This new 4.0L I swapped into our ‘02 XLT is giving me some rear main issues. I obviously chose to replace the seal prior to making the swap. I did everything by the book, too. Removed old seal, cleaned the gunk out of opening really well, then lightly coated the new Fel-Pro Seal (w/ metal sleeve) and bore opening w/ clean oil. Seated the seal using the OTC installation tool, and everything went great. Yet, upon finishing the swap, it still leaks. Only drips after the engine is warmed and only once the engine is off. I wind up with a puddle about the width of a tennis ball - give or take. The crankshaft didn’t have any noticeable nicks or scratches. I suppose it could be coming from the lower sprocket bolt on the rear timing cassette. But I put a fresh O-Ring on that, too, and torqued to spec. Leak is definitely originating from within the bellhousing, though; I’ve triple checked the valve covers, PCV, and intake manifold.

Does anyone have any thoughts on the Blue-Devil Rear Main Seal additive? I’m assuming it’s meant more for old seals that need to be rejuvenated, versus a brand new one. I reeeeeaaaaaallllllllllly don’t want to pull engine or trans again, but also hate having to constantly worry about oil loss and despise oil stains on my driveway!

 

Rear Main would only leak when engine has oil pressure, so when running, and it leaks A LOT of oil
And if it was sleeved then no nicks

The rear main bearing for the crank shaft is right next to the rear main seal so the seal has to block all the oil coming out of the back side of that bearing with oil pressure of 15-50psi flowing thru it

There is a Jack shaft seal which I think you changed, looks like a freeze plug, unlikely to leak but could leak and would not be a big leak

Could it be trans fluid, auto or manual both use ATF

Sure on the "stop leak" stuff, can't hurt, you can add a UV Dye to the engine oil just to make sure its engine oil that is leaking out, also pin points the leak if you ever pull the trans to find it

 

I'm not sure about the Rangers, but I have had a few Toyota do this exact thing. I found two fixes with these. What was causing it was the rear main shaft had a small wear groove in it, which caused the new seal to leak. I backed the seal out so it had a new sealing point and that worked on one of the 3. The other two, believe this or not, I put two seals in the plate. The first one backwards and the outer one in normal. Sealed right up. Hate to recommend this, but you may want to pull it all again to see if you do have a wear groove on the shaft. It will be noticeable. It's hard to believe that a rubber seal can do this, but it's true. Hope this helps a little. Last resort.... pore in the motor honey or that bs engine oil stop leak.

 

I think OP said he used a Sleeved Rear Main seal kit