Rear Main Seal Diagnosis


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. 



Revised 10-10-2018

4 thoughts on “Rear Main Seal Diagnosis”

  1. Took your advice and worked my way down. All the above items you mentioned were leaking along with the RMS. A couple of fel-pro gaskets and o-rings later she it leak free. The RMS replacement was a pain though, not as easy as those YouTube videos I watched!!!

    Thanks for all you do.

  2. Thanks for the update!! Glad you got it fixed.

    As for the RMS, now you know why I wrote this tip. Imagine how bummed a guy would be after all that work and it still leaked!!

  3. My 90 XJ at 190k was leaking out of everything. I slowly got all the oil leaks fixed. Both main seals, oil filter o-rings, valve cover gasket, pan gasket, trans pan gasket and thermostat housing. The trans seals were the only things not leaking. I learned you have to use thread seal on the thermostat housing bolts or they seep.

  4. My RMS leak symptoms were caused by a bad trans oil cooler hose connection….

    Thank you for this amazing source of information Cruiser! Every tip on your list proved very useful!

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