P0300 and C121C Problem - '06 XK 3.7L

[FLYCAV]

Thanks cricket and Matt for the replies. In addition, I switched around my ASD relay with another relay in hopes that maybe an intermittant relay was causing the interuption in fuel, but it didn't work either.

Going out on a limb here but on the Liberty's and on the Liberty forums, the 3.7 V6 has a problem of breaking valve springs, weak valve springs, etc. on valve springs. One of your Intake valve springs could be broken or worn so bad that the Intake valve isn't closing properly and could be throwing the miss fire code.

Of course that's a big if. How are your coils on each cylinder? Could one of some of them be bad causing the miss fire?

I hope it's not something that serious. I'd think if it was a bad spring or a bad coil that it would throw a specific cylinder code rather than the general P0300 code, but I'm not sure how the computer actualy identifies a misfire. I can't imagine multiple coils going bad at the same time. Although lately I'd believe almost anything.

P0300 is not a good code to troubleshoot on electronically controlled ignitions. More often than not, its an ECU issue when the plugs are fouled like that (otherwise normally a fuel supply issue).

Did you put new plugs in?

Have you changed the PCV valve? That seems like a log of excess blow-by in the intake.

I did change the plugs but not the PVC valve. I did run out tonight and picked a new PVC valve from O'riely's, I'll swap that out in the morning (I like the cheap fix options).

As far as it being an ECU issue, would that be fixed by reloading the factory tune (I was running the Hypertch tune) and in this whole process have reprogrammed a few times both the factory tune and the Hypertech tune. -OR- would a dealer have to reflash it? Or is it possible that the ECU itself is bad and needs replaced?

Matt, when you say fuel supply issue, are you thinking fuel pump issue? or injector issue?

One other thing I saw somewhere was that carbon buildup on the exhast valves can prevent the valves from seating and sealing completly and create the problem. Compression testing could help identify this issue. You don't happen to know what the cold compression numbers should be on a 3.7 and how much variance between cylinders is within standards do you?

Thanks again for the help.

[Matt]

I would definitely load the stock tune before doing anymore troubleshooting.

Normally ECU issues that result in P0300 result in an new ECU, not just programming.

Fuel supply issues could be anything from pressure bleed-off to the pump going bad. Doubtful that its an injector issue because you have a better chance of getting struck by lighting while you're on your way to collect your mega millions lottery winning than you are to have multiple injectors go bad at the same time to cause P0300.

I'm not sure what the compression numbers are off the top of my head.... but that engine would have had to been neglected for a long time for carbon buildup on the valves to do this. Again, it would have to be multiple valves at the same time to foul all those plugs.

How's your oil level?

[FLYCAV]

How's your oil level?

Oil level is good.

......... because you have a better chance of getting struck by lighting while you're on your way to collect your mega millions lottery winning than you are to have multiple injectors go bad at the same time to cause P0300.

I just wish the odds of winning the Mega Millions jackpot were more like the odds of my Jeep throwing a P0300 code on the way to pick up a 12 pack.

Normally ECU issues that result in P0300 result in an new ECU, not just programming.

I was afraid of that.

Fuel supply issues could be anything from pressure bleed-off to the pump going bad. Doubtful that its an injector issue because you have a better chance of getting struck by lighting while you're on your way to collect your mega millions lottery winning than you are to have multiple injectors go bad at the same time to cause P0300.

I just might have to invest in that service manual afterall.

[Matt]

Use this
https://dl.dropboxusercontent.com/u/...2005-WK-SM.pdf

[thance]

Use this
https://dl.dropboxusercontent.com/u/...2005-WK-SM.pdf

Do you know of a similar link for a 2010xk?

[Matt]

No, but they're practically identical except for the body and accessories.

[FLYCAV]

Thanks for the link Matt. Thats' awesome.

Replaced the PVC valve this morning and reinstalled factory tune. Still throwing the P0300 code. I guess I'm just about out of cheap solutions.

...and to kick me while I'm down, last night while online researching the issue more I got the dreaded "FBI Cybercrime/MoneyPack" virus and locked me out of my computer. was up most of the night and spent half of today getting that off my computer. (fingers are crossed)