Vacuum Test vs Pressure Test for '82 Mariner Lower Unit?

[minuteman62-64]

Got some water in lower unit issues w/1982 30 HP Mariner. Replaced the gaskets on the fill and vent screws and plan a run to see if there is still a problem. If there is, time for testing/remediation.

A couple of questions.

1. I've read a lot of posts that advise both pressure and vacuum testing to diagnose problems and verify an adequate repair. On my motor there are two seals on the driveshaft and two on the prop shaft. The flared/soft/sealing sides of all of the seals are positioned to the outside of the gearcase. To me that indicates that the primary function of the seals is to keep water out (as opposed to keeping oil in). So, in this case, wouldn't a vacuum test tell me all I need to know? IOW, why would I need a pressure test?

2. The seals are all original. If My test indicates a leak at one location (lets say drive shaft) wouldn't it make sense to replace all of them? As well as the boot over the shift shaft?

 

[Cogburn]

Re: Vacuum Test vs Pressure Test for '82 Mariner Lower Unit?

Got some water in lower unit issues w/1982 30 HP Mariner. Replaced the gaskets on the fill and vent screws and plan a run to see if there is still a problem. If there is, time for testing/remediation.

A couple of questions.

1. I've read a lot of posts that advise both pressure and vacuum testing to diagnose problems and verify an adequate repair. On my motor there are two seals on the driveshaft and two on the prop shaft. The flared/soft/sealing sides of all of the seals are positioned to the outside of the gearcase. To me that indicates that the primary function of the seals is to keep water out (as opposed to keeping oil in). So, in this case, wouldn't a vacuum test tell me all I need to know? IOW, why would I need a pressure test?

2. The seals are all original. If My test indicates a leak at one location (lets say drive shaft) wouldn't it make sense to replace all of them? As well as the boot over the shift shaft?

In my limited experience it is easier to find a leak with a little air pressure and some soapy water in a spray bottle. The last one I fixed was leaking from a scratch on the housing where the seals were pressed in and a vacuum test did not show it other than it would bleed off after a minute or two.

If the seals are really old and hard I would replace them but I have worked on some from the '60s and the seals were probably about as good as the replacements. If you were going to keep it a long time it may be a good thing to replace all the seals but there can be a number of them that are not easily replaced and are not likely to leak..