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Unread 09-17-2009, 02:33 PM   #1
Yarz
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4.2 leaking oil out of oil fill cap?

My 4.2 is leaking oil out of the oil fill capÖ It only does it when driving, Iíve never had any indication of this at idle.

The first thing I tried was a new filler cap thinking the seal may be bad. That didnít help.

I found searching online that in many cases that it is the pcv system. So I looked at mine, and I found and fixed a large leak in the system. Now, I can feel a lot of suction on from the pcv valve when I put my finger over it with the engine running. But, thereís still oil coming out of the fill cap.

I know that there has to be extra pressure in the engine for some reason that would cause it to force oil out, but I donít know what is causing it.

Anyone have any ideas?
Thanks in advance,

Yarz

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Unread 09-17-2009, 04:49 PM   #2
buckshot500
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Pressure from combustion is getting past the piston rings. Mine does it too, but I have a little red/chrome breather that gets oily. Change your oil often if it's getting dirty quick. Mine isn't bad enough yet to go in & put new rings in there.
Pretty major teardown involved.
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Quote:
Originally Posted by 5point1 View Post
Well after I drained the oil, I ran water through the engine to clean out the gunk. (the guy who owned before me obvioulsy didn't know how to maintain the engine).

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Unread 09-17-2009, 04:52 PM   #3
Yarz
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Does it usually happen quickly though? Because I had not noticed any problems, and then all of a sudden, one day there was a puddle under the jeep, which I traced back to the filler cap.
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Unread 09-17-2009, 04:55 PM   #4
Yarz
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And also, how can there still be enough pressure to force the oil out of the cap when the PCV system is sucking the pressure out, and the ccv (i think that's what it's called - the one on the back of the valve cover) is basically a breather hole that would let out the pressure?
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Unread 09-17-2009, 04:57 PM   #5
buckshot500
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Quote:
Originally Posted by Yarz View Post
Does it usually happen quickly though? Because I had not noticed any problems, and then all of a sudden, one day there was a puddle under the jeep, which I traced back to the filler cap.
Mine has been doing it for a few years now & hasn't got any worse.

You might have something else going on, which prolly is gonna be easier & cheaper than tearing down a motor.
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Quote:
Originally Posted by 5point1 View Post
Well after I drained the oil, I ran water through the engine to clean out the gunk. (the guy who owned before me obvioulsy didn't know how to maintain the engine).

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Unread 09-17-2009, 04:59 PM   #6
buckshot500
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Quote:
Originally Posted by Yarz View Post
And also, how can there still be enough pressure to force the oil out of the cap when the PCV system is sucking the pressure out, and the ccv (i think that's what it's called - the one on the back of the valve cover) is basically a breather hole that would let out the pressure?
Yes the breather should let it out. Maybe your breather has a one way device (flapper) in it. Also the 4.2l oil fill caps aren't very pressure-tight.
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Quote:
Originally Posted by 5point1 View Post
Well after I drained the oil, I ran water through the engine to clean out the gunk. (the guy who owned before me obvioulsy didn't know how to maintain the engine).

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Unread 09-17-2009, 05:36 PM   #7
Yarz
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Quote:
Originally Posted by buckshot500 View Post
You might have something else going on, which prolly is gonna be easier & cheaper than tearing down a motor.
That's what I'm hoping... Especially since it wasn't a gradual thing like I'd think the rings going bad would be. Plus, it's only got about 95000 miles on it. They shouldn't be going bad already, right?

Quote:
Originally Posted by buckshot500 View Post
Yes the breather should let it out. Maybe your breather has a one way device (flapper) in it. .
I'll have to double check it, but I don't think it does. I'm pretty sure that it's just a straight tube.

Quote:
Originally Posted by buckshot500 View Post
Also the 4.2l oil fill caps aren't very pressure-tight.
Mine never has been very tight, as you say is common, but it just started letting oil out within the last few weeks.
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Last edited by Yarz; 09-17-2009 at 05:56 PM..
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Unread 09-17-2009, 07:59 PM   #8
yellow2kgt
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Mine does the same thing. If your PVC system is in order and the gasket on the cap is good then most likely it is worn rings letting combustion gases pressurize the crankcase. Just the nature of an older worn engine. However, with the less then 100k your engine has(mine is pushing 170k) it hopefully is something simple. I would double check your breather hose as mentioned. I know on my YJ the new oil fill cap I picked up was still loose. I ended up making a shim(doubling the oil fill gasket) from some high density rubber sheet my buddy gave me.
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Unread 09-18-2009, 07:09 PM   #9
buckshot500
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Quote:
Originally Posted by Yarz View Post
That's what I'm hoping... Especially since it wasn't a gradual thing like I'd think the rings going bad would be. Plus, it's only got about 95000 miles on it. They shouldn't be going bad already, right?
This could depend on things like whether the engine was allowed to warm up before hitting the road.
Raw fuel that was not properly atomized, can wash the protective oil coating from the cylinder walls causing premature wear of the rings.

