I keep getting a CEL P0306 on my 2000 XJ 4.0 Sport.
I have replaced the coil pack, spark plugs, and injectors.
I then swapped the plugs and injectors with cylinder 5 hoping it was a bad part.
No joy, it comes back.
I have new O2 sensors going on today and tomorrow but how can they cause a miss in just one cylinder?
I have checked for vacuum and exhaust leaks. None.
It comes and goes. I can mess with it and it'll run fine for 2 days no CEL than Bam, rough running and a light. No warning.

I HAVE done a search and I find lots of threads that complain of the same thing BUT NEVER a final post on the resolution.

Any ideas? Anything I've missed?

 

Two things come to mind before moving onto more complex possibilities.

1. I've seen marginal fuel pressure create a single cylinder misfire. And I believe I've seen it on cylinder 6. It's called a "lean misfire". You definitely should test yours to rule it in or out. Must be done with a gauge which can be rented from many parts stores; call around. For a 2000, you should have 49 psi, plus or minus 5 psi.

2. Secondly is something internal can also cause a "mechanical" misfire. Never ignore the possibility of something internal. I always start with a simple compression test. Test all cylinders (dry) first. If any are low, add a tablespoon of oil to the cylinder and perform a wet test. Compression spec for the 4.0 is 120-150 psi, with no more than a 30 psi variation between cylinders.

Throttle position sensor is also an outside possibility, as well as something more obscure like a crack in a manifold. What plugs are you running? Best plugs for the 00-01 ignition are NGK copper plugs. I've seen isolated cases of E3 and Bosch plugs causing misfires; rare, but it can happen.

Good luck and keep us posted!

 

Hmmmmm Hadn't thought of No.1 but it makes sense.
Went to all the limited few places for parts here in The Great White North and found that they will not lend pressure testers for some reason.
Standard BS from Canadian retailers that have no real competition and then whine like the babies they are when we go south to spend our money.
Didn't want to buy their crappy tester for $60 bucks to use once to find out I need to spend another $100 for a fuel pump.
So as it's now a 13 year old vehicle, I'll spend the money on a new pump anyways.
Having said that any ideas on which manufacturer of pump I should buy? Seems like there is a ton of them out there.

Will test #2 when it stops snowing.

 

Sorry, I didn't see that you're in Canada.....you obviously don't have the same tool rental policies that many parts stores in the U.S have. An option would be to drive to your favorite repair shop and tell them that you'll pay them to just test your pressure. It shouldn't cost much as it will take only about 10 minutes and then you'll know. I think I'd do that before replacing the pump on a hunch.

But if you do decide to drop the tank and replace the pump, replace the ENTIRE fuel pump assembly; includes pump/regulator/filter. My favorite brand hands down for a fuel pump for the XJ is Bosch. Carter is okay. I don't have a third favorite. I will not install nor will I recommend an Airtex fuel pump. Too many issues with them; I was personally burned multiple times with them. Do not choose your fuel pump based on price. Unless you like doing things twice! You get what you pay for here.

Good luck and keep us posted!

 

Thanks TJ.
I actually asked the service dept at China Tire...... errr Canadian Tire how much too check it for me. $50! Just about shat myself.
I have one I saved from my 98 that I sent to the crushers. I didn't have any problems with it while it was running. Long story why it's gone.
I think when the tank is near empty, I'll drop the tank and switch them. If that solves it, I'll be on the look at for a new pump down the road.
In my further research I read that the problem is really the regulator at the top of the assembly. Will keep you all posted.

 

il be watching since i have this same problem right now but on cylinder 1.....if i get it figured out ill let you know what my problem was

 

subed. i have a 1 and 6 missfire, changed plugs, wires, cap, rotor, coil. it ran fine for 2 weeks and now the same codes are back . cant seem to find any direction as to where to go. if you guys find solutions please post back!

 

lol Crappy Service Department. A coworker of mine went there for an e-test and discovered that this alternator splash shield was no longer attached when he hit a puddle. I think Oil Changers has more clueful employees.

Partsource has a tool rental program, the local one didn't have a compression tester but you might give them a try.

 

There is an issue with valve springs on some of the later 4.0s.

Also an issue with carbon building up between the valve seat and valve face.

I will share with you the only thing that fixed my buddy's after extensive work including valve spring replacement, treatment with BG44K, and othe rfuel additives. He hooked up compressed air to the offending cylinder at tDC and got a hiss out the exhaust unless he pried the new valve spring up with a screwdriver.

I told him to try the old mechanic's trick below and it's never had a CEL or misfire again.
Pretty easy.

Get a cup of water, preferably something like a Dixie cup.

With the air cleaner off, hold the engine RPM at 2000 RPM or higher and dribble the water from the Dixie cup, which you've pinched to look like a spout on top, into the carb or throttle body. Go slowly and the engine will bog a bit. Stop and let it recover. Repeat until the cup is empty.

 

I remember doing this all the time to our old hotrods. Seemed to work then. Was actually thinking of doing that with Seafoam.
Will try the water first. Worth a try anyways.

 

Did the water trick. Wife thought the jeep had blown up. LOL.
SO cold today the steamy exhaust turned into snow.
I also changed out both O2 sensors. Yes it only has 2. Maybe it's an export thing as it's in Canada.
Took it for an extended run. Tried all kinds of different driving styles and no light.
Success? Don't know yet. It still runs rough at idle. Will change out IAC soon. Will follow up in a few days.

 

Thanks for the encouraging update.

 

Sorry about this............ It'ssssssss baaaaacccckkkk.

Almost made it to work. I do have a miss. I just cannot figure out why.
We get some nice weather around here I'll do a compression test (I still have my own tester) and maybe pull the valve cover. Needs a gasket anyways.

 

I vote map sensor

 

I'm pretty sure the 2000 XJ doesn't have one.
If I'm wrong someone please point out where it is.