Heavy Loss in Acceleration

[BIGSLVRXJ]

Okay guys got a couple questions here. My 90 XJ 4.0 AW4 has lost almost all it's acceleration recently. I noticed yesterday while driving, I was going 55 and I had the pedal on the floor and it wasn't going any faster. The only thing I noticed is when I attempt to "accelerate hard", I hear a hissing from in the engine compartment, like a large vacuum leak. I can't pinpoint where it would be from but the old Haynes manual says if there is a leak in the intake manifold the acceleration can suffer a lot.
The only thing I can think of is a...
Bad TB gasket
or... A cracked intake manifold- there are no visible cracks though.

Anybody have an idea? Thanks in advance.

[Dr. Dyno]

A plugged cat can cause this problem. Hit it with your fist. If it rattles like a can of marbles, replace it.

[BIGSLVRXJ]

I hit the cat and it just made a "ping" sound-didn't sound too abnormal. Any other suggestions?

[WobblesXJ]

The sucking sound may just be your intake. I can hear mine under load and floored. Is your transmission shifting correctly?

[BIGSLVRXJ]

I've got a K&N cone filter so that may be contributing to the hissing noise. My tranny was just rebuilt about 2k ago. My TPS is out of adjustment again but I think my tranny is still good. Any other ideas?

[Eagle]

Take it to a shop and have the catalytic converter checked. As Dr. Dyno posted, a plugged cat will seriously diminish performance. It's the high-tech version of putting a potato in the exhaust pipe.

[BIGSLVRXJ]

Thanks Eagle,I'll try to take it to someplace soon. I've never heard any noises from my engine and with just over 200k,from what I've heard, it should run for a while, so I doubt its internal problems in the engine but... I dunno. The only other thing I can think of is my fuel pump has been making the real loud whirring sound they make before they completely crap out so I wonder if its not putting out the right pressure. If the pump is not operating properly would that cause my acceleration to be crap?

[Rawbrown]

I had a cat go out within a year. get a new cat. I would be willing to bet that its clogged. it doesn't have to rattle when you hit it.
but an even easier check would be your air filter... that can cause a power loss also. if the air cleaner is good then replace the cat.

[BIGSLVRXJ]

The air filter is good because I converted it to a K&N cone and I constantly watch it. I've got a lifetime CAR-X cat on there so I'll probably take it back there sometime. I just dunno if they'll charge me for labor to check it or not. I suppose its time I probably got it checked anyway.

[ZmOz]

I've got a K&N cone filter so that may be contributing to the hissing noise. My tranny was just rebuilt about 2k ago. My TPS is out of adjustment again but I think my tranny is still good. Any other ideas?

If for some reason the tranny won't go out of overdrive, that would cause the problem you're having. An un-adjusted TPS could easily cause it not to come out of overdrive...

[WobblesXJ]

Check your kickdown cable. Also your tps may be bad and not telling your torque converter to unlock. Does it seems sluggish from a stand still?

[WobblesXJ]

wow you got to that just before me, lol

[BIGSLVRXJ]

The TPS is not bad its just out of adjustment. I just replaced it and haven't had enough money to get it adjusted.(I refuse to do it myself) I will check the kickdown cable tomorrow. Thanks guys.

By sluggish you mean? I never have good acceleration, no.

[WobblesXJ]

why not adjust it yourself. It takes all of five seconds and could be your whole problem since the tps tells the tcu when to shift, unlock the torque converter, and tells the computer how much throttle you are giving it. This could be the only thing wrong. Get a volt meter and adjust it to 4.2 volts. if you need instructions I can send you a pdf on it.

[BIGSLVRXJ]

I have had the thing adjusted before and it didn't help any so I put a new one on and now its out of adjustment as well. I can try to adjust it myself it just sounds like a PITA, but I may try.(I might need that pdf WobblesXJ) I am starting to really think it is the cat though because it feels like it is feeding pressure back to the engine almost. The pedal will push in, it just doesn't like to. I guess I kind of worded the thing wrong in the first place. The acceleration is fine, I mean if you struggle against the pedal it will go fairly quickly its just the fact that pushing the pedal is like pushing a 1000lb brick with your foot. Maybe that can give a better diagnosis. Thanks for all you guys helping so far.

[Sarge]

Sounds more like a problem with the linkage.

Sarge