Dreaded #7 Misfire

[AzDragonLord]

After 137000 miles, I got luck and had no AFM issues until now as my 2009 Sierra is throwing a misfire code for cylinder #7 (P0307). I swapped the coil/wire/plug with cylinder #3 and the fault followed, so I swapped the coil back to it's original #7 spot (plug wire got broke, so it's new) and the fault stayed with #3. This leaves just the plug, which is visually fouled.

My question is this... Assuming the fouling is AFM related, will disabling AFM now help with future fouling or is it too late?

[DaveH71]

At 137,000 miles you should be putting in new plugs and wires anyway. There is a decarbon procedure you can do to the cylinders to remove the fouling. This is straight from GM and i also do it at my shop fairly often.

 

Pick up some GM upper engine and fuel injector cleaner from amazon or wherever you choose, you will need 2 16oz containers. Pour 3-5oz of the cleaner directly into the spark plug hole of every cylinder and let it sit for 2-3 hours. Then replace the plugs and start the vehicle.

 

It will smoke A LOT so don't do this inside or near anything you don't want to stink.

 

Next if it where my vehicle i would disable the AFM, either by tune or by a RANGE AFM disabler. This will keep the carbon from coming back.

 

I also change the oil after i do this on customer vehicles just for piece of mind.

[Dnt]

Probably just burning a bit of oil. If this particular engine never had the valve cover changed to the new baffled style, that can contribute to the problem on the #1 3 5 and 7 bank. That is the side the PCV valve it tied into. Does the plug look like this one?

 

2009 #3 plug at around 85,000 miles

[AzDragonLord]

All plugs and wires were replaced around the 100,000 mile mark, and the plugs were looking good then.

 

Am I correct in believing the fouling is a combination of AFM with the PCV recirc into the left intake causing the unburned oil in cylinders 1 and 7?

[AzDragonLord]

 

Probably just burning a bit of oil. If this particular engine never had the valve cover changed to the new baffled style, that can contribute to the problem on the #1 3 5 and 7 bank. That is the side the PCV valve it tied into. Does the plug look like this one?

 

 

2009 #3 plug at around 85,000 miles

 

Looked a lot like that, yes.

[Dnt]

No way to diagnose from my armchair here but I can hazard a couple of guesses because that is a lot of fun LOL.

Double check that high quality Irridium plugs were actually used when the plugs were changed out at 100k and also make 100% sure the updated version of the valve cover is installed on the left bank.

Here is my current theory (guess): Best Current Guess #1 Irridium plugs ceramic can crack with the slightest impact (socket sideways etc) the ceramic may have had a hairline crack on just #7 when it was installed? Best Current Guess #2 Wrong plugs were installed at 100k and this is just the first one to foul out others will soon follow. Best Current Guess #3. Left bank valve cover was never swapped out for the new design.

 

Might be good to go ahead and do a compression test on all cylinders. That way we will have more information for formulating additional "theories" if needed.

[AzDragonLord]

I used ACDelco when I replaced the plugs and wires. My truck was at the dealers a few years back for warranty work, but I don't know if they touched the valve cover.

[AzDragonLord]

Thanks for all y'alls input. I tried cleaning the plug, but it still threw codes. I replaced it and all is right again. After 100k+ miles I had no issues with AFM, but I think I'll be disabling it now.