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P0308 Misfire? Could've been a valve cover oil leak.


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So, ever since december my truck began to misfire. Specifically the 8th cylinder. Looking at a bunch of things online, ...


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Old 02-15-2017, 10:51 AM   #1 (permalink)
 
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Default P0308 Misfire? Could've been a valve cover oil leak.
So, ever since december my truck began to misfire. Specifically the 8th cylinder. Looking at a bunch of things online, I arrived to the conclusion that it must be the plenum. Boy was I wrong.

While I was saving up money to get a plenum repair kit, I decided it was about time to change my spark plugs and finally fix the valve cover gasket leaks that were going on. Well, upon removing the 8th cylinder spark plug, this is what I found:


It's entirely caked in oil. No wonder it was misfiring!

So, after replacing it with Champions, it ran just like the day I bought it.
The moral of the story is to always keep your valve covers from leaking.


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Old 02-15-2017, 11:20 AM   #2 (permalink)
 
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Originally Posted by bluwarguy View Post
So, ever since december my truck began to misfire. Specifically the 8th cylinder. Looking at a bunch of things online, I arrived to the conclusion that it must be the plenum. Boy was I wrong.

While I was saving up money to get a plenum repair kit, I decided it was about time to change my spark plugs and finally fix the valve cover gasket leaks that were going on. Well, upon removing the 8th cylinder spark plug, this is what I found:


It's entirely caked in oil. No wonder it was misfiring!

So, after replacing it with Champions, it ran just like the day I bought it.
The moral of the story is to always keep your valve covers from leaking.
This has got me thinking that this might be the cause of my misfire on cylinder 6 on my 3.9L. I always get it if I push the truck about 70-75mph.


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Old 02-15-2017, 02:22 PM   #3 (permalink)
 
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Originally Posted by MCIntoDarkness View Post
This has got me thinking that this might be the cause of my misfire on cylinder 6 on my 3.9L. I always get it if I push the truck about 70-75mph.
mine was always around 45mph and above

turning overdrive off helped it stop, but I figured out the permanent solution thank god


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Old 02-15-2017, 11:32 PM   #4 (permalink)
 
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A leaky valve cover won't cause that fouling, that's a bad plug or excessive oil dump from the valves.
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Old 02-16-2017, 04:05 PM   #5 (permalink)
 
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^^^valve cover leak can't get into the cylinders
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Old 02-16-2017, 04:23 PM   #6 (permalink)
 
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If it was a valve cover leak there would be nasty oil residue all over the outside of the plug. Oil can't seep into the cylinder.
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Old 02-17-2017, 01:43 AM   #7 (permalink)
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Originally Posted by Gr8bawana View Post
If it was a valve cover leak there would be nasty oil residue all over the outside of the plug. Oil can't seep into the cylinder.
If it did, you have worse problems than a miss at certain speeds


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Old 02-18-2017, 02:35 PM   #8 (permalink)
 
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Wow, that is probably the worst fouling that I've ever seen on a spark plug. Even when my plenum was leaking, my #8 plug wouldn't foul that bad.

Pepper's right. Leaky valve covers at worst will coat the outside of the spark plug, but a leak there won't find it's way into the cylinders unless maybe the intake manifold gaskets are also toast.

If you're using oil, knowing that it's a Magnum engine, I'd plan on doing the plenum repair. If money's tight, people have had success with simply using a revised Fel-Pro plenum gasket for 20 odd bucks rather than shelling out the 160 for the kit.


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Old 02-22-2017, 12:48 AM   #9 (permalink)
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Plenum plate would be my guess as well. I recently bought a 96 off my niece which I know needs the fix. No vacuum, misfire on #8, oil consumption and leak down the back of the block, as well as if I let it idle long enough the miss gets slightly worse.
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