25hp 2 stroke lower unit leak, need advice

[rosscopeeko]

8113DA45-5611-492B-B335-0266169CEDE5.jpeg My motor is a 2004 Yamaha 25hp 2 stroke 2 linder. The motor has less than 100 hours on it and is in excellent condition. Today I went to change the lower unit oil after I ran it on muffs with the garden hose. Ran mint and pumped water drained the lower unit oil and it was murky. I removed the lower unit and prop, water pump and attached my bottle pump onto the vent port on the lower unit. Fill plug was in tight. I used my lungs and couldn’t get any of the shaft seals to leak. Researching a online I found out I was lucky to get 3psi through the lower unit. I could hear air leaking and saw bubbles coming up through where one of the water pump bolts go. The arrow in the pic shows where it is slightly leaking. What’s going on here?

[pstephens46]

Guessing part 22. If I am looking at your pic correctly. Looks like a gasket. Referred to in the diagram as lower casing packing. Another odd term for Yamaha....

[TownsendsFJR1300]

Plus one ^^^.


Many (not all) units are pressurized with 10 PSI and must hold it for 10 seconds, No oil in the LU when testing.

[rodbolt17]

and you have to have the housing bolted back in place. would be like pressure testing with the vent plug out.

[rosscopeeko]

Thank you so much for the replies. I bolted the water pump housing back up to the lower unit and used my vacuum brake bleeder to bring it to -10psi. Over a 1 minute period I lost .5psi. I didn’t have my new impeller installed, just the o ring, water pump housing etc. Anyone know what to torque the water pump bolts to? I just went until snug. The lower unit has no oil in it right now. My cheap vacuum brake bleeder may have a slight leak too, leading to me losing .5psi in my 1 minute test.

[99yam40]

Is that a negative 10 psi?
I would not think so because vacuum is measured in Inches of Mercury, but thought I would ask.
testing your tester may help you decide where the leak is located.

spraying with soapy water and watching for bubbles should help also

[rosscopeeko]

Here’s the setup I used. I think you’re correct it’s in mm of mercury. I took it to the outer 10. There is mm and in.

[rosscopeeko]

I haven’t changed the lower unit oil for a few years. The boat was only used maybe 3 hours over this time. Poor maintenance I know. Busy Dad life here. I have always cheaped out and never replaced the crush washers on the lower unit fill and vent plugs. I’m going to go and buy new washers, replace the impeller, and try to rig up a positive pressure system for testing, instead of my negative pressure setup. The oil in the lower unit was a little milky, not terrible, but it’s the first time I’ve noticed water in it.

[rosscopeeko]

Here’s the wrap up summary to help those in my situation. I went out and bought the mityvac 8500 kit and did the pressure test as per the Yamaha service manual. 14.2 psi for 10 seconds. Passed. Thx rodbolt for informing me to have the water pump installed. No bubbles present. I must have not had the lower unit bolts tight enough or the gasket was bunk. I did install new gaskets last time I did a fluid change. I remembered that after I reviewed my boats.net order box. My wife and I are doing a remote boat access camping trip in 5 days. I’ve checked and cleaned my spotless carbs, changed my impeller, new plugs, checked my lower unit seals, new fluid, and I’m going to adjust my air fuel screws tomorrow and then set my idle if it changes after my adjustments. I’m running a Yamaha mini water fuel separator with 50:1 AMSOIL mix and ring free. 45km in and 45km our with a camping load to support 4 nights, plus fishing and exploring. I’ve done this trip with Yamaha 2 strokes numerous times, and I trust them like no other. Thx for your help.

[99yam40]

before heading out on a long run like that ,I myself would want to take it out and run if for a good hard run to verify all is well.

but that is just me.
I hate it when things go wrong and ruin a trip.

[rosscopeeko]

I’m heading out tomorrow. I live in a city with 2 rivers. If I want to lake rest it I’m driving 1 hour. I think I’ll do that because the still water will help to adjust my carbs.