2000 XJ sport with 173K miles. Throwing an intermittent cylinder 4 misfire code (P0304). Doing it before swapping the cracked 0331 head and doing it again. Coil is new and swapped the injector with another one but no success. The injector has a nice ticking sound so I know its getting power. Compression is surprisingly excellent in all cylinders, ranging from 155 to 180 with cylinder 4 at 160 psi. Sometimes I can drive it for half hour on highway and no code and then as soon as I slow down behind the light or an exit the code pops up. Is there anything else I should look for? I've owned the Jeep since new so I know all the sensors are original. The engine runs pretty smooth with very little vibration and a there is a little skipping that can be seen/heard from the tail pipe. Appreciate any help from the experts.

 

spark plug ?

 

Sorry, should have mentioned the plugs are new also - copper core.

 

P0304 before the head swap, too?

 

Yes. That's what is baffling me. Prior to the swap the car ran rough with more vibration. All of that went away except a minor miss and the (same) code.
To be honest, I thought my leaking head gasket was causing the engine to run that rough.

 

Used, rebuilt, or new head?

 

Sorry - should have been more specific. Brand new unit from Clearwater.

 

Find a high end repair shop or dealership that sells BG 44K. Add a can to a full tank of gas and run that tank as low as you dare.

 

Thank you. Will do. Is that for cleaning the injectors?
Is it possible that the O2 sensor(s) cause a single cylinder misfire?
I'm getting so obsessed with trying to find the cause that I'm getting tempted to just take it to the dealer and have get it hooked up to their fancy engine analyzer and tell me the root cause....
By the way, I took it in for emission inspection and it passed with no issues. Luckily it did not throw a code while the guy was running it through...That made me think that the miss is fuel related as opposed to ignition otherwise the HC values would have been too high. Makes sense?

 

It is one of the best injector cleaners and carbon removers on the market. Not cheap but it does the job in one tank.

 

I had the exact same issue, id get multiple misfire code and misfire number 6 whenever I was slowing down fast off the highway or a stop light. Couldn't figure out what it was and nearly all the parts in the engine bay were new.

I ended up replacing some exhaust pipes due to a leak, and somehow it solved the problem. I'd take a look at your exhaust system and see what sort of shape your cats are in.

 

Talk to a dealership about recalls- I know awhile back there was a recall on newer XJ's because the exhaust manifold would heat up one of the injectors, boiling the gas inside it, and causing a cylinder 3 or 4 misfire, making the engine run really rough for awhile, that's what caused mine, they added a heat shield on the engine to help solve it.

 

Exhaust-related is also what I'm thinking. Cylinder 4 when you come to a stop... Hows your cooling? Efan, clutch fan, and shrouds in place? Could be heat coming up from the exhaust manifold if it's crazy hot. 2000 and 2001 have a TSB from Chrysler to wrap the #3 and #4 injectors with insulation. Its just some heat reflecting foil. Not a bad idea to do the whole intake manifold as well with all the heat thrown by the pre-cats.

+1 on cruiser's suggestion to get them injectors clean

 

Thanks gentlemen. Appreciate the input. Regarding the fuel injector(s) getting hot, I know about the "heat soak" issue and I'm pretty sure that's not the cause. The symptoms are very different. In fact, the dealer installed a "heat shield" between the intake manifold and the injectors one of the times when we took the car in for service a year or two after we had bought the car, I think back in 2002. I now have both injectors 3 and 4 wrapped snugly in a heat wrap. Checking the exhaust system for leaks is one thing I didn't think about and I'll definitely look at it. The entire system is still stock and surprisingly appears to be in a good shape, aside from some surface rust.
Coming to think of it, I should have inspected my exhaust manifold more closely when I had everything apart while replacing my doomed 0331 head.
Thanks again for all the responses. Truly appreciate it.