a friend has a 99 4.0 xj,, took it out on the highway and he said it doesnt wanna go past 50mph for some reason, shifted a lil funny one time, and its using a ton of gas,, it had no problems before,, any ideas? sensor? clogged cat?

 

i would check for codes first then go from there even if there is no cel

 

poss. the TPS or IAC valve. they control air and fuel and some tranny stuff as well. are there any other symptoms? are all fluids good? i doubt its the fluids cause its only an issue when you hit 50... umm... what else- the o2 sensors, coil, cap/rotor?

 

fluids are good, i told him to do a fresh tuneup and go from there,, i had a tps go bad and it wouldnt go outta first gear, so not sure if its that, what is the IAC valve?

 

he checked and no codes popped up

 

I had a similar problem,but not ALL the time.More so when i hit a puddle. My problem was the 02 sensor. where the wires go into the sensor,it was actually cracked. I would have 50% power if that and it would backfire through the intake. If i shut the motor down and cycled the key it would run good again. this was on my 92 though

 

wow i make sure he looks at that, well he did a complete tune up and a new TPS, and still the same,, maybe clogged cat?

 

Pull out the o2 sensor between the the cat and the and the manifold them take it for a drive.

It will probably set a code, but you will know if that is the problem.

 

Just saw this thread.
I have a portable fuel pressure gauge that you can put on the hood just ahead of the wiper arms and check fuel pressure while driving.

If the fuel pump cannot keep up with volume needed for the highway, it could explain a low top speed.
Another symptom would be backfiring (through throttle body) with the gas pedal floored at highway speed; the engine starts to run lean.

Most electronic stuff should not matter, but

I had a crank position sensor on a motorcycle that kept RPMs low, took three years to figure it out.