Maybe a rotted out oil pan or timing cover? Bad valve cover gaskets or leaking where intake intersects with heads / block junction?

Really no pressure on the front seal unless the engine crankcase breather is plugged up tight and crankcase is building pressure under the higher RPM. Then you would puke oil out all over the place at the higher RPM.

If it was the rear main seal, an oil filter or pressure sensor, it would be all the time. A qt an hour is a pretty big leak.

 

Hi Bill, thanks for the reply. As I alluded to, I'm not 100% sure it's the front main seal, for many of the reasons you point out. I have crawled and looked all over with the engine at various RPMs (but in neutral) looking for the leak, and have yet to see it 'squirting or pouring' as it must in order to lose anywhere near the 1qt/hr I mentioned. I find the arrangement of the engines too sardine-can-like to investigate while underway safely. I'd rather swap parts until it is fixed than injure myself or a helper. I have convinced myself it is the angle of the motor (nose up when at rest, nose down on plane, nose way down getting on plane) that explains why the front seal may be leaking underway but not at rest. I have seen significant oil behind the harmonic balancer.

However, I do appreciate you giving me some other areas to focus on.

All that aside, I am going to replace the front main seal before she is launched this season.

Can anyone give me tips on that? Is it as straightforward as removing the HB, prying out the seal, installing a new seal, and replacing the HB? I know there were various arrangements of front main seals on small block chevy's over the years, so hoping someone familiar with this generation will chime in.

 

not sure if yours has the plastic timing cover or not, which are single use. not sure if the seal is meant to be replaced but would verify that, and also check the cover and the crank position senor on the lower left of cover is not leaking.

if it only leaks on plane would suspect its either the pan having corrosion holes, dipstick, or rear main seal that only sees oil when angled back a bit. I would spend some more time with a scope camera and UV or pull the engine and run it on the ground if you really want to fix before the spring launch.

 

Does anyone have a service manual they would recommend for these motors?

 

PM sent

 

To help you find the correct parts, your engine serial number would be best to have... It seems there may be two options depending on what things add up to using your serial number vs what the service manual suggest.

Looking at a parts catalog for something around your engine year says the oil seal is part of the front cover and comes as a set meaning you cannot just buy the seal from Mercruiser but that needs double checking with your engine serial number.

The manual suggests you could just replace the front seal and shows you how to go about it but then says the front cover is not reusable and needs a new cover...Should the cover be damaged as some times they are and need new 'then the oil pan needs to be either dropped down pretty far or fully removed to do it right.


Looks like you may have received a service manual above but you'd be looking for Merc part number 90-864260 which there are a few places online you could pay to download or buy a used book from say Ebay at lower costs should the following link not work correctly as it fades out pretty quickly most times < if you don't pay but if you're quick enough, scroll to pages in and around 280 and it will help give you an idea what you're up against...It may hold the image that far down as it's working tonight for me to screenshot them for you.

Mercruiser Service Manual - 31 2001 - Newer GM Small Block V8 | PDF | Business | Technology & Engineering

Check out section 3A-90 of the oil seal info, Mercury mercruiser gasoline engines 5.0 l mpi alpha and bravo service repair manual 0m300000 | PDF


To install the seal correctly there is a tool...part number J35468 by Kent-Moore Tools found online with the number searched out.

Here's a few screenshots for you,

 

Amazing help guys, thanks very much (got the manual Bill!). I learned a lot reading through the above post & the manual!

Has anyone replaced the front cover in a 320 with v-drives without removing the engines? I'm away from the boat but questioning if there is room to drop the oil pan without removing the engine? Anyone?

 

A quick follow up in case someone else has similar symptoms in the future and goes searching the archives:

I successfully changed the front main seal in the plastic timing cover without changing the cover itself. I am not sure why they say it can't be done as it was straightforward. I suppose there is too much risk damaging the plastic cover.

Unfortunately, it turned out that wasn't the problem. It turned out that the silicone RTV sealing the seam between the oil pan and timing cover had developed a leak path. I was able to carefully pull the silicone out of part of this seam--it was loose in a ~3" area, not sure why as the area is free from rust. I then cleaned the area and seam thoroughly, and used a large syringe filled with RTV and a large hypodermic tip to inject RTV into the seam and give the entire seam a good bead of RTV. Dropping the oil pan to do the job per the book would have required removing the engine, so I felt this was worth a shot, and 6 weeks into the season, it is working perfectly, no more leak, hopefully the repair holds.