Originally Posted by cruiser54

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, people on Jeep forums who can’t see your Jeep from where they’re at, 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. "$hit 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.

Revised 02-26-2013

Originally Posted by redrooster

Cruiser54 question: I have replaced the valve cover gasket, oil pressure sending unit (with a Mopar), and oil filter adapter seals. Not a drop from above. Even did the oil cap mod by adding a spacer underneath the existing seal. Not a drop on the top of the engine, and I just completed a 4200 mile road trip.

Now - I have a rear main seal leak. Bad enough that I had to add about 10 quarts of oil over that 4200 miles. I noticed more leaking when I drove for extended periods (8-14.5 hours). The underside of my XJ is a black mess (Brake Cleaner to the rescue !)

I have seen that the rear main seal comes in two parts: an upper and lower.
I have seen the YouTube stuff on replacing it. Some forum indicate that the OEM seal was/is a 1-piece seal which requires dropping either the engine or transmission. What I want to know is - is it a 1 or 2-piece seal? This is a 1988 Jeep Cherokee Laredo 4.0 Renix; 210k miles; I change the oil EVERY 3k miles with Mobile One Extended-Life; new (rebuilt) transmission 3 years ago. Hope Cruiser54 drops by here as his information has been so helpful to me over the years. Thanks all!

Originally Posted by SatiricalHen

Have you checked to make sure the vents on the valve cover aren't clogged? My Cherokee used to build crankcase pressure on long highway drives and push oil out of seals, replaced them and it hasn't leaked since.

Originally Posted by redrooster

Vents not clogged. As a matter of fact, I recently did Cruisre54's valve cover mod. Is it possible that the mod itself - which forces more oil to the rear of the engine (firewall side), away from the air filter box - has caused more of a leak in a perhaps, already leaking RMS?

Originally Posted by SatiricalHen

It shouldn't. Just seems like a lot of oil in a 4000 mile trip. My Comanche burns it, leaks it, and spits it but still doesn't use that much oil.

Originally Posted by SatiricalHen

Have you checked to make sure the vents on the valve cover aren't clogged? My Cherokee used to build crankcase pressure on long highway drives and push oil out of seals, replaced them and it hasn't leaked since.

Originally Posted by cruiser54

You beat me to it!!!!

And the seal is 2 piece. To lose that much oil, I would suspect a crankcase pressure issue.
Is your air filter oil soaked also?

Originally Posted by redrooster

I'll get back to you real soon . . . I'll go check it.

Originally Posted by redrooster

Checked it - nada. The air filter is clean; just a few stains that are the usual.
So - no excessive crankcase pressure might be assumed.

Originally Posted by SatiricalHen

Or the oil is stuck in the crankcase and the pressure can't vent through there.

Originally Posted by SatiricalHen

You said you did the vent modification, but the tubes could be clogged. Can you blow through the tubes?

Originally Posted by redrooster

OK- I want to understand this and correct me if I am wrong.
The hi pressure in the crankcase goes up into the valve cover and is vented.
Since I don't have excessive (or almost none) blowbye, and since I don't have a blown head gasket (which could cause higher pcrank pressure from the excess gasses), then the following may be happening:
1) plugged breather valve/hose-the front one that goes to the air box;
2) faulty PCV valve or plugged PCV hose (the one in the back).

Do I have this correct? So by checking that I can or can't blow through those 2 hoses may answer the "excessive pressure" question?
Thanks for all the help everyone.

Originally Posted by SatiricalHen

Basically yes. If the tube is clogged the pressure can't vent and it will build pressure.

Originally Posted by redrooster

I checked both tubes (breather/front and rear of valve cover) and both are clear and no excess oil in the air cleaner or box (disconnected them and put the ends in my mouth and BLEW = new experience).

Top of engine is clean as a whistle; checked back of valve cover using a mirror and no oil whatsoever. No burning or smoke, except for what gets on the exhaust cross-member.

I ordered a new drain plug and copper gasket(s). Maybe it got leaky somehow and it is too hard to separate the oil pan/plug drips from the rest of the mess under there. The existing drain plug has that black-center piece which I believe is an emergency pressure-release stop; maybe it's leaky.

Next: I am going to put 10-30W Mobile One Extended Life and Liqui Moly 2020 Motor Oil Saver (German product); with new oil filter of course. Then I am going to have the undercarriage steam-cleaned and pray.