WHY VACUUM TEST THE LOWER UNIT?

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dthx

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I've replaced the seals in my lower unit.
I can see why to pressure test...presumably the heat causes expansion and then pressure...
but why the vac test?
 

Scott Danforth

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many times the seals may hold pressure, however not vacuum. when your sealed gear case is warm and the water is cooler, as the air and gear oil inside the case cool, the case is at a slight vacuum. if the seals leak just a bit, it will suck water in past the seals that may hold pressure only.

hence the vacuum test.
 

dthx

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That would only work if you left your boat in the water with the lower unit down after running the engine, right?
If you load your boat and get it out of the water right away, there would be no water to get into the unit due to cooling.
Do you agree...or am I wrong?
 

GA_Boater

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What if you run the boat to your favorite fishing hole and drop a line for even a few minutes? The water cools the lower unit in short order.
 

Vic.S

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That would only work if you left your boat in the water with the lower unit down after running the engine, right?
If you load your boat and get it out of the water right away, there would be no water to get into the unit due to cooling.
Do you agree...or am I wrong?
The trouble will be when you launch your boat on a warm day into cold water the air and oil inside will contract creating a slight pressure drop in the gearcase. If the seals don't hold a vacuum then water will be drawn into the gearcase.
 

dthx

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The water where I fish is 65 deg F at present....spring fed lake runoff....very cold.
So water could be drawn into the gearcase thru the prop shaft seal...or the shift rod bushing or the drive shaft seal....because water is common to those seals.
That would be a vacuum leak.
Then to leak out of the weep hole.... both or either the shift rod seal and/or drive shaft seal would have to leak, letting oil and water (water that has entered the gearcase thru the prop seal or others) to come into the casting gullies common to the weep hole.
I would have to have a vac AND pressure leak......
Do you follow?
 

Vic.S

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The water where I fish is 65 deg F at present....spring fed lake runoff....very cold.
So water could be drawn into the gearcase thru the prop shaft seal...or the shift rod bushing or the drive shaft seal....because water is common to those seals.
That would be a vacuum leak.
Then to leak out of the weep hole.... both or either the shift rod seal and/or drive shaft seal would have to leak, letting oil and water (water that has entered the gearcase thru the prop seal or others) to come into the casting gullies common to the weep hole.
I would have to have a vac AND pressure leak......
Do you follow?
You could easily have a seal which leaks both ways. Allows water in when you plunge it into cold water. Allows oil to leak out when it heats up in the sun..There may of course even be one seal allowing water in and another allowing the oil to leak out.
This is why pressure and vacuum tests are carried ou

For me stopping water entering is the more important because it would be salt water. Losing a little oil is not so important because it can always be topped up.

Dont forget the seals on the level and drain screws can also leak.
 

dthx

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Yes,,,,,thank for taking the time to reply and help me.
I understand all of what you are saying.
I will definitely do a pressure and vacuum test, somehow......the pressure test is easier than the vacuum test because of the equipment involved.
There is a prop shaft seal in a bearing assembly in which the seal is not sold separately.....could be the culprit.
The Assembly is $158.00 and I'm told that I can remove a washer to get to the seal.....and buy the at a seal house
I've seen the assembly and I'm not quite sure how to access the seal......dont want to damage the assy.
I'll eventually tackle it but I thought I'd as you if you've ever done that on a '76 Johnson.
The person on the forum that told me that originally ...I think has given up on me.
Thank you for the time it took to answer me.
 

Scott Danforth

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get a mity vac 8500 and pressure test and vacuum tests are easy peasy
 
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