My car wont go over 2000rpms in drive,PLEASE HELP!

My 1992 Infiniti q45 at first wouldn’t go over 5000rpms when i slammed the gas and now when i just push the gas pedal it wont go over 2000rpms.when its in park or neutral i can rev it up to 7000rpms but then it starts to drop down as i hold the pedal down.when its in drive Icant get it to rev over 2000rpms & when i try to it feals like it wants to go but somethings holding it back and then it starts to jitter & ill stop pressing the pedal & just press it enough to get the car to accelerate a little (around 1800rpms)anything over that & it starts to bog down.It will actually feel like im slowing down then going any faster when i try to push it past 1800rpms.when the car is first started i can get it to go upto maybe 4500rpms and it will accelerate upto that with no issues as soon as it gets a quarter of the way warm it starts with the issue of not wanting to go over 2000rpms.Also the exhaust flutters while idling(its a rough idle)the whole car bounces.under the car the exhaust from the c.c. back to the tail pipe is new.even before the c.c. is new.does this sound like it the c.c.?or something else?the air filter is new.This issue with the acceleration over 2000rpms started 2 weeks ago & is now at its worst.Please Help me figure this out.

Check the fuel pressure. When was the last time the fuel filter was replaced?
Why were the cats replaced? If you didn’t fix the misfire issue that caused the first ones to fail, then the new ones might be plugged too.

the cats werent replaced everything else was.the fuel pressure is doesnt just wont go over 2000rpms whiling driving.i can rev it to 7000rpms in park or neutral but when i drive it when its warm it wont let me go over 2000rpms.ill floor it and it will jump to 2300rpms and then bog and drop down to 2000rpms and it feels like it wants to go but has something holding it back.and the car has a rough idle and the exhaust sounds like it has an exhaust leak soemwhere like a ticking noise and the exhaust pipe at the end where the fumes come out sounds like put put put put put. i read on the internet that it sounds like the cats are clogged.does it sound like a maf problem ?

Can you rev to 7000rpm (gosh I wouldn’t do this) after it’s warm? Still sorta points to cats. You can tell with a vacuum gauge. You should have near full manifold vacuum at any no load rpm. The engine is just spinning out the air:fuel as far as it can …just like it’s doing at idle. When the exhaust backs up, as it would with a clogged cat, the throttle will have more of an opening without the corresponding rpm soaking up the available air:fuel. The reading will start dropping off more and more as the throttle opens.

It could be other things, but that’s how you can eliminate cats. Again, do a factoring out of variables when you are seeking to isolate things. That is, you say 7000rpm cold in N or P …but you don’t say N & P while warm …just in gear and warm. You’ve left a hole in the process that is void of data. Just trying to communicate a critical thinking format.

You’ve got the classic symptoms for a clogged catalytic converter.

At idle, put your hand at the exhaust outlet. Can you feel the individual bursts of exhaust? If not - exhaust obstruction.

You changed the air filter but did you look for a rodent nest in the air intake?

This car uses a chain to drive the cam. Has it been noisey? If so, possibly it is loose and has jumped a tooth or two. Another suspect a bad cam position sensor, but that should have set a computer code.

when its warm and in n or d it will rev up to about 5000rpm and then slowly drop down to 18000rpm and wont go any more then that.when i listen to the back of the exhaust it sounds like put put put.its not a nice flow its more of a stutter.and it sounds like somewhere in front of the cats it has a slight exhaust leak.nothing else is noisey.the engine and tranny are strong and without any stund…

I agree that this sounds like a classic case of a clogged catalytic converter.

However, a clogged cat converter may not be the biggest problem that you will face in the near future.
I refer to your statement:
“when the car is first started i can get it to go upto maybe 4500rpms”

If you keep doing that, you will need a new engine. This is a particularly bad idea with an engine that is already 17 years old.

DO NOT race a cold engine unless you are looking to replace it very soon.

i just found out that i have 2 mechanicl cats and 2 nonmechanical cats.what the hell?why does 1 car need so many cats?can i cut the 2 out and just have 2 instead of the 4?what does a mechanical cat do and whats the difference bewteen the other non-mechanical cat?

Forget the cats for now. Replace the fuel filter and have the fuel pressure checked.

sorry i ment manifold cats.the fuel filters fine and my fuel pressures exhaust is putting and my car has a rough idle.i can rev it fine in n or d.then as i rev it it starts to die down to feels like it can get the exhaust out,it feels like it wants to go but somethings holding it bringing it to the mechanic tomorrow and ill let you all know what he says and let me know what you all think…