM was so low the car sounded like grim death. Worse than before. I was able to rev it however it felt a bit sluggish. All other non-ac accessories turned on and the car felt fine.
So I guess I have some theories. Keep in mind these are theories from a person with limited car knowledge when it comes to anything aside replacing worn things:
1. The AC's bearings are shot and it's bringing down the engine? The AC has kind of clicking sounds coming from it. It functions however i've been told that the "ac clutch sounds like it's going out" by someone and that it's bearings might be going out by another person. Weird that it's worse AFTER alternator install which is also obviously driven by engine speed. RB25 alternator might require a bit more from the engine and it's bringing idle rpm the extra 100 or so down. Before the replacement it would do this but it wasn't as bad.
2. There is some other kind of a power issue. The battery is smaller than i'm used to seeing in a US car. However it cranks the car fine and I haven't had to charge it. Grounding issue? Short? I was able to drive it today after this install. Had the issue from the MAF? section once but it went away. Did not test AC while driving for fear of becoming stranded when I had work in 45 minutes.
This is as far as i've got so far. I've been working on this issue when i've had time/money for a while now and am not sure where to go next.
Don't let my comments lead you somewhere it shouldn't. I have little to no experience diagnosing issues like this.