Exhaust systems don't just get "plugged". TPS is my first thought, vac leaks seem unlikely from his description, a stock EFI Z won't idle smoothly at all if there's a leak large enough to hinder WOT operation that much.

RT, have you washed the engine bay lately or been in a downpour? Water can screw with the TPS, even if it's adjusted correctly. Follow Captain's advice and download the FSM and EFI guide.

I haven't washed my car in over a year, and it's been dry here for about a couple weeks. This car was recently resurrected about 1 month ago (from sitting for 1 year). I run it a few times a week, and I can even drive it aggressively with no issues up to redline. But last night is when I actually did super sudden WOT and that's when I found out the popping surging issues. But I'll do the TPS check. Thanks!

Every suggestion in this thread is worth checking, even if the problem goes away. Get a meter and a pen and paper and go through the tests in the Engine Fuel chapter. Almost guaranteed that you will find somethings that can be improved and your engine will run better when you're done. If you read through the chapter first you can put a table together that will show what the measurements at the pins at the ECU connector should be. For some reason Nissan broke the testing up in to testing continuity at the ECU first, then real numbers at the component itself. Easier just to measure real numbers at the ECU. For example, measure resistance at the ECU pins for the coolant temp. sensor, instead of just continuity.

Don't skip the fuel pressure check either, that would be another cause of lean running. You might have a weak pump. And a sign of a plugged exhaust is a lack of pulsing at the tail pipe.

I'm gonna change my vote to a major vacuum leak. The engine is running lean, you stomp the throttle, and it gets a big gulp of even leaner charge because the AFM isn't measuring all the air coming through the leak. Instead of acceleration enrichment you're getting acceleration enleanment.

I agree, exhaust systems RARELY plug themselves up, while catalytic converters demand near perfect mixtures to NOT get plugged up over time. One fouled plug can ruin a cat.

ALL of these scenarios will be RAPIDLY revealed by attaching a vacuum gauge.

So right now I"d be looking for a big fat crack in the bottom of the snorkel between the AFM and the intake manifold, a brake booster that's leaking vacuum, etc. I'd go spray all over the place with brake cleaner listening for an increase in RPM's.

good luck!

Usually vacuum leaks have much more impact at idle. Your typical vacuum leak is a fixed size hole (or accumulation of multiple holes) which doesn't change with respect to throttle position. That means the amount of air pulled through the leak is a greater percentage of the total air when you are at idle. When you're at WOT, the leak is still contributing air, but it's a smaller percentage of total and hence a smaller impact.

However, the L-Jet system, with it's AFM upstream of the throttle body, is a little wrinkle. If the problem is a vacuum leak causing problems at WOT, it's probably something upstream of the throttle body. As Wade mentioned you should inspect the snorkle between the throttle body and the AFM. There might be leaks in other downstream places as well, but they would have more impact at idle than WOT.

Just an update, but I think I solve the problem. I changed the fuel filter and I can WOT freely in the full RPM range. The old fuel filter had light brown gas coming out on both ends (inlet and outlet), kind of like tea.

I thought I had a vacuum gauge on hand, but it was just a compression gauge, so I didn't do the vacuum test yet. Also I had a fuel pressure gauge on order, so I'm waiting for that to come in. My fuel pressure is definitely leaking because when I unhooked my fuel filter, gas didn't squirt out as it should, but just dribbled out (I didn't bother to release the pressure prior to unhooking the filter because I already knew there won't be pressure after a few days of sitting).

We'll see how long this lasts. Thanks everyone!

Oh yeah. Something else that can cause a lean condition under high fuel delivery conditions is a clogged fuel filter.

Here's to hoping that it was a simple as that!

After I remove a fuel filter I blow through it in two directions, into a clean paper towell, to see what sort of debris is coming from the tank, and got past the filter.

You can do it with a rubber hose or an air chuck, but beware, compressed air + vaporized gas = FIRE RISK.

Normally there's VERY shiny metal, looks like gold, in the intake side, it's the remnants of the brushes / commutator in the fuel pump.

Rust is a very bad thing

If there's rust / debris on the output side of the filter, well, you might want to clean or replace your injectors. A single particle of rust can jam an injector open. At least run some injector cleaner through it.

whatever you do, don't ever start and run an engine on 5 year old, stale gas left in the tank by the previous owner. Drain and refill. It'll start once, alright, and then never again as the varnish locks the valve stems in place. Lawnmowers OR Z-cars.

Checkyoru fuel pressure, and do a vacuum test as well. KNOW you solved the problem and that it was just ONE problem instead of doing wishful thinking.