This is why I always recommend leaving the intake manifold's electric heater intact.



Quote:
Originally Posted by Yarz View Post
Mine never has been very tight, as you say is common, but it just started letting oil out within the last few weeks.
Rings wouldn't likely cause sudden leakage from the cap like that. Must be that the PCV system isn't working right. Maybe check the vacuum lines for blockage.
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Quote:
Originally Posted by 5point1 View Post
Well after I drained the oil, I ran water through the engine to clean out the gunk. (the guy who owned before me obvioulsy didn't know how to maintain the engine).

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Unread 09-19-2009, 08:51 AM   #10
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Quote:
Originally Posted by buckshot500 View Post
This is why I always recommend leaving the intake manifold's electric heater intact..
As far as I'm aware, it still is intact. The only thing I've removed is the two pulse air tubes going to the exhaust because they had both rusted through anyhow.



Quote:
Originally Posted by buckshot500 View Post
Rings wouldn't likely cause sudden leakage from the cap like that. Must be that the PCV system isn't working right. Maybe check the vacuum lines for blockage.
I am continuously looking at them, but a little at a time, so it's going very slowly..

I'm probably going to do a compression check this week (if I can get to it) just to be sure.
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Unread 09-19-2009, 09:42 AM   #11
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just clean up the area that the oil is coming from, take off the pcv hose/valve leaving that part of the valve cover, and take it for a short hard ride. if oil still comes out- its blowby.

if no oil comes out it might just be your pcv lines.

take your pcv valve out and shake it. it should rattle. wipe off the hose that connects to the pcv valve and blow in it. air should flow easily.

those are just some free and easy methods of checking it
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Unread 09-19-2009, 11:04 AM   #12
doherty80
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i had almost the exact smae problem and symtoms and my piston ring were beyond toast. hence my swap i'm doing right now
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Unread 09-26-2009, 09:19 AM   #13
Yarz
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Quote:
Originally Posted by yellow2kgt View Post
I would double check your breather hose as mentioned.
I double checked both the breather in the back and the PCV in the front. The breather is clear, and the PCV is sucking.

Quote:
Originally Posted by yellow2kgt View Post
I ended up making a shim(doubling the oil fill gasket) from some high density rubber sheet my buddy gave me.
I shimmed my new fill cap too, but I used a small piece of aluminum flashing my dad had laying around.. It still lets the oil out.

Quote:
Originally Posted by speepdaedeesi View Post
just clean up the area that the oil is coming from, take off the pcv hose/valve leaving that part of the valve cover, and take it for a short hard ride. if oil still comes out- its blowby.
I did this, and I think it still came out. It was a number of days ago, so I can't remember clearly. What I don't understand is how, if it is blowby, is there enough pressure to force the oil out, when there is basically an opening in the back of the valve cover and the PCV sucking out the air? I'm thinking that the combination of these would eliminate any built up pressure?

Quote:
Originally Posted by speepdaedeesi View Post
take your pcv valve out and shake it. it should rattle. wipe off the hose that connects to the pcv valve and blow in it. air should flow easily.
It rattles freely. And when blowing in it, the air flows well. I even replaced the hoses to be sure.
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1990 White YJ TBI350, SM465, NP231, SOA, RE 1.5" SOA springs, Ford 8.8, D30, 4.10's, JBConversions SYE, XJ shaft, Bilstein 5150s, jb4x4 steering, MORE SS brakelines, 35x12.50 15 MT's, '95 YJ Brake Booster, '78 Grand Marquis MC, ZJ Prop. Valve
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Unread 09-26-2009, 09:22 AM   #14
Yarz
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Quote:
Originally Posted by doherty80 View Post
i had almost the exact same problem and symtoms and my piston ring were beyond toast. hence my swap i'm doing right now
But how many miles did your engine have? I've got under 100,000. That's really why I'm trying to look at all of these other options first. In addition, it started doing this all of a sudden. Is that how yours happened? Can rings go bad all of a sudden? I thought it would be more of a gradual thing..
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