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15hp 4 stroke bigroot mercury making oil

J

Jhanratt

Regular Contributor
mercury 15hp 4 stroke bigfoot.

I started pulling my Trophy out of storage and noticed oil on the gravel underneath my kicker. I also noticed a drop of oil on the prop blade. It looked like engine oil so I pulled the dipstick. It is full to the very top. I opened the drain plug to check for water. No water just engine oil. It does not smell like gas? and has no water?

Ideas?

Obviously I will do an oil change and filter but what is going on?
I troll at idle all day on the kicker. It never sees WOT.
It has always run good but has a real flat spot between idle and mid range.
carb settings are factory.
It took on water one year only and that was 6 years ago. Never did find out why. Never happened since.
Engine oil is filthy this year. Many days of trolling at idle last year.
Main power is merc 125 2 stroke. They share the same fuel tank and water separator.

Ideas fuel wash down???

Thanks

JH
 
It sounds like you are onto a winner. A magic oil machine! So when you say the oil is filthy, is it a sort of pale tan colour? It could be that it has water in it but it is emulsified (mixed) up instead of a layer in the bottom due to the churning in the crank case. If it is water you either have an issue with your head gasket or the seal on the oil pump.

Stuart
 
It has been sitting in storage since last fall. I would think that if there was water it would have separated. It does not look milky at all to me. It just looks dirty brown. Could it be fuel and the light ends evaporated off leaving no odour?? I was in a hurry when I changed it last year but I can't see myself overfilling it either.

(storage is covered with a motor cover on the back of the boat outside in the weather)
 
Did you have the cylinders fogged when you laid it up?

Could just be the fogging oil that slithered out.
 
No I don't think I fogged it and we would be talking a pretty big volume from full on the dipstick to full right to the top of the dipstick hole.
Thanks tho.
 
From what I've just been reading emulsified oil does not separate again readily so if it is water it won't makes its way to the bottom of the sump. Can you post a photo of the oil in something clear?

Stuart
 
So today I dumped all the engine oil and drained the filter. It looks dirty but not contaminated to me. (I've been pulling wrenches on my own stuff for 25 years). I have seen gear cases and crankcases contaminated with water before and they have always looked either very milky or stratified. I grew up with an old OMC inboard/outboard so I worked on boats lots.

Anyway I changed the oil three times and finished with fresh oil and a new filter. It is sparkling clean now I will just have to watch it over the summer and see if it makes more oil.

thanks

Jeff

It did smoke for a long time so either being over full got oil into the cylinders or I fogged it last year and forgot.
 
I hope it works out for you. It certainly sounds like you've got the experience to know. There's one thing about motors, doesn't matter how much you know, sometimes they remain a mystery. :)

Stuart
 
DangarStu said:
I hope it works out for you. It certainly sounds like you've got the experience to know. There's one thing about motors, doesn't matter how much you know, sometimes they remain a mystery. :)

Stuart
Click to expand...

Don't know if you solved this but there is a serious issue with some 4-stroke Mercs. The fuel module has a vacuum line in some that ruptures and sucks fuel into the lower cylinder. If the vaccum line is ruptured severely it can hydra lock the motor with fuel loading up the lower cylinder. (that's what mine did, destroyed the starter and some other parts). If a small leak it could suck small amounts of extra fuel into the intake and gradually build up in the oil. You said it didn't smell like gas. Mine hydra locked and had an abundance of oil also (did it build up over time? ) the oil looked good and didn't particularly smell like gas, but was .5 to 1 quart on the high side.

If it was me I'd pull the fuel module and check the vacuum hose between the fuel regulator and intake manifold. If there is a small rupture you are sucking too much fuel into the lower cylinders and that has to be going somewhere?

Just sayin....................

Mercury did quietly issue a completely different pressure spec hose so your dealer can charge full price for the design defect with the low pressure OEM hose and Merc can duck all the downtime and repairs caused by the design issue.
 
Sorry to hear about your bad luck.

I am happy to report that I think in my case I was correct in my diagnosis. My carb was set so rich that fuel was washing past the rings and accumulating in the crank case. Last year I knocked out the carb adjustment plug which blocks access to the idle mix screw. I adjusted that screw a little leaner. The engine now has no flat spot and no stumble. It runs a LOT smoother when warm but the trade off is I have to use a little throttle advance to start it when cold. It runs SO much better for trolling though! So I watched my oil level through a year of unusually high usage and the level never moved a bit. When I changed it it also looked reasonably clean despite unusually high usage for me. When I pulled it out of storage this year it fired right up and there was no excessive smoke. I would say I have put 30 hours on it so far this year which is unusually low and it has not moved a bit off of the full mark.

So here is another interesting issue which is possibly related. My oil fill plug has been getting tighter and tighter. This spring it was so bad that I had to use pliers to turn it all the way out and all the way back in. I put in a new one and the issue is gone. I suspect the plastic oil fill plug has swollen from exposure to fuel. I can't think of any other reason it would swell up.

Thanks for the advice.
It is always helpful and appreciated.

Jeff
 
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