Whaler Central - Boston Whaler Boat Information and Photos

Posted by iankeller on 09/14/09 - 7:30 PM

I have a 1999 Mercury 90hp that's leaking oil from the cap when the engine is tilted up. The cap doesn't seal well against the neck of the tank. I searched these forums but didn't find any references to this problem.

Any thoughts on this besides replacing the tank? Maintaining a low oil level sounds like an invitation for disaster. Replacing the tank is an option, though perhaps a bit expensive (I'd love to know how much if anyone knows). Can the tank be refitted with a different top? Any suggestions welcome.

Edited by Joe Kriz on 07/29/16 - 7:03 PM

Posted by JohnnyCW on 09/14/09 - 8:11 PM

My 1990 115 Merc leaked oil from its tank since new. I had the tank and cap replaced several times. Eventually I just kept the level a little lower to avoid the mess but still about 3/4 full. With my motor that was enough oil for several tanks of gasoline.

Posted by number9 on 09/15/09 - 12:20 AM

Might want to try some self-fusing silicone tape on the filler neck threads. It should also prevent the cap from over tightening itself mentioned in the link you found. Read it's now available at Harbor Freight. Google it for more info.

Posted by awayland on 09/15/09 - 5:12 AM

I have the same motor and it does the same thing. I wouldn't put anything on the threads in fear of it dropping in. I read once that someone else had the same problem and just replaced the O-ring on the cap. that fixed it for them. I think the problem is the walls of the tank filler neck are so thin and flexible, that if that O-ring shrinks a little it's going to leak. I just keep the level lower than 3/4 and will get a new seal in the spring.

Posted by Tom W Clark on 09/15/09 - 8:49 AM

I have had the same motor and the same problem. As a workaround, I did not fill the tank all the way.

It is an almost universal defect among the Mercury motors so equipped. Just a lousy design.

The best thing you can do is replace the cap and seal, and do NOT over-tighten. It may still leak.

Posted by iankeller on 09/15/09 - 10:19 PM

Mine leaks like crazy when it's tilted up for trailering. I think it would have to be way less than 3/4 full to avoid leaking then, especially when the oil sloshes forward when I step on the brakes. To those of you who keep your tank partially empty, are you trailering it that way too?

Also, has anyone tried putting a hose clamp around the neck of the tank where the cap is seated? If the neck were under compression it might help to maintain a good seal and prevent deformation of the plastic. Any thoughts on this? The way mine leaks now I think I have nothing to lose by trying this.