Merc 125 2+2 Oil Reservoir Relief Valve Trouble

I have a 98 Merc 125 4 cyl. The oil tank on the motor, holding the 2 stroke oil, has a relief-type valve on the top of the tank with what looks like a little valve and a cover. Well, in rough water last week the cap came off and oil splashed onto the engine a little bit. I've cleaned this off and it runs fine, but I've had that cap come of twice now in the past week again. I've purchased a new cap and cleaned the valve and the slot real good, but it came out again anyway.

The cap doesn't seem to sit very tight, neither the new nor the old one (they act the same). Any ideas? The reservoir is only about 1/2 full. Is there another valve somewhere in the system that's bad and causing the problem? The engine appears to be using the normal amount of oil, based on how the tank has gone down.

I thought about getting a real small o-ring to increase the friction on the cap just a little bit. I'm at a loss, otherwise.

Re: Merc 125 2+2 Oil Reservoir Relief Valve Trouble

That's just a vent. The little bladder thing helps keep oil from leaking out when the tank is full and the motor is tilted, though it just slows it down a little. If neither cap stays, the problem is the tank. Drill a tiny hole sideways through the tank and cap right where a wire placed through the hole will retain the cap (about 1/16" from the top, just below the cap head).

Re: Merc 125 2+2 Oil Reservoir Relief Valve Trouble

Great idea. Thanks for the input, that should be easy enough!