93 XJ I6 hitting acceleration wall at 4000rpm

[bausman]

1041414

I have an 93 XJ with an I6, 120,000 miles and recently noticed a loss of power (not major, but noticeable.) I decided to wind it out one day and at 4000rpm, the engine will not go any faster, with the pedal all the way to the floor. It just keeps its speed but will not accelerate further. Even with no load (i.e. sitting in neutral) the RPMs go just beyond 4000 and then will not go further. Anywhere below 4000 rpm it seems to run fine. It has plenty of power on takeoff, even with A/C on.



I replaced the fuel filter and it has new coil, MSD wires, plugs, etc. K&N filter and flowmaster (done before this problem.) I cleaned the throttle body with Valvoline Syntech?? and it actually ran noticeably smoother, better takeoff, but wouldn't go above 4000rpm, still. No error codes on computer.



Speculations:



1) Not enough fuel pressue for wide open, but enough for lower RPMs - Fuel Pressure Regulator or Fuel Pump??

2) Crankshaft position sensor (I am really guessing here.)

3) Ghost in the machine?



Any ideas?



Thanks,

Carvel[addsig]

 

[balloo93]

1041415

This is a long shot, but did the muffler shop that did the flows chech to see what kinda shape your cat was in? One of the symptoms of a bad cat is loss of power.



Just a suggestion. [addsig]

 

[bausman]

1041419

No, I did that work myself. Nothing a good air chisel couldn't handle - it was my first and last muffler job.



Anyway to check without pulling it off? It passed emissions just fine - don't know if that would be a tell tale or not.

[addsig]

 

[berettajeep]

1041420

Is it an auto?

The transmission not locking the converter?

check for wires going to the transmission,maybe one got knocked off accidently when you did the exhaust.[addsig]

 

[MudderChuck]

1041427

Don´t know about the 93 but some of the earlier models have what they call initial timing, you gotta get the manual. A wire has to be disconnected to take the computer advance out of the loop and then the distributor is checked with a timing light to see if it´s close to spec. if it´s to far out, the computer runs out of adjustment potential (it can´t advance any farther). It kinda sounds like a advance problem in the distributor (computer) or possition sensor. But could also be low fuel pressure (volume) there is a test for this in the book. If I remeber correctly the motor only usually cranks to 4500 RPM, you arn´t far off. Try rebooting the computer, maybe the governer is acting up.

Another simple thing to look at is, the warm air flap at the inlet of the air cleaner box, if it is stuck in the hot position it could restrict airflow some, and cool air most always runs stronger than hot. Just a couple of ideas that might or might not work out.[addsig]