What changes at about 3000 rpm?
What sensors does the ecu stop looking at or whatever at about 3k ?

My 300zx runs like two totally different cars above and below 3 grand. Above 3k it runs like a champ. Pulls hard. Runs smooth. Etc etc. Below 3 grand, it breaks up. Makes no power. Just runs all around poorly. But. The idle is generally smooth. You can hear a very light miss in the exhaust at idle.
The engine in this car previously did the exact same thing.
Any ideas?

 

I don't know that ecu but it could be a open loop/closes loop issue. There are several set points that will determine whether the car is in open or closed loop such as throttle position, load and rpm.

You say it does this on 2 different motors with the same ecu. We're the maf sensors used the same also?

 

Different engines. Same ecu. A lot of the possible causes would have been swapped with the engine. The harness is the same though. Different maf. New tps. Bypassed knock sensor. Different cas. The more load, the more apparent the missing becomes.
You can shift to a higher gear under heavy throttle and it'll loose power and break up until the rpm's get to 3 grand and it's like a flip of a switch. It smooths out and pulls hard all the way to redline.
If you drive like a racecar all day with the rpm's up high, you'd never know there was a problem.

 

Ok. So those of you playing along at home, bypassing the fuel pump control unit seems to have cured the poor running under 3 grand thing. Only been a day and a half but now it runs like it just needs some adjustment. Spark time probably. Now the fuel pump gets an even 12v all the time like most cars. Well. Most older cars at least. So, to answer the what happens at 3 grand question, the ecu ignores load inputs and just tells the fpcm to step up from 6-7 volts to 12.
The fpcm manipulates the ground side of the circuit. I wonder if this went bad previously and that's what killed the old fuel pump or if the pump went bad and was trying to draw more voltage than it could get... hhmmm....

Now that it is getting more fuel, the idle will occasionally get hung up around 1500 rpm as the Revs fall. Hoping that's just a tps adjustment. But I have the aiv/iacv on the list to inspect just in case.

On a bit of a side note. Anyone got a good hook up on o2 sensors? By the time you pay what the parts store wants for two of them you might as well buy a wideband.

 

Partsgeek and Rockauto are my Go-to's for replacement O2 sensors.