2000 XJ 135,000 miles.

Yesterday after I had the oil changed and filled up the gas tank, my engine started misfiring something bad. I got it home and made sure there was nothing spilled on the coils. Everything seemed fine so today I went to the parts store and bought a new set of coils thinking they went bad. I replaced the coils and everything seemed better. The engine seemed to have a little more pep. I drove around town for the majority of the day with no problems. This evening I left my house to get a christmas tree and the misfiring started all over again. It started out very light and gradually worsened.

The misfiring isn't all the time and it never misfires during idle. It drives fine for a little while and then it doesn't want to drive anymore. While it's misfiring the rpm's jump up to about 3500rpm's and the transmission shifts in and out of gears. During the misfiring the transmission feels like it shifts into neutral. It has not yet thrown a code but I'm expecting it soon.

My engine temp, oil pressure and voltage are all normal. About 2-3 months ago I changed the spark plugs. My engine still has great power and until yesterday has been 100% reliable.

 

Start looking at components related to fuel delivery AND control.

1. Get a fuel pressure gauge and install it on the schrader valve on the fuel rail. For your XJ, you should have 49 psi, plus or minus 5 psi.

2. Test your THROTTLE POSITION SENSOR. It is responsible for smooth tip in of your throttle, as well as controlling shifting characteristics of the transmission.

 

UPDATE : tjwalker was right. A code was finally thrown. The TPS was not agreeing with the map sensors voltage. I went to the LPS and picked up a TPS for $44. I replaced it and now the XJ is running great again. Thank you for your help.