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vinnyb

50 yamaha mystery oil leak

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Here's the short version:

Boat has been on a trailer in my driveway all winter with the 2-stroke 50 hp Yamaha in the down position (just like every year for the past 10 seasons). The engine has not been started since winterization in December. A few weeks ago I noticed a fair amount of 2-cycle oil had leaked out of the bottom of the upper case, down the lower housing & dripped off the lower unit onto my driveway. I removed the cowling & sure enough there was a small pool of oil at the bottom rear of the case. I also noticed that the cap containing the oil level sensor had popped out of the oil tank (very strange). I cleaned up the oil & pushed the cap back onto the oil tank, making sure it seated properly. About a week later, the same thing happened but this time the cap was still seated on the tank. There was no evidence of anything actively leaking - it seemed that whatever occurred happened relatively quickly & then stopped. :confused: Again - I cleaned up the mess & this time I even removed some oil from the tank which had been pretty much full but not to the point of overflowing. (At this point the tank is about 3/4 full.) This morning it happened for a third time :confused::confused:. I tilted the engine up just to make sure nothing was coming from the caps (I can't figure how oil could leak from the caps with the engine in the down positon anyway but was just curious) and nothing seemed to be leaking. I returned the engine to the down position & kept the cowling off all day to keep an eye on it - no oil leaking. What the frig? Anyone have any ideas?

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My guess would be a check valve somewhere. Your oil system is always primed. At some point it's mixing with gasoline, which is a system that's open to the atmosphere, and therefore breathing as it gets hot and cold during the day/night. If you have a bad check valve somewhere, the fuel system could be siphoning oil from the oil system....possibly into the carbs where they're overflowing.

 

That's just a guess. Put a clean paper towel around the oil tank cap, under each carb, and under the oil pump, and check it every day to narrow down where the leak is coming from.

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My guess would be a check valve somewhere.

 

There is a check valve in the oil return line that leads from the top of the oil pump assembly up to the top of the oil tank. I'll check that valve to make sure it's operating properly. I'll also pop off the silencer & check out the carbs. As you mention, it seems that pressure is building up somehow & pushing the oil out.

 

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The oil will not siphon out of the secondary oil tank since the level of the oil is a lot lower than the main tank on the motor. Oil will spill out of the main tank if the cap is loose, the tank is full and the motor is tilted up. The fact that the cap seemed to pop out leads me to think that the secondary tank oil pump was running and overfilling the tank. For that to happen a lot has to go wrong. On the other hand, if the manual oil fill override switch is stuck in the on position the tank will overflow but even for that to occur, the motor has to be powered and that is not likely.

So let's go with the simple reason, the cap was lose and oil spilled out. Even with the cap on, oil often spills out of these motors because the cap does not seal well. If the tank overfills in general, more overflow will occur. This can happen when the sensor in the main tank (on the motor) doesn't sense that the tank is full and the pump on the secondary tank keeps filling the main tank. When the motor is running, oil is getting pumper ANF used so the overflow issue is somewhat muted. Once the motor stops running but the ignition is on, the tank will overflow. The oil pools in the bottom of the cowling and than drains when the motor is lowered.

 

Good luck

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Another "bullet proof" Yamaha having wierd problems.

 

Thanks for your help :rolleyes:. Engine is an '02 & has served me very well over the years with very few issues.

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The oil will not siphon out of the secondary oil tank since the level of the oil is a lot lower than the main tank on the motor.

 

Not sure I'm following this - as far as I know there's only 1 oil tank on this engine.

 

I did pull the check valve today & it seemed to be working ok - at least when I blew into it. The various lines associated with the oil tank do seem to be showing some signs of age so I've decided to replace all of them & the check valve as well. For 50 bucks in parts I figure it can't hurt. I also checked the carbs & there's no evidence of oil in them.

 

 

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Best bet vinny is to give the motor a GOOOOOOOD clean inside. Without knowing where the oil is coming from, it's hard to diagnose how to stop it. I know you don't see it, but how about this. Try cleaning it out as good as you can. Then spray it down with CRC 6-56, then wipe clean. It will likely leave a little residue, but not much. Then wait for a day(or however long it takes to "leak") then take baby powder, and "spray" it on the motor. Anywhere it's wet from oil, it will cling, and show you very well where it is coming from.

 

Once you figure out where it's coming from, it will give us a better idea where to go fixing it. I highly doubt it is the cap, as he said he replaced the cap twice, wiping clean the first time, and then when pulled again it was not wet. If oil had been leaking to a "pool" status in the bottom of the cowling, the cap would be wet. And as Vinny says, Keeler, this is the 50 hp, all it has is the single on-motor tank, and NOT the seperate tank inside the boat.

 

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