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

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bluwarguy

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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:
xqNjTYt.jpg

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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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:
xqNjTYt.jpg

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. :roflsquared:
 
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bluwarguy

bluwarguy

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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. :roflsquared:

mine was always around 45mph and above

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

dodge dude94

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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.
 

Gr8bawana

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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.
 

dudeman2009

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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.

If it did, you have worse problems than a miss at certain speeds :roflsquared:
 

Yeret

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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.
 

Shadow_Death

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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.
 

Johnny onnit

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Could this leak through the valve covers and into the plentum gasket fool u into thinking it was the valve stem seals.
 

Johnny onnit

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I was just thinking becouse im getting ready to do the valve stem seals on my 95. The 98 started showing oil at start up but has other issues , likes to die. Idle drops to low and dies. The only things left are the cam sensor and intake. If you saying oil can enter through the plenum gasket and fowl a plug could it also couse the smoke at startup
 

dapepper9

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That is possible yes. But that oil can't be coming from the valve cover like you initially asked. Valve cover leaks are strictly external whereas the intake is an internal leak. The oil comes from the lifter valley
 

Shadow_Death

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The only way I could see this being a valve cover leak is if it's sucking air in via leaky valve covers into the intake if you have the stock air filter box. The driver side breather goes to the intake housing. My engine always seems to suck oil from there and into the air filter... But your filter would have to be damn near gone for it to suck this much oil.
 
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dapepper9

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Exhaust won't have anything to do with it. The only way leaky valve covers end up in the intake is if they leak through the intake gaskets as well and into the intake port.

Bottom line, forget the valve cover seals for the misfire
 

Shadow_Death

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Exhaust won't have anything to do with it. The only way leaky valve covers end up in the intake is if they leak through the intake gaskets as well and into the intake port.

Bottom line, forget the valve cover seals for the misfire


Holy crap! I have no clue why I said exhaust. I need to fix that. Lol!

But yeah, that was my point. It won't suck enough oil through the intake to cause the issue he's having. I think he needs to just bite the bullet and look for leaks in the plenum.
 
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