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Old 06-06-2011, 07:33 AM   #1
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Elusive oil leak.

So I've been chasing an oil leak for about two months now. I'm nearing the end of my rope and just accepting that I'll have to add a little every now and then but I thought I'd post here first and see if anyone else has run into a situation similar to me.

I've got a 99 TJ with about 144000 miles on it. 4.0l and the 3-spd auto transmission. It has slowly leaked oil for about a year now and I decided I was going to fix it. There was oil all over the oil pan and bell housing. Pretty much the entire underside of the car had a layer of oil and dirt sludge. In doing research here and on a few other forums I saw that rear main seals were big culprits here as well as oil pain gaskets.

That being the case I went ahead and replaced them both. This was probably my biggest repair job done at home to date but I finally finished it up only to see that there is still a slow oil leak on the bell housing.

After cussing for a bit and cooling back off I did a bit more research and found that the valve cover gasket is also a common cause of leaks here. I went ahead and replaced this as well. The leak appears to have slowed, but there is still a small trickle of oil that shows up on the bell housing after driving the jeep. It won't leak if it hasn't been running, only if I drive it.

I'm not a mechanic so I can't really rule out that I made a mistake at some point in this process but everything seemed to go back together just fine, new gaskets and all. Any suggestions on what else could cause this sort of behavior? Should I take it in to the shop or just be content to keep an eye on it and add oil every now and then?

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Old 06-06-2011, 09:00 AM   #2
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Gotta start with a clean slate so to speak, when dealing with oil leaks.
Degrease the bugger so it's spotless. Now you can start tracing where the oil is coming from.

Oil is as sneaky as water when looking for leaks. Where you see it, a lot of the times is not where it's leaking.

Valve cover is the#1 leaker, followed by the timing cover/rear main/oil pan.

Get yourself a mirror and a flash light after you clean her up.

Take a look at the back of the valve cover. Way back by the bell housing.

I've seen it before. Previous attempts at tightening the bolts to try and stop a leak, can bend/deform the flange around the bolt holes.
You put a new gasket on, and those seals don't seat tight anymore. And bingo you have a leak still. It will follow the joint between the block and bell housing, and right to the back of the oil pan/rear seal area. And you think the pan/seal is leaking.

That valve cover has been a bane to jeeps since day one.

I did ours about a month ago, same thing, looked like the pan/rear main was leaking. Was so bad the skid plate, trans/t case back were soaked with oil.
De-greased it and started looking.

Nope, it was the valve cover. Pulled it off, laid a straight edge on it to check the flanges for square, checked everyone of the bolt holes and hammered em flat.
Then a thin coat of sealer to the valve cover and block, installed the gasket and seals, and it's bone dry top and bottom.

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Old 06-06-2011, 10:51 AM   #3
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Thanks Shelby that's good advice. So I may need to take the valve cover off again anyhow. I'll get a mirror and some degreaser and get the underside cleaned up and check again.
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Old 06-06-2011, 12:12 PM   #4
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Let us know what you find.
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God I miss that Jeep. Forgot the kinda Jeep, 80 SX4.
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Old 06-06-2011, 08:11 PM   #5
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On the front of the valve cover you'll find a plastic fitting with a hose that goes to the airbox. At the rear there's another one that goes to manifold vacuum. That's the CCV system.
If one or both of those hoses gets plugged, it can pressurize the crankcase, forcing oil out of many places.
At rest and at idle it usually doesn't leak, not enough pressure - but driving it there's more crankcase blowby pressure, forcing the oil out.
Remove the hoses, try blowing through them to make sure they are clear.

I just finished having the same problem with a Cherokee I just bought. Gum and crud builds up in the vents, mainly from doing extended oil changes. The PO bragged that he always used synthetic oil and changed it like the advertising hype said - seldom.
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Old 06-08-2011, 01:22 PM   #6
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I'll check the CCV hoses as well. I've been pretty diligent about changing my oil every 3,000 miles but you never can be too careful. I'm still trying to pinpoint the exact location of the leak, but provided i take off the valve cover and put some RTV down to make a better seal, will I need to replace the gasket again or should I be able to use the one that is on there? Also i noticed that my gasket did not come with replacement grommets, could that potentially be problematic as well?
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Old 06-08-2011, 02:36 PM   #7
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Strange it didn't come with the baggie of seals. The rubber gasket is supposedly reusable.
Yeah, clean up everything real well. Then apply sealer to both sides, valve cover and head. Just a thin coat, don't glob it on. It will help fill in any little imperfections that may be causing you grief.


Funny rrich was talking about extended oil changes. Had a guy come in the shop last week, bragging about this new oil he was using. 25000, yep twenty five thousand miles, between oil changes. Umm that's all well and good, but me I like my every 3000 mile changes. And 25000 miles, I can just imagine what the filter will look like inside at that mileage.

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