Hello all!! Looking for some guidance. 98 xj8 hesitates (falls on its face) under moderate to hard acceleration. I can back off the gas pedal and it picks back up, but as soon as I put the foot back in it, will not accelerate. Sometimes it feels like it is "searching" for a signal. Possible TPS or throttle body issue? Recent fuel pump, spark plugs, fuel filter. Runs great under light acceleration. Any input is greatly appreciated. Thanks!!

 

change or cleaning must be needed to the Mass Air Flow Sensor (MAF) and check the air filter while you're doing that.

 

I will give that a shot. Thanks for the input!

 

The fuel filter may also need to be replaced, and/or the fuel pump is failing.

 

I just noticed that the pump is new, but what about the fuel filter?

 

My hesitation troubles started with new fuel pumps. The primary provided good pressure at lower loads and idle, but the pressure started to drop as the throttle was opened, as more fuel was needed.

I diagnosed it by connecting a fuel pressure gauge with a long enough hose to place the gauge on the windshield so I could watch it while driving. As you open the throttle, the fuel pressure should actually rise, by action of the fuel pressure regulator connected to the inlet manifold vacuum. Mine would steadily drop.

Did this trouble start when the fuel pump was replaced?

 

Did you do all the work to fix this problem -- or did the problem occur at some point as the parts were replaced?

Most items trigger a code .. if you are not getting any codes ... I think you still have a bad pump or the hoses are leaking inside the tank.

You are going to have to test the pressure at high RPMs

 

the trouble did seem to start with the replacement of fuel pumps. My worry is the bad fuel situation and any damage it might have caused to the regulator. I did perform the fuel pressure test by way of the x308 workshop manual. Regulator checked out to those specs. My next action will be to drive with pressure gauge installed to watch pressure under a load. I am leaning towards a bad aftermarket fuel pump but want to cover all my bases as that job is not all that fun

 

Just as a side note from the bad fuel situation (water in fuel) - I replaced the fuel pump, fuel filter, and spark plugs after that event.