Question on LU Vacuum Test Results

[minuteman62-64]

1982 Mariner 30 HP. Did some work on the LU (replaced seals, impeller, gaskets). Yesterday p.m. applied 7.2 inches mercury vacuum to lower unit (no particular target - just ended up there after three squeezes of my pump). A few times rotated the drive shart and shifted through the gears when walking by. Held in the 7 inch range till bedtime. Small fluctuations probably a result of ambient temperature.

This a.m. the reading was at 5.5 inches mercury. Has been holding at 5.5 for the past 3 hours. It is a lot cooler now so I assume ambient temperature could be a factor in the over night change.

So, what do I have? Is there a leak somewhere in the system - either in the LU or in the vacuum pump/hose system used to apply the vacuum? If so, is it significant?

I'm headed to HF this morning to get some stuff to do a pressure test. But, I don't know if I'm actually looking for a problem based on the vacuum test.

Appreciate any thoughts.

 

[minuteman62-64]

Re: Question on LU Vacuum Test Results

Now I'm really getting confused. The vacuum dropped below 5" a couple of hours ago. I pumped it up to 5" and went to HF. Now I return and it has dropped to 3".

Would the higher initial vacuum (7.2") actually seal better than a lesser vacuum? As per the factory manual, the seals (the ones I replaced and the remaining original seals on the prop shaft) are all installed with the sealing surfaces to the outside of the gearhousing.

 

[Texasmark]

Re: Question on LU Vacuum Test Results

My manual says to use a pressure test. I can look it up, but it is in the 5-10 psi range and the set time is like no more than 1# loss in 15 minutes.

If you want the exact numbers I can look it up.

The vacuum test may not work due to how the seals are placed on the shafts. I don't know.

Mark

 

[minuteman62-64]

Re: Question on LU Vacuum Test Results

The vacuum test may not work due to how the seals are placed on the shafts. I don't know.

Mark

I was thinking the opposite based on how the service manual indicates the seals should be placed. The lips on both seals, at the driveshaft and prop shaft, face away from the gearhousing. Seems like that would make a better seal against something coming in to the gearhousing than going out - so I was thinking the vacuum test would be more relevant.

However, this is from a knowledge base of approximately zero.