I've got a 2000 XJ8 Vandenplas, and I've seen that other people have this issue, I get the restricted performance indicator and the code is always P0306 Cylinder 6 missfire. I've had it to three different mechanics, they all suggest a tune up or something to do with injectors. So far we've swapped coils, swapped injectors, changed plugs, run a flow test on the injectors, cleaned the injectors, etc. I've also had every type of compression test known to man and they come out "good". Regardless of what's been done the code is always cylinder 6 missfire.

I believe it is missfire because the plugs foul and the car idles rough. It's not all the time though, it can be as often as a few times a day and as in frequent as weeks in between. Normally I drive longer distances, say 40 miles 1 way on the highway (ave speed 65 to 70). It seems like I have more trouble when I drive around town, short distances, lower speeds (ave 35).

Has anyone solved this? Any suggestions? Any help is appreciated. Thanks,

 

I would be looking for a vacuum leak, perhaps the intake O ring seal for that cylinder, or a crack in the intake plenum.

I'd also check the valve clearances to make certain that there was not an obvious sign of a less likely problem.

Absent of any answers, I would perform a cylinder leak down test with particular attention to pressure loss into the cooling system. A head gasket leak which introduces a small amount of coolant into #6 is one possible explanation for your recurring missfire.

 

One of the compression tests was a leak down, again, according to the mechanic, it tested good, no issue.

I'm inclined to think at this point it might be valve related but I haven't had a mechanic ambitious enough to tackle it, they always brush it off as not likley.

Thanks for the suggestions.

 

Third mechanic, third strike. All three have determined that the fuel system must just be dirty, don't ask me how, they never seem to be able to explain it to me, and despite saying they've swapped injectors I still always get the P0306 (cylinder 6) missfire. Mechanics 1 & 2 cleaned the injectors, mechanic 3 cleaned the fuel system. In all three cases the car ran great for 3 or 4 days then started having the problem again.

There's another symptom that goes hand in hand, the car becomes hard starting. I have to put the gas pedal all the way down to get it to start, during the "4 day" grace period the car starts fine, then it becomes a little harder to start, then it gives the engine code and becomes hard to start thereafter.

Should I just buy an injector and install it. I'm tired of paying money to be repeatedly told the same thing and get the same result. Mechanic 1 was in another state, Mechanic 2 gave up when the clean didn't work, I've got to contact mechanic 3 tomorrow.

I swapped the coil myself so I'm pretty sure that's not it. Mechanic 2 & 3 claim they checked compression, running compression, leak down and traced out the electrical system for signal and ground issues. They both also said they swapped injectors and narrowed it down to the injector.

At this point I'm feeling frusterated. Am I going to have to take this on myself? I already short on free time. Any suggestions?

 

Don't know what to tell you, if you're getting accurate information then valve leakage and injector operation have been ruled out. Still no info about a vacuum leak, get a can of carb cleaner and spray around the plenum and the injector seal with the engine runnung and listen for a change in RPM.

 

Sounds like a bad valve . Take it to a place that has a fiber optic camera with LCD display. . Takes 10 minutes to visually check the valves : if not do the leak Down test as fit has said . Both test are easy and don't take very long to do

 

Yeah, if compression is bullet-proof, injectors have been switched with no change, coil changed, spark is fine, then you have a mystery. If it is loading up and fouling that plug, then something is telling that injector to inject when it shouldn't be. This doesn't sound plausible. Or you've got a vacuum leak around the intake on that cylinder, which sounds like the only thing left. Try what JagtechOhio said with the carb cleaner.

 

I talked with Mechanic #3 today, he's convinced that it's the actual injector in cylinder 6, he said that after he got it to fail in a different cylinder (5) he put it back in 6. I don't know if that's normal practice or not.

I'm going to try the carb cleaner test you guys recommended and then swap the injector and leave it in a different cylinder to see if it fails. Supposedly Mechanic #2 did this before but all I ever got was cylincder 6. By doing it myself, I'll be able to see for myself.

I really hope it's just the injector. I'll keep you updated.

 

so after a couple weeks of screwing around (swapping injectors, changing the seals on injectors, putting in a couple new injectors, etc) the thing is running pretty well, still the intermittant hard start but I've only gotten the light and code twice in 15 days where it used to be daily.

Now, my question is, since banks 1 & 2 are both numbered 1-4, which one is actually cylinder 6? I've totally confused myself, sometimes I think it must be the second one in from the front, sometimes I think it's the second one in from the back. Maybe it's neither of those.

Can someone help me out. I know this is a silly question, but any help is appreciated. Thanks.

 

number six is 2nd from the front drivers side US

 

Hey Bill,

I tried to catch you on another thread last night... a guy is looking in Dallas for somebody to pressure clean his injectors. PM me.