1997 240sx A/T 2.4L Hesitation and Stumble under any load. Can Not figure out!

Ok. so ive been racking my brain on this thing. any and all help appreciated!
symptoms: idles well, but as soon as you get on throttle it stumbles and sputters. If you ease into it, it will go normally.
codes p0340, but distributor and wiring has been checked and swapped distributors.

What ive done: checked timing chain, distributor timing, firing order, compression, TPS signal and adjustment, MAP signal, Knock Signal, ECT sensor, o2 signals, and im sure im missing alot more.

Ive had a scope on it and verified spark happening on time. the only thing that i found that was off of what nissan specified was maf signal. it was fine on low load and idle, but when you rev to 4k rpm the voltage does not reach what is specified. I checked the wiring and tried a known good sensor and still no dice. Also it seems as soon as you get into throttle it falls into open loop mode until you let off again. as soon as it hit open loop under load it fucks up. But i need to know what is causing it to go into open loop. Am i doing something wrong???? Missing something????

I'd start by checking fuel pressure, just because it is easy to do, after that, I'd start logging data with a scanner and look for anything that falls out out spec just before the open loop mode.

BTW, if you are just using a generic scanner without OEM specific enhancements, you may want to have a dealer to scan it for you.

Ive checked fuel pressure and on using a snap on ethos pro scanner. On a scan tool nothing seems to drop out of ordinary before open loop comes on. Temperature stays constant and o2 waveform keeps cycle.

Just for giggles, what's manifold vacuum at idle?

When you say you "checked the timing chain", what exactly did you check?

Vacuum is about 17in hg on a compound gauge and I checked timing marks on the cam and crank sprockets, as well as with a timing light for ignition timing. The pics I posted above were of cam signal and coil voltage on negative of coil. Also on scan tool ignition advance is 20° which is what nissan specifies give or take 2.

Thanks, Apologies for my having missed that post. I'll think about this further.

I'll take a WAG based on the symptoms...fuel delivery issue (clogged injectors) or ignition issue (weak spark). Also since the pcm is complaining about the cam sensor, it could be wigging out under load?

Firing voltage is around 7 to 10k. I dont remember burn time and oscillation off the top of my head but they were all equal across all cylinders. I had scope hooked up on a test drive and amplitudes didn't change when it started messing up. Voltages were still constant and waveforms were still in time with cam angle and spark event. I do think theres a slight leaking injector as when I pump up pressure and lock off supply the pressure bleeds down slowly. But I mean like a few psi in half hour slow. I dont think that would cause total loss of power on load but revs fine in park or neutral. I've also tried swapping a good ecu in from another running car and same issue. I notice the code hasnt come back since new distributor but still same issue. This is absolutely ducking my brain. I have been thinking about the open loop operation. The only think I can think of that I didnt check is o2 heater. Coolant temp reaches 190 to 205 ish, o2 waveform cycles 100 to 900 in about a 100 milliseconds. But what if the heater circuit has an issue. It would default to open loop. I think sensor is new but I'll verify with owner and check circuit and see what I can find.

Being very careful, can you drain some fuel and check for contamination, age, odor, etc.? Maybe do an alcohol test.

Of course, if this has been going on for several "tank fulls", it is not likely fuel quality.

If you scoped the ignition coils showing clear burn times etc then likely not an ignition issue. So in my mind that leaves fuel and air, what are the fuel trims reading? Does it smell rich? because a stumble can be caused by both lean and rich conditions.

Under WOT most (many/all?) PCMs will ignore the O2 input and instead use a fuel map (usually based on MAF, ECT etc) to regulate the fuel. However, you said 'under any load' so it's worth checking the O2 signal too. I assume you've checked for any tears in the air intake hoses/boots which can sometimes only open up enough under load as the engine rocks.

My first thought were a fuel delivery issue - but as the MAF isn't peaking to the max voltage on a throttle snap and you have already swapped in a known good MAF I would do an exhaust back pressure test.

Have not checked the fuel quality actually now that I think about it. It has ran for a little while after he bought it properly and then it started doing this so it is a possibility fuel contamination. I can try an alternative fuel source. We have already tried swapping in a known good mass air flow. And I don't believe back pressure is an issue because a vacuum gauge is showing steady 17 not dropping also it's straight piped. I've smoked the intake already and verify that there are no intake leaks. Including checking for the PCV. it does smell rich. But the fuel trims are saying that it is running properly they aren't exceeding six. The rich smell is probably the crack in the gas tank that is letting Vapors Escape. Now that I think about it also moisture could have entered into the gas tank through that crack it has been rainy lately. I will definitely try the fuel source

Also tested o2 and circuit. Its good.