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I have a 1999 90 hp 2 stroke that leaks out the oil tank fill (when tilted up). I would like to fix this but there seems to be no fix.
Mercury for sure knew about this problem the first year this style of oil tank was used yet they continued to make it this way for many years.
This seems to be blatantly irresponsible to not recall this poorly designed tank and replace with a non leaking design. It is at best a big mess for every owner of these
motors with this tank design. At worst it is environmentally terrible ( oil in splash well and then into water). It also makes motor boaters look bad to others.
I am by no means an environmental nut but this is inexcusable. I am really surprised no one has brought about a suit for Mercury to fix this.
And they wonder why people are buying foreign brands of outboards? This is just the kind of thing environmental extremists point to when wanting to ban motor
boats on lakes and ponds as well. Mercury really dropped the ball on this one and showed just how little they are concerned about the environment and there
customers. They would do very well to even at this late stage acknowledge and correct this problem. They are a marine and american icon,
I would like to see them stay that way!

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Why not just replace the tank and cap. What have you done to fix this? Have you contacted a certified tech or Mercury customer service? Just trying to get a feel for your problem and what you've done to fix it.

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My 1994 90 hp Merc had the rectangular style door on the tank and it was totally worthless. I replaced it with the one with the round fill neck and the expanding O-ring on the plug. The only thing I added was to put a hose clamp around the fill neck on the tank so that it could not expand when the cap was tightened. I also fill the tank to the bottom of the fill neck with the motor tilted and check the oil level more often. This has worked out fine for me so far. No more sloppy mess.

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If I could replace the tank and cap with new ones, and not have it leak, I would do so gladly. My local Mercury dealer says that they all do this (leak),
and buying new ones will not correct the problem. The best solution I have found is to keep very little oil in it. The problem with that is you are always needing to
add oil. I just wish there was a real fix for this. If there is I would love to know what it is.
Thanks for the replies, any further help would be appreciated as well.

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Auburn

Have you inspected the cap and o ring? Given it's age, replacing the o ring and cap may solve the problem.

I would visit the parts manager at the dealership, not the service desk, and ask them to look up the tank and cap on the computer and see of it the same or of it was changed. It would be worth it to change the tank

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It is a sad situation, and I have seen it many times. I have seen at least 3 different designs used my Merc, and they all sometimes leak. It's very similar to the leaking air boxes on many engines...leaving an ugly mess in the splashwell every time you tilt the engine up to the trailer postion. No excuse for this, just poor design.......and I've seen this on foreign brands as well, it's not an exclusive Merc problem.
A new tank and cap assembly will probably solve the problem for awhile, but you have to be careful not to overtighten the cap.
Ditching the oil injection and premixing does eliminate the issue...in lieu of that I have tried carefully stuffing some oil absorbing pads under the airbox such that it will collect any oil/gas dribble out around the airbox, and a minor oil cap leak.

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My Yamaha 40 2 stroke with the under cowl oil reservoir never leaked a drop.

My Merc (1996) 75 2 stroke leaked often. Odd, but not always. I could not figure it out. Sometimes it leaked. Sometimes it didn't.
I replaced O-ring,, and same results.
Just a bad design. I actually sold the motor and got a 90 4 stroke because of the aggrivation!

FISHNFF

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I had the "drippy carb" problem along with the leakey oil cap when I first got my boat and motor. I drilled and tapped a 1/8" pipe thread in the bottom of the air box cover. I installed a 90 degree, barb fitting into the hole and ran a piece of clear oil injector tubing from there out through the lower cowl into a small plastic bottle in the splash well. This caught the oil that dripped from the carbs and ran to the bottom of the airbox when the motor was tilted. Since that time I have learned that if you run the motor at an idle for a few minutes after each trip before tilting it up the "drip" from the carbs is reduced to practically nothing. I still keep absorbent pads under the bottom of the air box cover but have not had to change them since I made the modification. This plus the clamp around the oil tank fill neck as explaned in my previous post has solved the "sloppy mess syndrome" to my satisfaction.

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The reason the oil tank caps leak is because people over tighten them. They only need to be seated loosely to not leak. I was warned of this by my Merc dealer when I bought them.

I have twin Merc 90's and they don't leak.

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I would bet the one he has that is leaking is the square type lid that snaps down, not one that screws down. I had the same thing on my 1996 100 hp merc. when the level warning sensor went out on the tank I had replaced with the tank mentioned above with the screw down top. it even has a dip stick attached.

I can not help with who made the tank right now but if you can not find any info on the replacement tank I will be glade to run by the shop and get you some info. send me a e mail

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I had a leaky cap this year, Changed the O ring and no problem. Like Finnegan said don't over tighten - 1999 90hp merc.

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I have twin 2003's on my Revenge, when I bought her, I wondered why the previous owner didn't tilt the motors up all the way, I quickly figured out why. I quickly bought new o-rings for the caps and actually doubled them up on the caps, since then, no leaks.

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My 90 is an '05 model with the screw-in w/dipstick cap. Never a leak.(yet)
Mike

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My 90hp 2002 leaks oil also. The dealer gave me a new one, but I haven’t replaced yet because I haven’t figured out where the oil is coming from. Is it possible oil could leak into the carburetor and then go through the air box?

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where is the best place to get the new Orings?

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I got my o rings straight off the mercury website... don't remember the web address though.

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For a fix to the leaking oil tanks & caps on Mercury outboard 2-strokes check out www.ebay.com for "Merc BUDDY". You can also go to www.robbco-llc.com for more info on this solution. Solves the oil leak problem - provides for a better way to add and check oil and provides for a backup in case you loose your cap & dip stick in the water.

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I see I responded to this thread 2 years ago just after I replaced my O ring. In a short period of time my cap did start leaking again. So I added a zip tie instead of a hose clamp and it has not leaked since. Almost 2 years.

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I have learned so much from some of these sites I figure it's time I helped someone else out. I am fairly new to boating but feel mechanically inclined to attempt to fix most reasonable issues. I have had both the leaking oil cap and the leaking carb when tilting the engine up. About the leaking oil from the cap when tilting, my fix was a simple fix from home depot called a Fernco 2" PVC cap. It's black rubber and has a adjustable clamp. Works great no leaks. Need a screw driver to loosen and tighten but I can top off the tank now and not have to keep looking to add, adding all the time. Problem fixed. Great 2 dollar fix.
I also have a leaking bottom carb on a 4 cyl 1996 115hp merc. I wanted to say, that is a great fix by drilling a whole in the silencer and running a tube to a jar to collect the gas/oil when tilted. I haven't done it yet but my bottom carb unloads everything when tilted up. I know the problem and know it's a pain to fire back up once this happens, whether it's hot or cold and playing with the choke and bulb. I'm always fishing low water and drifting so I like to keep the motor up some if not all the way. I know what to do now after calling sea tow once and researching the cause but you are right about trying to explain it to anyone else like family etc when this problem occurs. If I'm not aboard there's going to be a problem LOL. I was thinking about rebuilding the carb but seems I hear and read this all to often and may be wasting my time and money. Think I may just go ahead and try that fix and keep a jar under the seat. I hear it is a big fine if they see oil or gas hovering around you engine.. They should be fining Mercury instead from all the pollution this issue has created. Thanks for the tip. Hope I helped as well on that oil cap issue.