1998 8HP 2-stroke vacuum-testing lower unit - how much vacuum?

It's serial 0G590511. A new lower unit was purchased and installed in early 2014.
Used during the 2014, 2015 and 2016 seasons (gear oil was changed each year).
Drained the gear oil a few weeks ago and put it in a glass jar. There is some dark tan
fluid that has settled to the bottom. A pressure/vacuum test shows that there
is a very slow leak. I've read on this forum that a vacuum test is the most important.

My pressure/vacuum tester has a meter on it....how much vacuum should I apply?
How long should it hold for? Is even the smallest leak worth following up on?
My Factory Service Manual says nothing about pressure/vacuum testing!

If it's not holding a vacuum it needs a reseal

That dark stuff is sludge. Just changing the oil more often will eliminate that.

Any vacuum being held is a good sign; if the reading slowly drops, it's seal time. I use a Migthy-Vac hand tester (Amazon fairly cheap) with a simple adapter.

Jeff

Vacuum should hold 6 "Hg merc for about 15 min. And 15 "Hg merc for 15 min. Use soapy water to find any leaks.

Thanks for the vacuum figures and info!

Well, I used my Craftsman pressure/vacuum tester on the lower unit with some "bubble solution"
I picked up at a toy store. There was definitely some bubbles forming at the edge of the stainless
steel impeller cover. So, I removed the bolts holding the water pump in place to see if the drive
shaft seals were leaking. Removed the impeller cover and impeller. Put the bolts back in.
I pulled the drive shaft up and down a bit and rotated it...didn't see much of anything.

Under pressure of 20 psi it is now declining about 1 psi every minute. I decided to check the
tester....turns out it has a small leak. Need to get out the manual.

In the meantime, the tan fluid in the bottom of the jar has largely disappeared. There may
not have been much in the way of water. The cap has been on the jar.

At this point, I don't know what to do. The engine is heavily used during the summer season
and we can't afford to have a mid-season breakdown. There definitely was a leak.
It seems to have largely disappeared.

As long as l/u holds any vacuum you are all good. 3 to 15inches of vacuum, It should hold overnight. Just a thought ,when metal gets warm it expands. BTW 20 psi pressure test is a bit too high i believe,wait for other responses

Well, I found the leak in the pressure/vacuum set up.
I had some vinyl hoses mated with plastic connectors that
were leaking at the junction. Now the 15" Hg vacuum holds all night long!
The bubbles I saw were probably from turning the driveshaft slightly
and the impeller pushing some air out underneath the impeller cover.

The tan fluid in the bottom of the jar is gone...it must NOT have been water.
Rather sludge as suggested by "fastjeff".

I came within a hair of ordering a new water pump kit complete with new seals
and gaskets that would have required a lot of installation work. The impeller
blades have quite a set to them...so, I'll replace the impeller.

The moral of the story is: check your pressure/vacuum test set-up for leaks
(put your finger over the end and see if the reading on the meter changes)!

well done, But i would replace entire water pump assembly,housing,wear plate, impeller, gaskets, keyway, and orings,sealer on both sides of wear plate. Place a little grease or dish soap on impeller for ease of installation and breakin:rolleyes:

Meant to add: I've found that little seal on the shifter shaft to be a major leaker on these motors. Why, since the shaft doesn't spin like crazy, is a mystery, but there it is.

Jeff