Lower Unit Oil Leak

[JimS123]

2019 Mercury 4-stroke 150 HP.

Looks like a minor leak emitting from the rear end of the lower unit, where it bolts on. The motor has very low hours and is always garage kept. Fresh water only. I noticed a slight drip of oil after sitting all Winter. The oil is puddling on top of the AV plate and dripping off the end. After 6 months of sitting I estimate the leak at less than an ounce.

My thought is to tighten the 5 bolts holding the lower unit on, check to be sure the gear oil is full, and monitor the leak.

Anybody suggest a better approach?

(The pic shows the motor tilted up - it was horizontal all Winter)

 

[Mc Tool]

1st thing to do is to be sure its gear oil and not engine oil ......no chance you spilt oil whilst topping up the engine . You can usually tell by the smell of the oil what it is .
Tightening the lower unit mounting bolts wont help .

 

[JimS123]

1st thing to do is to be sure its gear oil and not engine oil ......no chance you spilt oil whilst topping up the engine . You can usually tell by the smell of the oil what it is .
Tightening the lower unit mounting bolts wont help .

My plan was also to check the engine oil. But if it was, how did it get down there?

A spill is not feasible. I wipe everything down meticulously. I'll give it a try, but I don't think there is enough there to get a good smell.

Why won't a bolt tightening not help? I'm assuming some came loose after 6 years.

 

[jlh3rd]

those lower unit bolts have nothing to do with sealing anything. The just hold the gearcase on.
The nuts are nylocks so not prone to loosening.

 

[roscoe]

Tightening won’t help because that is not what seals the gear oil in.
There are seals on the driveshaft and the shift shaft.

“I'm assuming some came loose after 6 years.”

They should have been loosened/removed when the water pump impeller was replaced. If you haven’t done that yet, it’s time.

When you remove the gearcase to service the water pump, you can inspect to see if it is engine oil or gear lube, and whether a seal has failed.

 

[airshot]

Would be kinda unusual for gear oil to be above the cav plate....Have you checked the gear case oil for water intrusion or low level ?

 

[JimS123]

Would be kinda unusual for gear oil to be above the cav plate....Have you checked the gear case oil for water intrusion or low level ?

There was no water intrusion last Fall when I changed the oil. It has not been in the water since then.

I can see where the oil is seeping out. It's from the seam where the lower unit meets the mid section, which is just above the AC plate. I don't believe there is a gasket there. Isn't it true that the 5 bolts are what brings it tight together?

 

[Chris1956]

Jim, if it is gear oil, drop the gearcase and find the leaking seal. It could be the shift shaft seal or the driveshaft seal. If the leaking point is not visible, you could drain the gear oi and pump a few PSI of air into the gearcase and spray those areas with soapy water.

I have not had any Merc newer than 1993, but gear oil leaks have occurred over the years. My '93 Merc V6 has leaked twice at the shift rod seal. My '77 Merc IL6 never leaked there, but the driveshaft seal in the waterpump base was the leaking area a few times.

 

[Mc Tool]

That gap is where the lower unit bolts to the mid section . The lower unit is sealed to contain the oil inside itself . The engine sit atop the mid section ....also sealed to contain oil , but there is no oil in the mid section , however the engine main seal may leak down onto the mid section and out that gap . The lower unit can leak out of either the drive shaft seal or the gear change shaft seal on top of the lower unit .....and the oil will show up at the same gap .....so you have to suss weather its gear or engine oil .
That gap doesnt need to be oil or waterproof ,its just there where the sealed lower unit bolts on .

 

[JimS123]

Gotcha. I never pulled a lower unit before. I just assumed that there was gear oil back there.

It's not green, so I'm guessing it's engine oil.

For it to leak out there, wouldn't a large amount of engine oil have to leak down there to fill the cavity?

The oil was changed and topped off last Fall. As soon as I can get access I'll pull the boat forward and check the oil level.

If it's just a small leak from the engine main seal and the engine is kept topped off, what are the consequences of not replacing the seal?

 

[Mc Tool]

Gotcha. I never pulled a lower unit before. I just assumed that there was gear oil back there.

It's not green, so I'm guessing it's engine oil.

For it to leak out there, wouldn't a large amount of engine oil have to leak down there to fill the cavity?

The oil was changed and topped off last Fall. As soon as I can get access I'll pull the boat forward and check the oil level.

If it's just a small leak from the engine main seal and the engine is kept topped off, what are the consequences of not replacing the seal?

Oil leaks tend to wick along surfaces and gaps and this can make it hard to figure where its coming from , usually you would give it a clean and then see where the oil appears 1st . The oil doesnt need to fill the cavity .....only needs a small leak to wick down the inside of the mid section and show up at that gap .
If you can pinpoint the leak you may decide that just topping it up is an option but such leaks never get better and usually get worse . Im still hoping that oil was spilled either during draining the old oil out or filling with new oil , just that when you get a new issue it is often related to the last thing done to the problem area and as you have an oil leak after an oil change ......... Some engines are just messy to change the oil no matter how careful you are, like anything with the filter mounted horizontally is gunna spew oil out as soon as you loosen it .....and sometimes all you can do is clean it up when it drips out the bottom of whatever.
Getting the lower unit off is not hard ,basically all you do is disconnect the gear shifter linkage ( google it ) and remove those bolt and the whole gearcase ( inc pump and drive shaft ) will just about fall off . This is a pretty simple thing to do as you have to do this to service the water pump ,can be a bit fiddly to reassemble untill you have done it once ,a spare pair of hands is helpful here , again google you tube and Im sure some budding movie star/ director has done either the exact same motor or one so similar ( around same model and year ) it doesnt matter .

 

[M9.9]

2019 Mercury 4-stroke 150 HP.

Looks like a minor leak emitting from the rear end of the lower unit, where it bolts on. The motor has very low hours and is always garage kept. Fresh water only. I noticed a slight drip of oil after sitting all Winter. The oil is puddling on top of the AV plate and dripping off the end. After 6 months of sitting I estimate the leak at less than an ounce.

My thought is to tighten the 5 bolts holding the lower unit on, check to be sure the gear oil is full, and monitor the leak.

Anybody suggest a better approach?

(The pic shows the motor tilted up - it was horizontal all Winter)

View attachment 415464

You might check the Gear Oil for water contamination, first.

 

[jimmbo]

Perhaps some Oil was spilt when it was filled

 

[JimS123]

I guarantee that not a drop was spilled. This is the easiest engine I ever did an oil change on. The drain plug is on the side of the leg. It has a fitting to attach a hose and it drains right into a container, not a drop to spill. Even the oil filter has a trough under it, also fitted with a hose fitting.