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Losing Drive Oil

2060 Views 39 Replies 11 Participants Last post by  man-of-war
I can't figure out where my drive (gear lube) oil is going. I have filled it up twice only to have it empty. I can't see it leaking anywhere obvious. Any ideas? Bravo 1 by the way.
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I just had the same problem. Mine was the little orange rubber pressure valve in the top of the cap. I changed it for a new one and problem solved.
Could it be leaking out the drive seals into the water !?
Bop, did you by any chance change the OD oil then this happened? I recently changed my drive oil and that reservoir bottle was down around an inch below that line that says DONT OVER FILL.

Philcat said:
Could it be leaking out the drive seals into the water !?
Good point, but Bop an old captain I'm sure that's the first thing he checked,looking for that haze of oil on the water :confused:
I did see some haze on the water but not much. How do I check the seal?
Leud Conduct said:
I just had the same problem. Mine was the little orange rubber pressure valve in the top of the cap. I changed it for a new one and problem solved.
That would cause it to leak out the top of the resevoir/cap wouldn't it? There is no oil inside the boat that I can see. I did however loosen the screw on the bottom of the drive to check the oil before I put it in the water. There was oil at that time. Could there be a hose loose to the drive and when I trim it up it leaks out? The only reason I say that is it only seems to leak when I go out in the boat not at the dock.
Check the torque on your top cap bolts on the drive. Sounds like it is leaking out only when pressurized. Also did you replace the o-ring on the drain plug when you removed it? Good luck hope you find your issue.
Had this same problem was loosing about qtr cup a day when in use, real thin film on water when I put on dock. Turned out to be the wire tie on the hose barb on water side of outdrive, tie had worn off, replaced and loss of fluid stopped.
If you don't find any external leaks there is one place the drive oil can leak and you'll never see it. It can leak into the boot that houses the u-joints. There is a seal just aft of the u-joint that is like a front engine seal. The seal gets worn and sometimes simply pushes out causing a slow leak of drive oil into the boot area.

Good luck!
Have you changed your drive oil this year? If you did and took the drain plug out from the lower unit or the plug in the middle to vent you could have damaged or lost the o-ring or some have a gasket in place of the o-ring, there are 2 different styles.
Here a theory......

you loosened the screw on the bottom to check the oil (not sure why you went about it that way, but thats another thread). when oil came out, some metal shavings and small chunks started to come out with the oil. You may have tightened the screw up on a piece of metal not letting that small gasket on the drain plug re-seal.

I'd change the oil, replace both gaskets on the upper and lower screws and go from there.
Agree with Mark... I always changed my drive fluids every year or between 50 to 100 hours... I never wanted to loose a drive, preventative maintance I guess, that gear oil is expensive though....woo..

Two summers ago I ended up changing it 3 or 4 times since I had a leaky seal/drain plug and some water got in there... not good.
I'm going to pull it out of the water tonight and we are going to check and replace the gaskets Wednesday night. Hopefully that will fix it. Before I do that I will check the drain plug to make sure the 0-ring is still there and functioning properly though. I sure hope this is a small problem and not a huge one. Thanks for everyones help!
sometimes the oring will stick on the outdrive side....not the screw side. make sure you dont double them up cause you think its missing. if its on the drive, take it of and put it back on the screw before reinstalling.
There are two kinds of drain/vent plugs, and two kinds of plug O-rings.

The drain/vent plugs with the short threads and long magnet are for Mercury I/Os (all Bravos).

The drain/vent plugs with the long threads and short magnet are for Mercury OB motors.

If you inadvertently put an OB's plug into a Bravo drive, it WILL leak. I had a shop screw this up on mine, and lost drive oil because of it.

Also, make sure the 6 nuts (3 each side), and one bolt (under the aft anode) of the joint between your upper and lower are tight. If these are loose, you can also lose fluid. The studs for the 6 nuts can also come loose over time.

I have used the little blue O-rings and the Yellow/Silver O-rings. They both seal the same, BUT, the Blue ones have a tendency to tear apart when you are removing them. MAKE SURE you get ALL of the old O-ring out of the drain and vent plug openings before you insert the plug back in with an O-ring.

This is about all that you can to check without removing the drive. Your next step would be removing the drive and having it pressure tested. You cannot pressure test with the drive fully installed on the boat. You can pressure check if you loosen all of the drive mounting bolts, and pull the drive out about 1/2 inch. This will allow the ball valve in the drive to press out and keep pressure.

Good luck.
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I wish I could remember who gave me this tip to give them credit. Go buy an extra reservoir cap and a tire valve fitting that has o-rings from a tire store. Drill the cap out and install the fitting. Now put that cap on your reservoir and use a hand pump to pump it up to abot 5 lbs. Now look for leaks.

I found a u-joint seal leak this way after scratching my head for weeks. Took awhile for that one because the bellows have to fill up. But you can quickly check the reservoir,drain and vent plugs, prop seal, etc using this method.


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The person that surveyed my boat last fall, had a contraption like that for pressure testing it and pumped up up wiht a hand pump, it looked like it worked to me....

I think it was All States Marine, and the owner/surveyor was TOP NOTCH... Although I think he came from NJ and I am sure he charged appropriately.
Found the leak. What is happening here? Tell me this isn't horrible news please.


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Looks like the o-ring seal on your back cap could be gone. Drain out your lube and pull the back cover off. Check it for cracks also while you have it apart. If you do have a cracked cap let me know I think I have one kicking around. That bolt is not loose correct?
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