I have been doing a few bits of work on my Dec 2001 XJ8 4.0. Last job before a 300 mile round trip, was to replace leaking cam cover gaskets. All went OK and no more leaks. After getting back I parked the car outside to unload the luggage. An hour later I went out to put it in the garage and on starting, it sounded to be running a bit lumpy. Blipping the throttle didn't clear things. It could be plugs, ign coil, injector. I cleaned the MAF sensor this morning and it made no difference, it wasn't dirty. I find that if I depress the throttle pedal slowly, it will rev up as cleanly. If I quickly blip the throttle, it sounds like I am getting a backfire in the exhaust. I am trying to get hold of an OBD tester to see if and codes are present. If anyone can help I would appreciate it. Thanks Bill W.

 

sounds like you have a air leak. Check your part load breathers on each side and the intake tube and connections

 

+1 rlj's comments. Double check the connections from airbox to throttle body. Also clean the throttle bore and plate (carefully). Run some good quality fuel system cleaner through a tank (SeaFoam or Gamout's look alike).

Dirty connector, air leak, dirty throttle, most likely. But an OBDII readout for fault codes would id misfires.