Gear case 1987 Yamaha 70BET 6H3

[lelesaita]

Hi all,
yesterday I tested the gear case on my Yamaha. It failed the leakage test: should hold 730 mmHg for 10 secs but i cannot even get to that pressure. Maybe it's the homemade tester (I got the idea from a YouTube video) but I could not hear any air leaking from there. Is it worth going and check the water pump, gear and prop sealing? In what order, given that it takes time and money? Prop first?
The old oil that I drained looks old but no water in it.
Thanks

[robert graham]

The first thing to check/replace would be the drain plug washer and the vent plug washer.....then remove prop and visually inspect prop shaft seal for fishing line around it causing seal leakage and/or seal damage. The other seals are the drive shaft and shifter shaft seals....if old drained oil isn't milky looking then maybe no water in it? It's a good idea to drain/refill lower unit yearly at the end of the season so fresh oil will be in there during lay- up period...

[TownsendsFJR1300]

Although most testing requires vacuum testing as you did, I would first remove the prop and drain the oil.

Then put 10 PSI pressure INTO the LU and spray soapy water at the prop shaft seals looking for bubbles (leaks).

If none there, remove the LU, the WP and access the shifter shaft and drive shaft.

Do the same with pressure. You'll see major bubbles in one place or another. You've found your leak

[pstephens46]

Motor so old probably not a bad idea to replace the seals no matter what. Buy a Mity Vac for 50 bucks....

[almetelo]

Hope this can help

https://www.youtube.com/watch?v=RD4u38pbBV0

[panasonic]

I would not go looking for trouble where there isn't any. If the oil in the LU is water free...great, all is well, seals are not leaking. Keep clean fresh oil in there.

I think you're homemade tool maybe at fault for the failed test.

Now if you are going to drop the LU and inspect/replace the water pump then this might be a good time pressure test the LU and check the driveshaft and shift shaft seals. But then again why? NO water in LU...NO problem to deal with.

my 2 cents

[99yam40]

OP are you sure the manual calls for a vacuum test?
Yamaha lower unit seals are set with both seals on each shaft in the same direction to keep water out, so they may not hold well when you are pulling a vacuum.
Double check what the service manual calls for on your motor

[walleye1]

Psi test is all you need, I test them at 14 psi for 5 mins.