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RPM problems, Transmission? Fuel Pump? Exhaust? Can't tell

I’ve got a 1994 Nissan Pathfinder SE, its got about 220k miles on it. Its a little junky but I try to keep decent care of it. Until recently its been having trouble running. It will start fine, turns over and everything but after a few minutes of use and accelerating it will reach a certain rpm, usually around 2000, and it will kick back down, unable to exceed it. After this happens, the maximum rpm I can reach will continue to decline until the truck will no longer accelerate. After turning it off, it will turn over but it will not start for some time (couple hours). Is this a problem with my transmission? I initially thought so and replaced my trans cooler after seeing a trans fluid leak from below the radiator. This didn’t fix it though and I’m wondering if I should look into a new fuel pump. I also replaced the fuel filter but have yet to check the fuel pressure.

Will the engine rev freely in neutral?

Checking the fuel pressure would be a wise next move.

Certainly could be a fuel pump. Check the pressure at idle and while driving. Tape the gauge to the windshield so you (or a passenger) can read it. (Don’t bring it into the car with you) Accelerate until the problem occurs and see if the pressure drops below spec for the car. It could be an electrical problem in the ignition but you didn’t say how long you run the truck before it acts up. If it only acts up when warmed up it could be electrical. Either problem is likely to strand the vehicle so don’t go too far.

I made sure to try revving it in neutral, and no it does not help. I’m glad to hear people validate my suspicion, unfortunately I do not know how to check the fuel pressure yet. I will reply again when I have figured out how and made time to do so. 24 hours tops.

Clogged cat?

All good comments above. Anything that prevents the free flow of intake and exhaust air and fuel could be the cause. I second the clogged cat idea. If the other ideas don’t pan out, have the cat back pressure tested. Fairly inexpensive test. Another idea: A sticking EGR valve could be the culprit too.