[JSH]

I see a few drops of oil on my garage floor under the rear of the engine, so let's assume it's a rear main leak.

What are the most common causes? Crankcase pressure is surely the most common. Mine has an MM Race.

Is Bar's Leaks Rear Main Seal Repair worth trying? I don't want to pay for a shop to repair the seal unless I know the cause.

[Tim M]

I doubt stop leak is going to fix if indeed the rear main seal. Believe their most common failure is rolling the lip outward (rather than pointed inward) and there is nothing you can do to fix other than replace...this was my issue after excess crankcase pressure occurred.

Just a new seal is required - not the entire cover (must maneuver with oil pan then!).

Mine:

[RobZL1]

Man that stinks. Loose oil filter maybe?

Small chance you have loose pan bolts? Doubtful on your built engine, I guess, but worth a look. I don't know if it would even leak with a loose bolt these days with the RTV gaskets?

Maybe it's just the oil filter

[KingLT1]

Check the oil cooler and pan bolts. They seem to like coming loose. I had to tighten all that up on mine at a few thousand miles.

[Z OH 6]

I doubt stop leak would work nor would I want to use that in a boosted application.

[Deadpool Z]

I agree with everyone else, check the pan bolts. When my shop did my oil pump who was a GM technician for 10 years learned that the oil pan bolts on the stock LT4 were to be found loose.
This is exactly what he had found on mine, several of them being finger loose. He also told me I even had a oil leak. I never knew it because I found nothing on my garage floor.

[ZLRob]

Originally Posted by Deadpool Z

I agree with everyone else, check the pan bolts. When my shop did my oil pump who was a GM technician for 10 years learned that the oil pan bolts on the stock LT4 were to be found loose.
This is exactly what he had found on mine, several of them being finger loose. He also told me I even had a oil leak. I never knew it because I found nothing on my garage floor.

Makes me really glad I did my build if that's the case. I don't believe any of my oil pan bolts were loose on disassembly but now that everything has been gone over absolutely zero doubt in my mind exists that anything engine wise as far as fasteners are concerned are loose.

As for the OP I would not use stop leak or bars or anything stop leak related. They usually tend to do more harm than good more often than not. True story, years and years ago I had an LS2 GTO that I did pretty much all of the gaskets and seals on prior to the actual head gaskets. The thing was a real leaker but potent for an NA. I had the rear main seal done on it while I was out of state for work by an old co worker and when I got back and got to driving it it had a leak that appeared it was from the rear main again. He did it again for me and the leak persisted. Factory rear main too with the cover and all. Frustrated at that point I really got to checking the thing over with a fine tooth comb. Come to find out the valve cover gaskets that I just did were leaking from the passenger side and making it seem like it was the rear main. I replaced them and it never happened again during the rest of my ownership of that junker car.

Anyway, there are three ways this plays out for you, I don't know if you have an M6 or A10 but your tail shaft seal could be leaking, your oil cooler as mentioned, your valve cover gaskets could be the problem, or the rear main could indeed be the culprit. Most simple, and hopefully actually the issue, your oil filter is loose, but I'm not too confident that's your actual cause. Regardless, if you got some race ramps or access to a lift, an inspection is definitely in the works for you.

[george]

Originally Posted by JSH

I see a few drops of oil on my garage floor under the rear of the engine, so let's assume it's a rear main leak.

What are the most common causes? Crankcase pressure is surely the most common. Mine has an MM Race.

Is Bar's Leaks Rear Main Seal Repair worth trying? I don't want to pay for a shop to repair the seal unless I know the cause.

Could be a seal around oil pan as well. Especially if youve had engine work. I've seen the rear portion of the valve cover gaskets leaking.

[JSH]

The leak is definitely at the rear of the block, so I assume it's the seal. The problem is, I don't want to pay the shop to fix it (how many hours, 6-8?) when we don't know what caused it. It has an MM Race CC now. What more can I do to eliminate pressure?

[LiqTenExp]

I don't bother with catch cans. I blew out my rear and front seals over a couple events. Both returned to normal and continue to not leak after the event luckily. The vent line on the air intake really was too small to capture the multiple ports worth of crankcase air. One event was due to it being pinched accidently after an install of another component (my fault).

Since then I've had them both replaced with LS-3 seals (stiffer) and installed of of these: https://metcomotorsports.com/mbr0003

Years later and no more issues. It has a 1 way valve to prevent any unmetered air from coming back into the engine. Do this!

[jlaudio11]

Originally Posted by JSH

The leak is definitely at the rear of the block, so I assume it's the seal. The problem is, I don't want to pay the shop to fix it (how many hours, 6-8?) when we don't know what caused it. It has an MM Race CC now. What more can I do to eliminate pressure?

I understand you have the race can but what size lines and where are you running them from? We're similar power levels and I have actually too loose of ring gap so I get some blow by, but I'm just going to a generic catch can with 5/8 lines off the stock valve covers. I did put an ls3 front seal in when I had my engine apart. I think I actually reused my stock rear main seal. Cant remember right now.

[KingLT1]

I run metco breathers on my engines. That being said the MM wild catch can has a breather so it should do the same thing, but you might try adding the Metco breather on the Valve cover.

I had a rear main go out on a new 5.3 LS(mild build) that was less than 1k miles old. Replaced seal added Metco breather, no issues since.

If you don't want to try the Metco breather, One thing you will want to make sure is that your breather lines are all upgraded to -10 and routed correctly for the MM wild can to vent crank case pressure efficiently.

[LiqTenExp]

Originally Posted by KingLT1

I run metco breathers on my engines. That being said the MM wild catch can has a breather so it should do the same thing, but you might try adding the Metco breather on the Valve cover.

I had a rear main go out on a new 5.3 LS(mild build) that was less than 1k miles old. Replaced seal added Metco breather, no issues since.

If you don't want to try the Metco breather, One thing you will want to make sure is that your breather lines are all upgraded to -10 and routed correctly for the MM wild can to vent crank case pressure efficiently.

That catch can breather depends on the lines going to it to be able to manage the volume of air in the crank case. Catch cans and their long small diameter hoses likely offer much more restriction and hold pressure in the system longer.

This breather is a large 1" size opening to the crank case with significantly less pressure drop. It can handle large volume quick impulse crank case pressure events much better than any catch can could (shifting, snapping off throttle, etc). When vacuum returns to the crank case it seals off the vent with the floating disc.

[KingLT1]

Yea I agree which is why I run the Metco breathers on my engines. But the MM wild does work well if you increase the breather line(s) diameter to -10 as I mentioned prior.

That is one nice thing about Whipple's. Is they upgrade the factory vent lines with their kits. It definitely becomes an important thing to address as you turn the boost up and make more power.

And I still believe catch cans are important to keep oil from being ingested. That will cause detonation and power loss. Stock LT4's get quite a bit of oil blow by through the PCV system. I have seen oil pooling inside the supercharger lid.