I am having trouble with my 91 NA Z , drives. Great for about 20 min then starts to bog down under load. Rpms max out the more you press the accelerator. If I let up rpms creep back up .
If I pull over and turn car off for a few min, then drive again it’s good for a while again.
Things I’ve done. Checked fuel pressure. Runs about 40 , but when bog happens drops way down. , changed fuel pump, changed MAF, changed Regulator, New Spark NGK plugs,
All fresh 91 octane gas , added fuel injector cleaner additive and octane boost. Checked code 55. No faults
Any ideas?

If fuel pressure drops during this issue and pump is new/working then it might be the fuel pump controller. Located behind the drivers seat for a 2 seater. The can fail when hot due to the age of these. Look up FPCU bypass, I think you just ground out one of the wires to the unit. Cant remember it's been too long since doing it.

Is this the part you are talking about?
Where is it on a 2+2

Yes, look here:

https://conceptzperformance.com/wiki/in ... pass_(2x2)

Thanks for the help. Unfortunately I’m still having the the same issue after bypassing the FpCU.
Car runs great for 20 min then begins to bog .
The fuel pump gets louder the longer I drive.

Hmmm, Now its getting hard to internet diagnose.
The only other thing I can think of that would cause fuel to drop is a failing Fuel pump relay.
Located in the foot well near the fuse box.

Ok. What is that green relay next to FPCU?
Do you know any z experts in the Denver/ CO springs area?

Woody75(Denver) is his username on another forum, I don't know him personally but hear he is great.

His name is Tim O'conner:

http://www.twinturbo.net/net/directory. ... r_id=22423

Hi. I have a ‘91 NA with an 11lb flywheel, aluminum crank pulley and a lightweight one-piece driveshaft on its way. Still the stock 270cc injectors cause upgraded ones aren’t/won’t be needed for this build, but I do have and am trying to install a dual intake kit that I’m currently just sitting on. Right now the car is running like absolute piss and not revving any higher than 3k rpm before bogging, backfiring or making a “CLUNK” sound within the engine. Now I’ve spent countless hours watching YouTube videos, reading forums and wrenching my hands into rhino skin not to mention hundreds if not thousands of dollars on parts and vehicle fluids/lubricants. Please tell me, will an eprom chip fix the problem and end my suffering? If not what should I do next? A response would be greatly appreciated. Thank you.

Have you checked compression and done a leakdown?
Have you confirmed fuel pressure at the inlet and outlet of the rail?
An Eprom will very likely do nothing for your issue, you have something wrong with the car. On the off chance the EPROM is damaged it could be causing some weirdness but if its a stock ECU that's never been opened the chances of that are very low.