1985 Yamaha 90 HP Oil Tank Pressurizing

[Crispy Taco]

I have a 1985 90 HP Yamaha 2 stroke outboard in which the oil tank keeps pressurizing. When it pressurizes it pushes against the linkage and it will not go out of gear. I can untighten the oil cap and re-leave the pressure allowing the linkages to free up. I have removed and and cleaned the check valve on the tank which I think allows air in to compensate for the oil leaving the tank.

[boscoe99]

The parts catalog shows the use of a breather on the oil tank. It is not called a check valve or relief valve, which leads me to believe that air should be able to flow freely in both directions.

Given that the tank is filled manually, at which time air can flow out as oil flows in, I am not understanding the tank being pressurized. Some tanks have oil pumped to them in which case the tank will balloon outwards if it is not vented. If the breather is clogged then ambient air pressure changes might be causing the tank to balloon outward.

Now if the breather was clogged then the tank should be collapsing as the oil flows out of the tank while air is not being allowed in, while the motor is in use.

[Crispy Taco]

I disassembled the breather and it was in good working condition only allowing air into the tank. Like I said, I think it allows outside air to enter the tank to keep it from collapsing. Could something be going on in that pressure is coming back from the oil return hose? Is the tank pressurized to force oil into the oil pump or is it just gravity fed?

[DennisG01]

I have no practical experience with that setup, but looking strictly at a parts diagram, the oil appears to be gravity fed to the pump. The tank could be pressurizing as it heats up. BUT, there should be a relief valve to stop that. If that breather tube does not vent in both directions, then the only other thing I can think of is that the oil cap serves that purpose with a built-in valving. I can't think of anything else?