smelly and black lower unit oil

[airshot]

I have a 1991 40 hp Johnson in excellent condition, runs great, well maintained. I recently changed the lower unit oil and found it to be very smelly and black. It was changed at the end of last season as always and only run on muffs so far this year as it has not been on the lake. I decided to freshen everything up with the potential of using it and found this black gear oil. Been around outboards for 40 years and never saw this before, anyone have any idea what has happened? Thanks in advance for your input.

Airshot

 

[Will Bark]

Re: smelly and black lower unit oil

My oil always comes out smelly and black but this is out of 24 year old 150 Johnson; what I'm looking for is water and metal shavings if none then I haven't worried about it maybe I should. Is this the first time for this motor?

 

[kfa4303]

Re: smelly and black lower unit oil

yeah, smelly and black isn't so bad, or uncommon. I agree with Will Bark, that you want to look more for metal shavings and/or white/milky colored oil more than anything. You should be good just changing the oil.

 

[Faztbullet]

Re: smelly and black lower unit oil

smelly and black isn't so bad

The reason gear oil turns "smelly and black" is that it is is decomposing from heat and shear. The smell is hydrogen sulfide a by-product of oil breakdown. Usually the main reason this happens is the wrong weight oil is installed or a cheap automotive gear oil was used in the unit. Marine gear oil has anti-foaming agents, extreme pressure additives and emulsifiers to help prevent oil breakdown in case of water intrusion. Auto/trucks use a vented gear housing so foaming is not a critical problem and are not subject to exhaust heat transfering thru gear housing.

 

[Borgey401]

Re: smelly and black lower unit oil

The reason gear oil turns "smelly and black" is that it is is decomposing from heat and shear. The smell is hydrogen sulfide a by-product of oil breakdown. Usually the main reason this happens is the wrong weight oil is installed or a cheap automotive gear oil was used in the unit. Marine gear oil has anti-foaming agents, extreme pressure additives and emulsifiers to help prevent oil breakdown in case of water intrusion. Auto/trucks use a vented gear housing so foaming is not a critical problem and are not subject to exhaust heat transfering thru gear housing.

Wow, very good info to know. When I first purchased my boat I drained the gear oil and it was exactly like this. Very black and smelled awful, it smelled so bad that I could smell it inside of my house from outside. I obviously went to West Marine and purchased some gear oil and changed it but I am tempted to check it again just to see what's going on in there.

 

[airshot]

Re: smelly and black lower unit oil

Faztbullet thanks for the scientific explanation, while it may be hard to believe I have never had this before. Some mild discoloration only but this time it is very dark charcoal gray/black. I have only used premiem synthetic gear oil in my lower units and have have never experienced this before, especially when it has not been used. Thanks to everyone else that has had the same issue at least I feel better about it.

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[Reelistic]

Re: smelly and black lower unit oil

I also had a similar experience with the foul smelling gear lube... I noticed mine while in the water. I thought maybe I had a leaking seal so I checked for water in the lube... no water, but metal filings!! I thought the smell was a result of all the metal filings. I'm wondering if there is a connection? Also, anyone know if lower units fail abruptly, or are there gradual symptoms. My motor was running flawlessly when I began checking.