This has been going on for about 3 months, every week or so, CEL goes on after rough idle. Check codes with Foxwell NT510, and I get P0302, P0300. I clear the codes, and the vibration and Restricted Performance goes away... Car is usually good for a few more days, and will happen again, I reset the Codes and again its fixed. Sometimes the CEL blinks, sometimes just on all time. Today, I got Cyclinder 4 P0304 and P0302 and P0300... Any Help here? Coil Pack? Fuel Injector?

 

Clear code. Swap coil with adjacent cylinder. Road test. Determine which cylinder misfires. If no change swap plugs. The reason it clears after clearing codes is the injector on that cylinder is deactivated after excessive misfires to protect the catalytic converter. Most likely the coil. Good luck.

 

Thanks for the explanation, wondered why Clearing codes seems to fix everything. I was worried about clogged fuel injector etc. Does the XJ have individual Direct Ignition coil packs per cylinder or bank? I will search the forums for any relevant threads

 

Yes, these are individual "coil on plug". As was mentioned, swap the coils with the adjacent cylinders. If the misfire codes change, the C.O.P. is bad. If the code remains the same, it is likely a bad plug, or possibly but less likely, a defective harness or connector. When moving the C.O.P. look at the boot. They can harden or crack, causing arcing, and subsequently, a misfire.

 

Replaced coil pack and spark plug, and problem persisted. The connector looked fine. Also saw a hit on cylinder #4 once. Direct injector replacement was next. Took car to a euro shop, chrged $1800 to replace #2 & #4 Direct injectors with Bosch equivalent DI modules. Problem fixed... they replaced Supercharger belt, for only the cost of the belt.

 

yeah the injectors can carbonize eventually and foul, even with good gas

 

Agree, specially in diesels. I have used Lucas upper cylinder lubricant small amount once a month in to the tank, to keep my (diesel) injectors in shape.
262t km on clock and smoke test at MOT shows still 0.00.