P0300 and C121C Problem - '06 XK 3.7L

[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)

[Matt]

Do you have a scanner that you can see which cylinders are misfiring?

I'd also check the PCM connection. Disconnect it, check the contacts, use some electrical cleaner, and reconnect.

With the engine running spray some carb cleaner around the throttle body, valve covers, etc to see if there's a vacuum leak. You'll know if there is one because the engine will rev up when it sucks in the carb cleaner.

[FLYCAV]

Do you have a scanner that you can see which cylinders are misfiring?

I'd also check the PCM connection. Disconnect it, check the contacts, use some electrical cleaner, and reconnect.

With the engine running spray some carb cleaner around the throttle body, valve covers, etc to see if there's a vacuum leak. You'll know if there is one because the engine will rev up when it sucks in the carb cleaner.

All I have is the Hypertech unit which will just show the DTC's and it has only thrown the general P0300 code no codes identifying a specific cylinder.

I did disconnect the PCM plugs and cleaned them out with electrical cleaner and made sure they were secure again.

I didn't check for a vaccum leak but I'll try that in the morning.

If it's the PCM and I end up replacing, would I need to have a program loaded at a dealer or would it come ready to plug and play? I was actually Googling the part number for the PCM last night. The first search result I clicked on was what cause me to get the computer virus...That would be a little ironic if the PCM was responsible for taking down my Jeep and my computer.lol.

[Matt]

The PCM's not plug & play, it needs to be programmed to your vehicle.

[FLYCAV]

Went out this morning and did the vacuum leak check and there doesn't appear to be any leaks.

A buddy came over to see if he could be of any help. He's a Ford guy to the core but as long as he's not visible from the street he'll help work on a Jeep -LOL. We picked up the loaner fuel pressure tester and a compresion tester from the parts store and first checked the fuel pressure. The spec is 58+/-2 psi and the tester was indicating between 60-68 psi...not sure if it was really that high or if the loaner was just old and tired and not reading accurately. We started going through the service manual and thought maybe the MAP sensor was bad causing it to send bad vacuum data tot he PCM so we swapped it out with a new one. Didn't fix the problem. When we were installing the new MAP sensor I pulled out the dipstick to put a little of the oil on the O-ring and noticed what appeared to be water or coolant on the dipstick. I opened the oil filler cap and removed the insert and there was a lot of moisture foam residue lining the fill tube. I can't believe I hadn't even check that prior to this. Here are a couuple pics.





On the list for tomorrow is to do a Cylinder-to-Water-Jacket test and a Cylinder compression test. I'm assuming at this point that there is either a leaking head gasket or a cracked cylinder head or block.

[Matt]

Oh man

Keep us updated.

[lekmedm]

D'oh!




Out of your list of suspects, I hope it's the head gasket!

Good luck.

[stites-xk]

Good luck man! Hope its not to bad!

Sent from my DROID RAZR using Tapatalk 2

[1USMC]

Good luck man! Hope its not to bad

Out of your list of suspects, I hope it's the head gasket!

^^^^^^^^ ..Ditto.. ^^^^^^^

[criket]

Yikes!!!