HELP!!!! Strange Misfire I can't fix.
 

Okay a few months ago I bought an 01 XJ after totaling my Rubi. It was a mid build buy. So far I'm fixing all the OEM things getting her running right before I make improvements. So I've read a few hundred threads and haven't found an issue like mine. I'm pulling a #6 misfire. But, here's where I'm pulling my hair out. It only misfires when I decelerate quickly. I pull over turn it off turn it on runs smooth again. If I start it and let it idle it will idle smooth as glass all day until I drive it then it roughs up and the misfire starts. If I turn it off and on smooth again. I have replaced all the boots on the coil pack I put in iridium plugs and swapped plugs from other cylinders to see if I got a bad one but still does the same thing. I checked the cylinder pressure and it's damn near perfect. I swapped in a known good coil pack from a friends jeep and same thing. I have sprayed damn near a gallon of starter fluid at ever vacuum line connection I could find and not even a hiccup of a change. I also swapped injectors tosee if that was it and same thing only good ole 6. The engine is brand new the intake and exhaust is brand new and they have less than 15K on them. That was the mid build part. I am stuck I like to think I'm not an idiot but this XJ is proving me wrong. Please help me.

It's always #6 and only #6?

"I checked the cylinder pressure" :popcorn:

Bad intake manifold gasket? Or bolts not torqued?

The code is for vacuum leak and #6 misfire. Also when the plug is pulled there's fuel on it.

Why popcorn DFlintstone? I'm not a master mechanic but I figured checking compression seemed like a logical step.

I was just marking the thread so I'd have that little arrow on it. Never heard it called "cylinder pressure", and normally the compression # is most important relative to the others.

I'm no help, except to mention Radi is pretty sharp, and myself I might try regular plugs...(for lack of a better idea)

Just left the shop and it wasn't much help. There were no irregular HC readings in the coolant and no chocolate milk in any oil. So head gasket seems fine. After talking with the tech in the shop he suggested the cam or the ECU. At this point I'm lost. I'm thinking of taking it to FNJeep or Colorado Jeep CO, I thought about 4 Wheel parts but I've heard mixed reviews about them. Anyone know of a good Jeep shop in or around Colorado Springs.

You mentioned a build...what of the engine is stock and what is not?

#6 plug is coming out wet with gas? And exactly which codes are you getting? Do you have the actual numbers?

You can actually call it a Powertrain Conrol Module. (ECU 87-90).

I'm no 'lecrics teck. I HAVE successfully repaired 1/2 dozen or so gadgets because often you can see when a "cap", blows. That has a bunch of them. >> https://www.google.com/search?biw=10...22.4NSRBN-qWhg

DFlintstone is keeping me on my toes with my terminology. No lazy posts round these here parts with slang or whatnot.

Yes it was mid build, the motor is a stroked crate from what I was told. I haven't been able to tear it down or test it enough to figure out sizes of injectors or things like that. So as far as whats stock or not I'm still building a parts reference guide. I've pulled part numbers where I can but it's really a mystery in areas. I got such a good deal I couldn't turn it away but it was and is a mess.

So far I replaced all fluids
flushed the entire cooling system
replaced both pinion seals
new plugs
new rear main seal (it had a small leak because the top half was backwards)
new thermo, housing, fan clutch, water pump
new belt
new front rotors and pads (didn't do the back yet because I'm switching it to discs)
fixed the blend door and dash foam
fliped the tie rod
oil pan and trans pan seals

So now I'm stuck on this damned misfire. The plug does come out wet when I get on it brake hard into the garage for testing. Normal driving it's not wet. Only when I was intentionaly causing it. I'll get the code when I get home though and post.

I haven't said it before but thanks for the help fellas. This miss is testing me.

Can you find a ECU to swap, just to try. It rule it out or fix it.

I also was chasing down a misfire on #6. Mine turned out to be a burnt valve.
However something I came across while doing my search was a weak valve spring problem. It has a TSB (Technical service bulletin) on just this type of problem. In this rebuild did they reuse the original valve springs?


I bought 3 new springs for $16 and a valve spring tool for $25. You can change them while the head is on the block. Just need air pressure in the cylinder. Cheap way to trouble shoot this.

I do have a spare ecu I pulled from the junk yard. I planned on ahving checked to see if it's any good before I plug it in though. I'll be wrenching all weekend trying to get it right though. I'll update this as I get or find anything else.

Have you done a compression check?

In rare cases the problem is the valves/valves seats causing low compression.

Happened to my 01 XJ. Rebuilt the head and the problem never came back.