Alright fellas, here's my issue. I noticed just before a trip I had left on, that my XJ was hesitating when I would accelerate. If I would gradually give it gas, it would begin to accelerate, then it's like it would almost hit a wall, even as I continued to push the pedal, and I would have to keep pushing until it would almost jump into accelerating again. Where as before this issue it would accelerate smoothly without me having to bear down on the throttle. Went on my trip, drove 300 miles while pulling a small trailer, and my gas mileage was normal around 15.5 MPG. So I figured the problem worked itself out. Filled her back up for the driving around that weekend, and I noticed the hesitation was back (never really went away actually) and I was getting horrible MPG, around 9-10 MPG. thought it was the fuel pump, so I replaced it (just the pump, not the entire module) and it still didn't fix it. Drove back home, and got the normal MPG again. I'm assuming the cruising was what made my MPG normal. But I noticed something today that might help pin point the problem. I had my AC on full blast, and was having to give it some serious gas while driving up a hill, and my AC stopped blowing hard, was still coming out, but not the force it usually has on high. Then when I let off the gas, it went right back to full force again. Any ideas? Vacuum hose issue? Thanks a bunch!

 

In order to help you out, its handy you let us know your model, year, automatic or manual etc first.

There are many threads on stalling issues (the hesitating during acceleration/driving) as well as loss of power of several components. Many different approaches, but don't start throwing parts like fuelpumps etc at it before you got some more diagnostics done)

 

My 95 will do this to me occasionally if what your describing is the same thing I experience. Its like I have to almost press the pedal to the floor to speed up. It seems to happen to me the most when the engine is running a tad warm. Most often it drives fine. I haven't had the drop in mpg however. I originally looked into the throttle body and the cable to make sure nothing was blocking the cable from moving correctly. A guess at this I could see it being the TPS going bad. Like the throttle is opening up but the TPS isnt registering the throttle opening therefore not upping the fuel ratio. Maybe the TPS gets warm and is intermittent? Just a guess but seems like a possibility.

 

+1, hopefully if the OP replaced a good pump with a cheapy they kept the old pump around. Lots less expensive to pull the the codes first, and have the start of a plan.

 

It's a 2000 4.0 automatic