I haven't had a chance to check anything other than the codes but the symptoms, to me, sound like a MAF problem. The DTC codes however would indicate otherwise. This is my new beater 4th gen btw.

The codes I pulled:
Knock Sensor
EGR High/Low flow detected
Downstream O2 sensor fault


But here are the symptoms:
When the car is cold, just after a cold start in cold weather, it doesn't exhibit any problems. As it heats up though the stumbling increases and when its at full operating temp, it's pretty bad.

Once fully heated up, the car will stumble very badly below 2000rpm at any throttle setting. Between 2000 and 3500, the car will stumble if more than 1/3 throttle is applied, and will barely accelerate AT ALL. If you floor the car, it can not make it past 3500rpm. However at part throttle (where no stumbling is occuring) it will go past 3500 just fine.

I always thought that a knock sensor problem just cause severe power loss, but not stumbling and driveability problems. Hence I assumed it was a bad MAF. However the codes say bad KS and nothing about the MAF. When it gets warmer I'm going to start testing things.

Opinions.

 

Could your injectors be dirty or maybe a bad fuel pump?

 

I replaced my KS with a new one, couple a days later I got the EGR code., my problem was resolved by replacing EGRC solenoid valve. Both KS and EGR codes never returned.

Also check this
How to troubleshoot EGR problems.

Credit to Dave SZ.

http://img52.photobucket.com/albums/v158/evilBIGD/EGR/

Also: Maxima Repair manual.

http://members.dslextreme.com/users/maxima/esm/

 

Swap all of the coils with your other car first.

 

injectors? spark plugs maybe foul? MAF may be shot, I can't offer anymore than what you probably already know.

 

I'd say injectors. I'd pull all the plugs look for fouling. Any plugs that look funny should have the associated coil pack and injector tested. Do have a FSM or other shop manual?

I'd also closely inspect the MAF elements and clean them and install a new fuel filter. I'd also check the fuel pressure with a gauge to verify the pump isn't flaking out.

 

Definately the MAF...Had this same thing happen to me like a month ago. Check your codes!!!

 

Edited. Problem was mostly fixed by swapping coilpacks, but is still evident just a little bit. See the next post for description.

 

Actually it's not totally fixed. The issue is DRASTICALLY improved, probably 90% gone, but if I go full WOT, i can still tell as I'm rolling on the throttle that it's hesitating a little bit. The car is faster at 90% throttle than it is at WOT, so something is still going on, but its a million times better than before. No RPM issues now, it just seems to have to do with the throttle position now. any ideas?