01 XJ #4 misfire (nonheat sink)

Due to some family issues resulting in a total lack of time (like not enough to hardly sleep) my XJ has sat for 3 months unloved and was just fired up today. But let's no go into all of that...

Here's what we have.
01XJ - 85K miles, not the crack prone block (331?)
Last used to pull a small duck hunting boat every weekend Nov 1st - Jan 12th.
Running like a top with absolute 0 problems. 75 miles each way. Pulled great in 4wd when needed.

Last filled with stabilized (with sta-bil) winter grade regular unleaded on Jan 18th.
Driven 9.2 miles after fill up to circulate fuel stabilizer throughout system before being parked.

Fired up today on what was left of the battery on the first try. Outside temperature was about 68 degrees. Alternator output appears OK, battery came back rapidly.

Immediate rough idle. Check engine light on shortly after start. Check engine occassionally blinked on acceleration below 10mph, no blinking check engine light after first 2 miles. No dents in cat and it didn't even take a light hit in any of the 'recent' trips, so I don't think it would have any reason to be clogged.

Hesitation under load initially below 2,000 rpm. Now (10 miles later) hesitation primarily under 1,500. Now a smooth idle at stop that settles into a rough idle @ 750rpm. The longer it idles the greater the possibility of hesitation. (I had to go to auto zone for the scan, I don't have a scanner...yet.) Only code revealed was a misfire on #4. No other codes detected.

No oil in coolant.
Oil looks fine.
Pulled #4 plug and gap was .035. Plug looks fine with no fowling and .035 is what I gapped it when it was installed.
Just a shade over 10K miles on plugs. The plugs are Champion 4412.
Compression numbers unknown.

I've got half a mind to fill it with injector cleaner and run it on the highway for a while to burn down the tank then top off with summer fuel and go again. However, this baby has 3 cats and it really scared me when the check engine started blinking. When it first got underway I though I may have heard a very small backfire into the manifold cat, but it could have been my clunky front suspension too. (I need to do a little work.)

Any suggestions?

I would try draining the gas tank and refilling. It could just be bad/old gas. The fact that it seems to improving as you drive it could mean it's gradually cleaning itself out. I don't really know, but fresh gas couldn't hurt. Check your cap/rotor and plug wires while you are at it . . .

No cap/plug wires on an 01

x2 on fresh gas and injector cleaner

Fuel filter possibly? Just a suggestion since ga$ is up there to refill an entire tank, a filter is around $12 at Wal*Mart around here. Personally thats what I would do if you are unable to remember when it was last replaced. :dunno:

New filter wont help bad gas. If its bad, its bad.

I'd drain the old gas out before you screw up your injectors/foul your plugs. Good luck!

Bad gas had crossed my mind, but I fill everything up at the same station and I've been using the same fuel stabilizer for 12 years with no problems. The stabilized fuel is supposed to be good for 6 months but I always burn out the tank and refill at least every 3 months if it hasn't been emptied for any other reason.

It just dawned on me the fuel filter is the only thing I haven't replaced. doh!

The fuel filter is in tank. not on the frame rail. And if im not mistaken its going to cost a little bit more than $12.00

Hit it with a can of B12 to the tank tonight and there was no joy in mudville. If anything the problem became worse. It didn't make it around the block without the CE flashing.

I've got a fancy OBD2 scantool coming in that will monitor the O2 sensor and such in real time. (Finally took the plunge.)

I'm having 0 luck siphoning anything out of the tank. Anybody have any suggestions? Since I have to take the aftermarket tank skid and trailer hitch off, I'd really prefer not to drop it. They weren't all that easy to mount the last time and if I have to lay aside time to do it, I'll probably get to it about October.

This really isn't acting like old fuel. I'll know more when I get the scan tool. If it turns out that #4 is all that's misfiring I'll probably start by replacing plug 4 in case there's some crack that I didn't see.

try pulling the #4 injector connector off while its running.

I wouldn't run that thing for more than a few minutes with that misfire. I've had a rough idle on my 00, and replugging the throttle position sensor stopped it. never had #4 misfire, thats usually an early sign of the cracked 0331 cyl head.

But if the plug is clean, and theres no sign of green staining in the cylinder, its possible the injector is clogged with some crud the fuel stabilizer loosened up.

Sounds like dropping the fuel tank is unecessary, unless there's multiple misfires. (That in-tank fuel filter sure is a bitch)

Thanks, I'll try pulling the #4 injector connection while I'm working on it tomorrow.

I have triple checked for a cracked head and I'm not seeing anything that leads me to think that.

I haven't lost any coolant. The oil is clear and there isn't any brown sludge when I look inside the oil filler cap. Of course since it used to run like a top and suddenly went caput, that doesn't mean that it's not cracled. It just hasn't progressed far enough to see.

Well, I let it sit for a day so all of the oil would drain out of the top and I could see in clearly.

We have a winner.

Cracked head. No sludge, no crust where it's coming through but the top of the head looks green through oil hole in the vent cover and there is a visible crack now that oil isn't sitting over it. The oil on the dip stick still looks clear with no abnormal contaminats so hopefully it was catestrophic enough that I didn't drive around like that for a while.

I'll quickly spin it around in the garage tomorrow so I have some room to work.

I think I'll try that new 0331 casting from Alabama Cylinder Head. Now I just have to get some time to work on it.