Mercury Four Stroke oil/gas sheen – SOLVED

Hey All,
I run a 2004 115 Mercury Four Stroke EFI. I have a sheen that appears when my prop is in the water. The sheen will appear after I shut it off from running and will also appear when I hop into my boat after it has sat for a while in the water trimmed down (The boat rocks around from my weight and then the sheen appears out back around the outboard). The sheen DOES NOT occur when the prop is trimmed out of the water. There are currently no signs and were no signs of oil or gas leaking while on the trailer as well and under the cowl is clean and dry, nothing wet.

I brought it to a referred boat shop and they replaced the shift shaft seal stating they found the seal leaking after pressure testing the lower unit. Took the boat out and it still has the same problem (Probably 5 hours of run time since repair). Brought it back but they couldn’t pin-point much but think the base gasket could be bad. They also said it could be oil mess left over from the shift-shaft seal repair. I’m not about to throw more money towards a non-confirmed part problem. I could throw parts at it myself.

The motor itself has always ran well, the occasional stall while shifting from neutral to drive but that’s it. I do not think this amount of sheen is normal as I do not see other Mercury Four Strokes doing this but I will listen to other experienced operators. For starters I am going to blast and scrub the outboard and lower unit to remove any old oil. I would greatly appreciate some help if there is a Mercury Four Stroke guru out there and am hoping this is a common problem/issue. Former diesel technician so I am mechanically inclined but not an outboard expert. Included is a picture after a 5 minute 3/4-full throttle ride around the lake. Thank you!

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Have them do the pressure test again and ask if you can be there when it happens. If ever a place tells me they found the problem and fixed it, and I still have the problem, I want to see it.

On a side note-Ever boat I’ve owned seems to leave an oil or gas sheen of some type…nothing that bad but a tiny drop will make a huge sheen. Maybe I’m bad and adding oil and gas…

ClownColor wrote:

On a side note-Ever boat I’ve owned seems to leave an oil or gas sheen of some type…nothing that bad but a tiny drop will make a huge sheen. Maybe I’m bad and adding oil and gas…

Well, this is most likely the case. One tiny drop of gas or oil makes an outsized sheen on the water. Just a little unburned fuel from cranking before ignition is going to be pushed out the exhaust on almost every engine.

Not something I’d worry about.

TheFamousGrouse wrote:

Just a little unburned fuel from cranking before ignition is going to be pushed out the exhaust on almost every engine.

Not something I’d worry about.

X2. Most likely unburned fuel from your exhaust. Instead of leaving a black drip on your garage floor it’s going into the water.

JiffyRude wrote:

will also appear when I hop into my boat after it has sat for a while in the water trimmed down (The boat rocks around from my weight and then the sheen appears out back around the outboard). The sheen DOES NOT occur when the prop is trimmed out of the water.

Do you just have oil in the housing, from an oil change, maybe? Not sure where your drain holes are but it sounds like you are bumping some moisture out of the housing. I only have a newer 60HP but this is basically exactly what would happen when I hopped in the boat. I took off the cover and sprayed soapy water all around the bottom and flushed with a hose several times and that took care of it… until I change my oil again

once you get everything cleaned up and it still leaves a sheen on the water, that is not normal.
check your oil drain plug, make sure its tight and not seeping around the gasket area.
after you have ran the engine and park it, pop the hood and look at the oil filter to make sure its not leaking there, and, check the oil pressure sending unit.

I do have a question, does the motor smell like its running extra rich or is there heavy gasoline odor present after you shut it off??

That used to happen with my Merc 2/stroke 100. Always worried I had a fuel leak but it never caused an issue.
Sometimes it would go away for a trip only to return next trip.
Ran the motor for 25 years w/o any major issues and compression in every cylinder was about 145 after 25 years.

I have a 1989 25hp Mercury tiller that sits in my backyard with the motor down and recently saw drip marks running down the skag from the bottom screw- was dripping milky watered down lower unit grease. Changed it and took care of of the screw. Now it doesn’t leak but there is grease now dripping out of the prop.

Anybody have any thoughts? Not trying to hijack the thread but it made me think of it so I piggybacked

I do have a question, does the motor smell like its running extra rich or is there heavy gasoline odor present after you shut it off??
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iowaboy1 – I have thought about it running rich but I do not smell excessive gasoline. I’ll check this again when I have it out next. What are your thoughts here? Thermostat stuck open? injectors?

As I said make sure all of the oil is cleaned up from top to bottom and as LabDaddy1 mentioned check the gaskets on the drain/vent plugs.
I have also seen where the dipstick doesnt fit like it should and it will leak around its base.
If you are still seeing a sheen then I would suspect a fuel problem, stuck thermostat, failing pressure regulator, and or a leak in the hoses.
fuel sheens go away fairly quickly unless its two stroke fuel, if the sheen lingers for a long time I would still suspect an oil leak.

Turns out the remaining excessive sheen was caused by motor oil leaking from the cylinder block and oil pan gasket. I was finally able to see oil leaking down the lower unit while on the trailer after burning a few gallons of gas. While I was in there I also replaced the oil pump O-rings. I think the gaskets dried out over the years as the previous owner did not use the boat much. Found out the motor only had 82 hours on it when I first brought it in to the shop this summer. I assume it had around 50 when I bought it last summer. Oh the joys of being a boat owner! I only wish I had more time to get it out as this fall was a bust. Thank you to those who replied.

BOAT –
Break – Out – Another – Thousand

Glad you figured it out and got it fixed !
thanks for bringing this thread around and letting us know.

Nailed it. With every boat there is always something at sometime.

My 2018 115 had a bad base gasket and oil pan gasket that were replaced under warranty at ~120 hrs.

Hope the parts were revised, but I can’t be sure. Only time will tell

Follow up for anyone with the same/similar motor or problem – I have probably ran it for 5-6 hours this summer and no oil/sheen on the water

Thanks for the updated info it will help others in the future if they have a similar problem. IF this was a google search it would have the words “SOLVED” in the heading.

Fair is fair, supercat!

I can add that to header if you want. Let me know.

-J.

Hey J, it does not appear I have that editing option. I would appreciate it if you added that to the header. Thanks!