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How-To: Replace a Jeep 4.0L Rear Main Seal

Posted in How To: Engine on April 8, 2018
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If oil isn’t dripping from beneath your Jeep TJ’s inline-six, it’s because it’s either brand new or the rear main seal has been replaced within the last 40,000 miles. That’s just a fact of four-liter life. A leaking rear main seal (RMS) is almost as common among TJs as a leaking valve cover gasket. Both are annoyances that leave the undercarriage a mess, as well as oily spots wherever the Jeep is parked.

Oil seeping from around the back of the valve cover and running down the rear of the engine block can be easily stopped by either re-torqueing the valve cover bolts or removing and replacing the old gasket with a new one. A leaking RMS, which typically rears its ugly head with leaking oil originating from between the transmission and the engine, is a little more time consuming.

Fixing the leaking seal entails dropping the exhaust, oil pan, main bearing support bar, and rear main bearing, and then replacing the two-piece seal. It’s one of those jobs that shops we work with typically charge at least a few hundred bucks. The good news is that doing the RMS repair yourself is relatively easy, and the cost can be less than $50 for the seal and sealants. The six quarts of oil and a new oil filter of course will add to that.

It can be done in a couple hours—providing the exhaust down-pipe bolts come out without breaking off. (Lube them well with penetrating oil the night before.) A basic metric socket set, along with a good torque wrench and a round-nosed punch, are the only tools required. You will need tubes of anaerobic (seals in the absence of air) sealant and RTV sealant, though.

To learn how, follow along as Dunks Performance (in Springfield, Oregon) techs replace the RMS on a customer’s 2006 LJ. The 4.0L had just turned 57,000 miles and it had been seeping oil for six months. The owner of this TJ Unlimited was having a new 4-inch suspension installed, which provided the perfect opportunity to address the oil leak because the front axle assembly and exhaust would already be conveniently out of the way, allowing easy access to the oil pan.

The first step in replacing the rear main seal is getting the exhaust out of the way. The exhaust was removed from the 2006 4.0L to access the oil pan. Squirting the four exhaust pipe bolts with penetrating lubricant the night before was a big help.
A fistful of 11mm bolts holding the oil pan on was removed. Then we carefully peeled off the old pan gasket.
Nearly a year’s worth of leaking oil from a bad RMS had accumulated a good coating of residue on the flywheel inspection cover. We had to scrape away the accumulation before cleaning the area with solvent.
A dozen 14mm nuts had to be removed to drop out the 4.0L inline-six’s main bearing support bar before we could remove the rear main bearing cap.
The correct way to remove the rear main bearing is to gently rock it fore and aft, but not pry it side to side.
Here’s what the upper half of the rear main seal looks like once the bearing cap is removed. This one is worn and leaks because the sealing lip at the outer edge facing the crankshaft (left side of the seal) is gone.
The lower main bearing sits in the cap and can be easily dislodged. So for good measure remove the bearing and set it aside before removing the old seal and giving the cap a very thorough cleaning, including the grooved surfaces where the seal sits. (Do not damage the cap’s metal surface where the seal sits.)
The old seal was gently and carefully tapped with a small round-nosed punch so that it would slide around the crankshaft enough to be grabbed with pliers for removal. Be very, very careful not to gouge or scratch the surface the seal rides on or the new seal will leak.
Once we had the old seal slid around about an inch, it was easy to grab it with needle-nose pliers and carefully pull it the remainder of the way out of the groove.
Here’s how the old seal (left) and new one compare. Note the lack of sealing edges on the old one, which allowed oil to seep out around the back of the rear main bearing. These seals typically wear out around 50,000 miles on the Jeep inline-sixes.
We used a flat screwdriver to gently keep light pressure on the new seal as it was pushed into the groove around the crankshaft. The trick here is to make sure the paper-thin sealing lip, which faces toward the engine, doesn’t get cut by the sharp edge of the groove it slides into. Some seal kits come with a special plastic insertion tool. Ours didn’t.
It’s very important to make sure the bearing cap is thoroughly cleaned, including the groove where the seal sits. Use a toothbrush or fine nylon brush to remove all residue form the surface without damaging the metal. This is critical to maintaining a good oil seal.
We gently inserted the lower half of the new seal into the cleaned rear main bearing cap, making sure the lip of the seal (seen in close-up photo) faced toward the main bearing. Both bearing and seal got a light coat of new engine oil before reassembly.
It’s very important to put one drop of anaerobic gasket maker/sealant (Permatex 51813) on each end of the main bearing cap to prevent oil from leaking around the mating edges. Spread the sealant out so it only covers the area between the seal and the groove on the opposite side, as shown in the close-up photo. This is per Jeep’s instruction manual.
Dunks replaced the rear main bearing cap and then torqued the bolts to 80 lb-ft per Chrysler specs for the 4.0L.
After the mating surfaces for the oil pan gasket were cleaned, we put a dab of RTV silicone sealant at the critical areas of the mating surfaces and on the new one-piece oil pan gasket. Using RTV to fill gaps at the corners where the timing cover meets the block, and at the rear where the main bearing meets the block, helps to prevent oil leaks on the 2000-and-newer 4.0L six-cylinders.
With the oil pan cleaned inside and out, we lifted it into place and torqued the bolts to factory specifications (7 lb-ft for 1/4-inch x 20s and 11 lb-ft for 5/16-inch x 18s). Now the RMS on this TJ Unlimited 4.0L should be oil leak–free for another decade.


Dunks Performance

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