LS rear main seal leaking

Hey guys, trying to diagnose a leak which I am guessing is either the rear main seal or the cover itself. It does drip but it doesn’t leak bad, in other words the level on the stick stays fine, I haven’t had to add any oil yet. So I am partly trying to decide if it’s even worth it yet, and should I expect it to get worse?

MY MAIN QUESTION, is the oil level in the pan above or below the pan gasket, and is it above or below the seal???? Trying to determine if it leaks only when running and maybe shortly there after …. Any ideas ????

K5Rob said:

Hey guys, trying to diagnose a leak which I am guessing is either the rear main seal or the cover itself. It does drip but it doesn’t leak bad, in other words the level on the stick stays fine, I haven’t had to add any oil yet. So I am partly trying to decide if it’s even worth it yet, and should I expect it to get worse?

MY MAIN QUESTION, is the oil level in the pan above or below the pan gasket, and is it above or below the seal???? Trying to determine if it leaks only when running and maybe shortly there after …. Any ideas ????

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The oil level is below the pan rail and way below the main seal. What often happens is that someone doesn't properly align the rear cover when replacing the rear main seal and then it leaks at the corner where the rear cover and oil pan gasket meet. I had the same issue on mine when I did a rear main seal replacement while rebuilding my transmissin.

Have you diagnosed that it IS the rear main seal, for sure?

Many places on the Gen 3 & 4 block leak oil that will run down the back of the engine and around the bell housing to make you think it’s the rear main.

See if you can look at the back of the block, flex plate/flywheel and torque converter / clutch thru an inspection port to see if it has oil on it? If not, start looking at oil cooler lines, or cap, if you don’t have an external oil cooler. Look at the OPS, valley pan, and valve cover gaskets, etc etc.

A little searching around may save you a lot of work/headache

vr1967 said:

Have you diagnosed that it IS the rear main seal, for sure?

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I have not. Driveway is still very mushy. Definitely could be (and hoping it’s) leaking somewhere else. Just talking hypothetically right now.

Main question: Where is the oil line at rest?

K5Rob said:

I have not. Driveway is still very mushy. Definitely could be (and hoping it’s) leaking somewhere else. Just talking hypothetically right now.

Main question: Where is the oil line at rest?

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Below the top of the oil pan, if you look on the PS of the pan you can see where the oil level sensor is. Oil level is close to that.

I am in the middle of doing a rear seal on a GMT800 L33... If you're going to do all the work to get in there you might as well do both the cover gasket and the crank seal.

After all the work to tear this greasy SOB down as far as I have, I've decided to do an oil pan gasket, oil pick up tube O-ring, and crank position sensor too. I was going to put a new starter on because it's such a PIA to get out but have decided to settle on a used one I have laying around to reduce the cost.

CalSgt said:

Below the top of the oil pan, if you look on the PS of the pan you can see where the oil level sensor is. Oil level is close to that.

I am in the middle of doing a rear seal on a GMT800 L33... If you're going to do all the work to get in there you might as well do both the cover gasket and the crank seal.

After all the work to tear this greasy SOB down as far as I have, I've decided to do an oil pan gasket, oil pick up tube O-ring, and crank position sensor too. I was going to put a new starter on because it's such a PIA to get out but have decided to settle on a used one I have laying around to reduce the cost.

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Ok well that tells me it is leaking only when running / shortly after, would you agree?

IF it is the rear main on further inspection, I will buy the cover with seal already installed. The seal that is in it now is new and i think i may have messed it up when installing. It was a PITA, I do remember that ….

Even if it doesn't have a cooler, it could still have a block off/bypass plate that has a gasket that could leak.

K5Rob said:

I have not. Driveway is still very mushy. Definitely could be (and hoping it’s) leaking somewhere else. Just talking hypothetically right now.

Main question: Where is the oil line at rest?

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How many miles on it? VLOMs are notorious for the bolts coming loose and leaking after 150K and running down the back of the block. Could also be VC gaskets leaking out the back and running down. Oil cooler plate gasket next to the oil filter. Oil sender could be leaking as well.

Looking inside the bell hosing inspection cover it could look bone dry but the oil could be flinging off the back of the flywheel. Hard to see in there.

Some of these engines also have something called a dog bone in the back of the block that should be replaced and that cover can leak as well.

Unfortunately they're right, there's several places at the back of the engine that like to leak on LS's. The oil cooler plate/block off plate has weeped or leaked on every LS I've ever had. The rear corners of the pan if the sealer gives up or was not applied, the valve covers will leak back and down. In my C10, the 5.3 was leaking from the rear main but I didn't know it. It looked bone dry looking through the inspection cover until I got it apart and got the flexplate off, it was just barely weeping past the seal, following the crank flange and getting flung around by the flexplate. All around inside the bellhousing was wet, but directly under the rear main was dry. Go figure.

K5Rob said:

Ok well that tells me it is leaking only when running / shortly after, would you agree?

IF it is the rear main on further inspection, I will buy the cover with seal already installed. The seal that is in it now is new and i think i may have messed it up when installing. It was a PITA, I do remember that ….

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Yes, Most likely leaking while running and immediately after shut down.

@89Suburban makes a good point about the oil cooler block off, I replaced that one too. I also did a front harmonic balancer seal and valve cover gaskets which I could see were leaking but still had a leak.

I bought a bottle of that U/V leak detection dye and went all around looking for my leak with the special flashlight. Note: it does need to be really dark for the light to work well. I couldn't see any dye on the upper part of the engine block, nothing coming from the front or from the new valve cover gaskets but I was getting a fresh neon green drip from either side of the block where the bell housing covers are. I was able to see some neon green while looking through the torque converter inspection hole, I was able to just barley see the bottom edge of the rear main seal was glowing. That was enough for me to be certain I had found the last of my leaks.

89Suburban said:

How many miles on it? VLOMs are notorious for the bolts coming loose and leaking after 150K and running down the back of the block. Could also be VC gaskets leaking out the back and running down. Oil cooler plate gasket next to the oil filter. Oil sender could be leaking as well.

Looking inside the bell hosing inspection cover it could look bone dry but the oil could be flinging off the back of the flywheel. Hard to see in there.

Some of these engines also have something called a dog bone in the back of the block that should be replaced and that cover can leak as well.

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Almost forgot about that Dog bone heard it called a barbell as well might look into an aftermarket metal one to replace the plastic junk they use in there.

Yeah, but the barbell is UNDER the rear cover.

If it's a Gen III be aware that the cam position sensor is also mounted in the rear of block from the top. Sensor is fitted with an o'ring type seal that will not leak from top of block, but rear of block. Learned that while working for the general years ago. At the time the o'ring was not serviced separate of the sensor. Good luck.

Fix it if the leak bothers you if not wait until you need to pull the trans.
As far as the barbell it is a solution to a problem that doesn't exist, spend your money on something else.