Lower unit leak

[brielly1]

I have a 96 yamaha 250 hp S50turu There was a small bit of water mixed in the gear case at the end of the season. I just pressure tested and vacuum tested. It held 10 psi of pressure steady for over 10 minutes. I sprayed soapy water on it as well and no bubbles. The vacuum test on the the other hand.... it really couldn't get above 5 on the gauge and I could here hissing from the prop shaft seal. I'm assuming that seal needs replacing but I want to be sure before tearing in to a major job like this. Any advice or repair tips wold be greatly appreciated.

[pstephens46]

I have a 96 yamaha 250 hp S50turu There was a small bit of water mixed in the gear case at the end of the season. I just pressure tested and vacuum tested. It held 10 psi of pressure steady for over 10 minutes. I sprayed soapy water on it as well and no bubbles. The vacuum test on the the other hand.... it really couldn't get above 5 on the gauge and I could here hissing from the prop shaft seal. I'm assuming that seal needs replacing but I want to be sure before tearing in to a major job like this. Any advice or repair tips wold be greatly appreciated.

You have specs for a vacuum test for that motor? My F150 manual only specified pressure test. BOSCOE!!!

[brielly1]

Unfortunately I don't have the specs. I'm moving in a few days and my manual is packed away along with most of my tools.

[pstephens46]

It held 10psi for 10 minutes!!!!

Maybe a bad gasket?

[scofflaw]

Nothing major about changing out the prop shaft seals. Forget about pulling the carrier, just R/R seals only, take less than an hour. Long 1/8" drill bit, deck screw, vise grips, hammer and a pc. of pvc tubing the proper diameter

[pstephens46]

It held 10 psi pressure for 10 mins according to OP. It that test was done correctly there should not be a problem. The vacuum test is the one I am curious about. Did he apply excessive vacuum and damage the seals?

If the pressure testing was correct, where did the water come from?

I would pressure test again (no vacuum test) and see if they will hold 10 psi for the same amount of time. If it passes, change the oil and gaskets, and run the hell out of it for a few hours. Dump the oil and see if there is water intrusion.

Changing the seals using the hack method is risky. Nick the prop shaft and it is toast. I took the risk and was successful, btw. He may not need new seals.

[scofflaw]

It held 10 psi pressure for 10 mins according to OP. It that test was done correctly there should not be a problem. The vacuum test is the one I am curious about. Did he apply excessive vacuum and damage the seals?

If the pressure testing was correct, where did the water come from?

I would pressure test again (no vacuum test) and see if they will hold 10 psi for the same amount of time. If it passes, change the oil and gaskets, and run the hell out of it for a few hours. Dump the oil and see if there is water intrusion.

Changing the seals using the hack method is risky. Nick the prop shaft and it is toast. I took the risk and was successful, btw. He may not need new seals.

If water is in there, reseal the whole unit, shifter included. $60 bucks in parts and a couple of hours of work. Nothing hackish about that method, unless of course you are a hack. Done several, them seals pop right out of there.

[pstephens46]

Could be a $1.50 oil drain gasket too.

[brielly1]

There seems to be conflicting info on the web whether or not the vacuum test is effective due to the orientation of the seals. I pressure tested again today bringing it up to around 13 psi. Held fine and again no air bubbles with the soapy water. There wasn't much water in the LU, just some very slight streaks of tan color mixed in with the blue. The prop shaft seal was replaced 4-5 years ago and the driveshaft seals look to be in good shape. I'm hoping it was just the plug gaskets.

[TownsendsFJR1300]

Did you DRAIN all the oil before pressure testing?

The LU must be dry/empty of all oil.

10 psi for 10 seconds, DRY, is acceptable...(see below from my Yamaha service manual)

Did you spray the shifter shaft, the seals under the WP as well as the 0-ring that seals that part under the WP impeller?

Did you spray the prop carrier large o-ring as well?


I have found the drain plug seals DO tend to loosen up after initial installation. Re-tightening, IME, does usually get a little more torque on the screws..

On the upper driveshaft seal, (visible after the wear plate is removed), is the small stainless steel TENSION SPRING still there?

I gather you looked closely for any fishing line in the prop seals as well?

[pstephens46]

There seems to be conflicting info on the web whether or not the vacuum test is effective due to the orientation of the seals. I pressure tested again today bringing it up to around 13 psi. Held fine and again no air bubbles with the soapy water. There wasn't much water in the LU, just some very slight streaks of tan color mixed in with the blue. The prop shaft seal was replaced 4-5 years ago and the driveshaft seals look to be in good shape. I'm hoping it was just the plug gaskets.

I am thinking you are good to go. Just change it more frequently to reassure yourself in the short term. Oil is cheap.

[fairdeal]

There wasn't much water in the LU, just some very slight streaks of tan color mixed in with the blue.

that doesn't sound like a problem.
When you really have water in the LU, the gears emulsify it with the oil
and it looks like this:

[99yam40]

yep, my guess would be a little left over in there hiding got mixed in maybe.

[scofflaw]

He gets to 5 lbs on his vacuum gauge and hears hissing at his prop shaft seals.

The advise he gets here is change his fluid....WTF really?

OP...use some common sense

[99yam40]

all of the vacuum gauges I have seen were in inches of mercury not pounds or PSI as that is not a vacuum reading.

OP was looking for info if a vacuum test for his lower unit was a good test because of the seals orientation.
Did you answer that for him?

I do not know myself, but I would not think so if the seals are both in the same direction