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Temperamental rear main seal leak?

paulhead

I overtighten $#!t
Hour and half drive to the trail, 4.0 got a little hot but not terrible, stopped to get gas at trail head and oil's dripping from what looks like the rear main seal. Looked underneath and it sprayed pretty good. I checked oil filter, behind the valve cover, every well else and came to the conclusion it was the rear main seal. Stopped dripping, stick sitting at half a quart I decide to take it wheeling any way because it was a short trail and I wasn't alone and had oil. End of trail, no leaking, didn't lose anymore oil. Drove it home another hour and half no sign of leakage. Ideas?
 
CRUISER'S MOSTLY RENIX TIPS
REAR MAIN SEAL DIAGNOSIS
OCTOBER 31, 2015 SALAD 3 COMMENTS


I’d be looking up ABOVE first, and VERIFYING the source of the oil leak YOURSELF.

Everybody, who doesn’t own or have to pay for or perform your vehicle repairs, loves to poke their noggin UNDER the Jeep and come out bearing the false bad news that your RMS is leaking. Many mechanics, friends, and good old Uncle Bob seem to enjoy telling you it’s the rear main seal. Has a catastrophic ring to it, doesn’t it?

A simple leak at the back of the valve cover or other source could produce the same symptoms. You don’t need to be a mechanic to figure this out. If you have good eyesight and a dim flashlight, you’re good to go on your own. Don’t jump on the RMS/oil pan gasket bandwagon right off the bat.

Almost any oil leak on your 4.0 is gonna drip from the RMS area for two simple reasons:

First off, the engine sits nose-up and any oil will run back to the RMS area.

Secondly, the RMS area is also the lowest point on the engine. Simple physics and the old plumber’s adage apply here: “Crap flows downhill”.

Valve cover gasket, oil pressure sending unit, oil filter adapter seals and distributor gasket, in that order, have to be eliminated as possibilities first. A little tip here. Rather than use a dizzy gasket, use an o ring instead. NAPA #727-2024. Tips 12 and 13 will help you get your distributor back in place correctly.
 
If you have too tape a piece of paper towel on a coathanger and rub it behind the back of the valve cover.
 
I think I did a pretty good job reaching behind the valve cover/block and not finding wet oil. Oil sending unit looks dry too along with the oil filter. Could a clogged PCV cause pressure build up to squeeze it between surfaces? Also I did check the oil pan bolts and everything was tight. (edit) :) I guess it can
 
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The small vent line at the rear of valve cover running to the intake helps vent the engine and can get plugged, there is no PVC valve on an XJ that I now of.

I have had a front seal leak and the oil travels down the top of the pan rim all the way to the back of the motor (my guess is the airflow helps with this). Stick your finger on top of the pan rim and see if they feel wet with oil.

Side note it is maybe too late now but be careful how tight you get the pan, overtightening and old gasket can split the gasket and cause leaks. The best way I've found is to squirt out the pan bolt holes with brake cleaner, let it dry or use a small piece of paper towel to dry the hole and then put a tiny drop of Loctite on the thread and snug it and note really tighten it. I've got a 1/4" drive torque wrench I use or in a pinch, I choke up on the handle of a 1/4" ratchet to tighten. One of those jobs you have to do with feeling and not strength.

It may be you have slow seep at the rear seal and some oil collected in the bell housing, it can happen sometimes. There is a casting ridge along the bottom of the bell housing that makes a little bowl that collects oil. You park on a downhill slant or drive down a steep slope and the oil sloshes out. It may look like a bigger deal than it is.
 
The small vent line at the rear of valve cover running to the intake helps vent the engine and can get plugged, there is no PVC valve on an XJ that I now of.

I have had a front seal leak and the oil travels down the top of the pan rim all the way to the back of the motor (my guess is the airflow helps with this). Stick your finger on top of the pan rim and see if they feel wet with oil.

Side note it is maybe too late now but be careful how tight you get the pan, overtightening and old gasket can split the gasket and cause leaks. The best way I've found is to squirt out the pan bolt holes with brake cleaner, let it dry or use a small piece of paper towel to dry the hole and then put a tiny drop of Loctite on the thread and snug it and note really tighten it. I've got a 1/4" drive torque wrench I use or in a pinch, I choke up on the handle of a 1/4" ratchet to tighten. One of those jobs you have to do with feeling and not strength.

It may be you have slow seep at the rear seal and some oil collected in the bell housing, it can happen sometimes. There is a casting ridge along the bottom of the bell housing that makes a little bowl that collects oil. You park on a downhill slant or drive down a steep slope and the oil sloshes out. It may look like a bigger deal than it is.
Right on. Yeah I changed both elbows and grommets to include the little one in the back, front wasn't a PVC, you're right, old brittle rubber/plastic that needed it anyway, blew thru the skinny line to make sure it wasn't clogged. I got a long drive today again, I'll keep an eye out on the spots you recommended, thanks.
 
I won't tolerate any leaks so my garage floor is Aircraft hanger epoxy, if anything drips I know immediately. The rear main is not a big deal to fix, just plan for a couple of days.
 
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