extrashaky

I have read the generic answers, but I'm hoping that someone who has had the exact same symptoms and fixed them will chime in with what worked for them.

2000 XJ, 4.6L stroker with approx 10K miles, aftermarket 0331 head, automatic transmission

Codes: P0300, P0301, P0304, P0305, P0306 (random misfires, which leads me to believe it's not a compression issue)

Symptoms: Jeep idles rough, accelerates smoothly with plenty of power, misfires at cruise with flashing CEL. The misfire happens any time I attempt to maintain a constant speed, whether it's 30 MPH or 70 MPH. The CEL stops flashing when I give it gas or let off the accelerator and coast.

I have already done most of the easy stuff.
Has anybody else had this exact same set of symptoms (specifically, misfire at cruise but smooth acceleration otherwise)? If so, what worked for you?

extrashaky

ufg8tr93

Did anyone ever help you with issue? Or were you able to resolve? Having same issue

extrashaky

No, but my symptoms have changed. Now it has settled on a consistent cylinder six misfire, P0306, still with the flashing CEL at cruise. I have a couple more things I'm going to do to try to diagnose it. After that, I'm going to take it to a shop and hope I don't get repeatedly raped on labor and parts as they throw things at it that also don't work.

cruiser54

Injectors?

extrashaky

Yes, it has some of those.

This weekend I did get around to pulling the fuel rail and swapping the #1 and #6 injectors. I cleared the code and took it out for a run. Got back with a light on. Same code, P0306, so I don't think the injector is the problem. I didn't think to put the noid light on it again this weekend, but I put it on there a few weeks ago and had steady flashing on all connectors as far as I could tell.

I also pulled the plugs again and tested the compression. Despite having at most 15 miles on it, the #6 plug came out half (almost) white, half dry carbon black, like a black and white cookie. That seems to jive with the way it has been behaving. Apparently that cylinder is running lean sometimes and very rich other times. The rest of the plugs looked fine. I swapped in another plug for the fouled one with no improvement.

Compression on #6 was 195 psi both times (stroker!). Cylinders 1-4 were all 190 to 200 psi. Strangely, cylinder #5 came in at 174 psi, which shot up to 240 psi with a squirt of oil in the cylinder. So apparently I might have an issue with the rings on #5. I can't see how that would cause a misfire on #6, though, so I'm going to wait until this misfire thing is figured out before I contact Chad Golen to see what he considers out of spec for this motor.

Just for giggles I tested the fuel pressure again just to be sure. It came in rock steady at 48.5 regardless of what I did with the throttle. That .5 is probably within the error for the gauge.

I would pull the valve cover, but to be honest unless something is shattered into pieces I doubt I'll know what I'm looking at.

Thoughts?

cruiser54

do you have a vacuum leak near #6?

extrashaky

I don't think so. I sprayed some carb cleaner around the area looking for a vacuum leak and got no response.

I'm out of town at the moment, but I can try again this weekend. Any place in particular I should look?

extrashaky

BTW, I didn't say above that the symptoms were sudden onset, not gradual. I was in the final hour of an 8 hour trip when it started. It had been running fine most of the way. Then I put my foot in it to get around some annoying traffic and noticed it running rough as I coasted up to the next red light.

Not long after that the CEL came on. Then it started flashing as soon as I tried to hold speed. I pulled over and had P0300, P0301 and P0306. I limped the remaining 30 miles home and had P0304 and P0305 when I pulled codes again in my driveway. After doing all this fiddling with it, it has finally settled on just P0306 with none of the other codes persisting.

I have looked for vacuum leaks with carb cleaner, but I haven't tested vacuum with a gauge. My gauge is packed up in a box somewhere. And I'm not 100% sure how to do it.

cruiser54

A vacuum gauge doesn't really help find vacuum leaks.

Pull the dizzy cap and have a peak in there.

extrashaky

It's a 2000.

cruiser54

That sucks.

extrashaky

Not until now. I like my coil rail.

cruiser54

Doesn't diagnosis seem to be a PIta though?

extrashaky

Yep. That's the thing about the coil rail motor. It works and works and works and works. Then one day it just doesn't.