Lower unit oil overflowed out of water intake vent. Problem?

Hi, I'm a newbie outboard owner servicing my Yamaha T9.9 high thrust/long shaft.

I have the Yamaha service manual and was moving smoothly through all the procedures, until I got to the section about changing the gear oil in the lower unit.

I removed the drain and check screws and drained all the gear oil, then misread the manual. Instead of leaving both screws out while refilling the oil, I put the drain screw back in, then attached the pump to the check screw hole, and started pumping in gear oil, using the pump sold to me by my Yamaha dealer.

It was hard to how much oil to put in, but I figured if I put in a bit too much, it would drain out of the check screw hole when I removed the pump.

To my surprise, gear oil started coming out of the water intake vents. I stopped pumping, let the overflow drain out, and then put the check screw back in.

My question:

It doesn't make sense to me that the gear oil reservoir and the water intake vents are connected, so as to allow gear oil to overflow out of the vents. How could this happen?

I don't think the little pump I used could have created enough pressure to blow a seal, but that's what I'm worried about.

Any help, much appreciated.

Gear oil is always filled from the bottom hole till it comes out the top hole.------------So run to your favourite repair shop and ask for a pressure test on the lower unit.-------If this is OK then fill the lower unit with oil.

racerone said:

Gear oil is always filled from the bottom hole till it comes out the top hole.------------So run to your favourite repair shop and ask for a pressure test on the lower unit.-------If this is OK then fill the lower unit with oil.

Click to expand...

Ahhhh... fill from bottom hole. Now that makes sense. Thanks.

Okay, so I built a pressure tester this morning and tested the lower unit.



It's not under pressure in the photo above. I was just taking a pic of the set up. The tester itself is air tight and can hold 15psi if it is not connected to the lower unit and the end is just plugged off.

Now, according to the Yamaha manual, the lower unit is supposed to hold 14psi for 10 seconds.

When I apply 14psi to the unit, it gradually decreases to 8psi and then holds. It takes 5-7 seconds to go down from 14 to 8.

Question: is this good enough, or do I need to replace some seals?

Thanks: John

Just tested an OMC 50 HP lower unit.-Held 5 PSI overnight.------------Tested a 9.9 OMC a while back and it never lost pressure for 4 days.---------I think you need to look at the seals, perhaps they have shifted position----Very good work to make up your own tester !

Thanks. I'm not feeling very clever after making this mistake, but I figured I'd need a pressure tester to fix the problem, if it needed fixing. My local plumbing supply house had all the parts for $30.

Well, I guess I'm going to pull the lower unit and see what there is to see. Thanks!

I've been studying the manual and I think what I probably did was push the two oil seals that go around the drive shaft, up a bit. This allowed the oil to flow into the water pump and out through the water inlet vents.

So, that at least makes sense. I'll pull the lower unit tomorrow. Maybe I can just put the seals back in place and that will do the trick. Worth a try, anyway. It doesn't look too difficult.

Just to close this thread off for anyone who finds it in the future...

I dropped the lower unit and disassembled the water pump so I could see the main oil seals around the drive shaft. After pressurizing the gear case and brushing the seals with soapy water, I could see that the oil seals were indeed leaking.

However, the gear case was still holding pressure at 10psi.

I visited my local certified Yamaha mechanic to see if he had the parts I would need to replace the oil seals. I explained what I did, and he asked me if I vacuum tested the gear case. I said, no, I'd only pressure tested it, as the official Yamaha manual had called for.

He told me that the manual had been mis-translated, and instead of pressure testing the case, you really needed to vacuum test it. This is because the oil seals are all meant to keep water out, not oil in.

In other words, the seals around the drive shaft were there to keep the water in the water pump -- which is under a positive pressure when the motor is running -- from being pushed down the drive shaft into the oil. So the seals are oriented to seal in that direction. The gear case never has any pressure in it, so oil can never be pushed up through the seals, unless you are dumb enough to pump it through the seals, as I did.

He vacuum tested the case for me and it held 14 psi no problem.

He also said that most shops don't bother to do any testing at all, unless there is water in the gear oil. No water, no problem.

Bottom line: my seals are fine. I reassembled the outboard, and it is running fine. Case closed.

When you shake a pop bottle you get pressure.---When a gearcase is operating you have pressure.--When a gearcase sits in the sun you have pressure built up in it .--Case closed.