Can a Z run rich?
 

This question is directed to ZLover cause he seems to be very reliable for technical questions (No offense to everyone else)

So I'm trying to get this `90 NA to start running smoothly. It's tuned to the point where I can get over 6500rpm and eliminated neutral stalls but when there's a real load on the engine it hesitates above 5000. It's acting like it's running really rich. I even had some fuel on the sparkplugs. All the cylinders appear to be firing. I've got good compression and spark as well. it just FEELS rich.

I know this car auto adjusts it's fuel and there's no way to adjust the mixture without changing the ECU so that should be fine.

What do you think? MAF or O2 sensor? The problem only occurs when I have the pedal floored which means the ECU should be using the MAP instead of the O2 sensor. That would mean there might be a vacuum leak in the manifold.. ok now I know where to look next but any other ideas?

have you tried running a diagnostic?

Can't be the O2 sensors - they are inoperative over 3krpm and also at WOT (they exist solely to regulate air/fuel levels while cruising).

Do a boost- (or in your case, vacuum-) leak test and run an ECU self-diagnostic as b300z said. Also test the resistance of your injectors... should be 10-14ohms across the leads. NA's run lean more frequently than rich because when they have an air leak, it will draw in extra unmetered air - inversely, TT's will run rich under load because the turbos are drawing in air that is escaping. So it could be a bad sensor or an injector that's dumping too much fuel (less than 10ohms).

if nothing else turns up, a good cleaning on the MAF helps out often too. it made a noticeable difference on my car. in case you didn't know, this cleaning consists only of spraying the sensor with MAF cleaner, not scrubbing it or anything. you would break it. running rich makes the plugs very black too. so if they are wet, but white, that might be a different problem.