pressure testing lower unit

[blackboat]

Correct me if wrong here but doesn't pressure testing only diagnose half of the possible problems? If it passes the pressure test,yeah...it will not leak oil. What about the other half which is water coming in? Seems to me that a vacuum check would also be in order. There are two seals on the shafts,one facing in which holds oil in,one facing out keeping water from entering case. Every LU I have checked that was full of water,there was no oil leakage whatsoever. So...pressure testing if the LU has water in the lube is basically meaningless if this testing is what I understand it to be.

[jtracc]

I was wondering the same thing. I just noticed the other day I have some water in my lower unit. However, there is no leak what's so over. I will have to have it looked at.

[EuropeanAM]

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While many other manufacturers have recommended vacuum testing also, Mercury traditionally does not due to single-directional seals in some of their cases.

MOST leak scenarios can be replicated by testing at various pressures (example, 2, 5, 9, 11 psi) and rotating all shafts while at each pressure.

[Reel Life]

I always use a mini-vac to test my mercury V6 gear cases to determine leak tightness. 90% of gear cases leak by sucking water in as they cool after a hard run and they heat up. The seals on the propshaft and driveshaft cannot be properly evaluated if you don't check them under vacuum because there are 2 seals... each providing isolation in the opposite direction. Them only sing le seal is on the shiftshaft, which has very limited movement.

[Bamaman]

Testing the seals on lower units is a maintenance job that's so often overlooked. I'm not saying you need to have the pressure test every year, but you don't need to ignore the lower unit indefinitely. They are very expensive to rebuild. And it's really a job for a professional.

[06 SB]

Pressure testers are not expensive and easy to use. It allows me to check things at home to see if there is a problem vice it sitting a dealer to get around to it.

[revlimiter]

I always use a mini-vac to test my mercury V6 gear cases to determine leak tightness. 90% of gear cases leak by sucking water in as they cool after a hard run and they heat up. The seals on the propshaft and driveshaft cannot be properly evaluated if you don't check them under vacuum because there are 2 seals... each providing isolation in the opposite direction. Them only sing le seal is on the shiftshaft, which has very limited movement.

Exactly. I always wondered why the positive pressure test was the only one done. I generally don't worry about the seals though if there's no water in the oil. That's the true test right there.

[blackboat]

I always use a mini-vac to test my mercury V6 gear cases to determine leak tightness. 90% of gear cases leak by sucking water in as they cool after a hard run and they heat up. The seals on the propshaft and driveshaft cannot be properly evaluated if you don't check them under vacuum because there are 2 seals... each providing isolation in the opposite direction. Them only sing le seal is on the shiftshaft, which has very limited movement.

Sucks water in when cooling...makes sense. If the LU has water in it,IMO, pressure or vacuum tests are never going to be conclusive. Do the tests,yup,it's leaking. You already knew that.Really hard to determine exactly where the leak is by these tests. Best bet is to replace all the seals and gaskets,drive shaft-water pump,prop shaft,shift shaft. I bought Merc knockoff tools for the prop and shift shafts for cheap that work great. Easy enough to do if you're halfway mechanical.

[EuropeanAM]

Really hard to determine exactly where the leak is by these tests. Best bet is to replace all the seals and gaskets,drive shaft-water pump,prop shaft,shift shaft.

A spray bottle full of "kiddie bubble mix" (or real soapy water) can be sprayed on every sealing area and quickly allows you to locate the lion's share of leaks.

This is important as not all leaks are going to be solved by replacing every seal in the gearcase... some come from worn shafts, bent shafts, distorted carriers, or worse yet- a Pitot Passage that was left plugged, FROZE, and burst into the lube area.

Now that last one is a real booger to find... and it happens more often than folks realize.

[blackboat]

Dang it Don! Something new to worry about. Frozen pitot passage...never would have thought of that. Could one assume that if the water pressure speedo is working,the passage will drain and not freeze?

[EuropeanAM]

It should- provided that it's not plugged/blocked.

[RickCamilleri]

I was wondering the same thing. I just noticed the other day I have some water in my lower unit. However, there is no leak what's so over. I will have to have it looked at.

I've used a simple bike pump, with a blue angled adapter. attach to that the tube you use to fill your hgearcase pup. I found a shift shaft seal leak in my previously owned 150 optimax, and this year i found bearing carrier seal leaks on my 200 EFI. its a handy lil tool and cheap!

[EuropeanAM]

I've used a simple bike pump, with a blue angled adapter. attach to that the tube you use to fill your hgearcase pup. I found a shift shaft seal leak in my previously owned 150 optimax, and this year i found bearing carrier seal leaks on my 200 EFI. its a handy lil tool and cheap!

Good job making use of what you have. This would work where you are certain that there is a leak... a gauge-type setup is useful when trying to determine that there is, or is not a leak at all.