TOPIC: lower unit pressure test?

I know I'm getting water in my lower unit. I'd like to do a low pressure air test to determine what is leaking. Can anyone give me some basics about that? Like, are there pressure test kits out there that can be adapted to the merc fill fitting? And, how much pressure should be used? I've noticed that some of the auto parts stores have loaner pressure test kits for radiator caps and such. Wondering if one of those might be adapted to the fill fitting. They use a small hand pump so pressure could be kept low. Any suggestions appreciated.

Hi Norm,

I am interested in this topic also. I had posted a few weeks ago about replacing gaskets in my 1964 500 lower unit. I had some water in it last fall when I winterized. Some suggested a pressure test, before haphazardly replacing gaskets. Nobody elaborated on specifics of such a test. I just talked to my buddy / boat motor helper last night. He said we will put low pressure air to the vent hole. He said it will not really use the threads of the vent hole but just a make shift rubber gasket around the air supply tip, enough to get a reasonable seal for low pressure air. If there is a leak, lower unit oil will ooze out from one of the potential leak sites. He claims it will be obvious where the leak is at.

I have never done this and have no real specific info to offer. It sounds pretty straight forward, but with antique motors, what sounds simple and quickly turn into a nightmare.

I am not recommending others do this. I post this to help stimulate the conversation. I am hoping others with experience at this will post more here and say this is wrong or here is how to do it.

Joe

Hi Joe, thanks for the reply. I found an informative youtube video with a pretty interesting test fixture. Here's the link:


If you read the poster's description below the vid, he gives some good tips. He suggests draining the oil first, then pressurize and test by submerging the unit and looking for bubbles. He also suggests running both a pressure and a vacuum test since some of these seals are doubled up back to back. I have a mity-vac hand pump that would suffice for the vacuum test. Also important to keep the pressure low so more seal damage isn't done.

A seal job on my 650 is really the last thing I want to tackle, but in my area outboard techs are pretty rare. I'd much rather ship it to someone with the knowledge and tools to do the job right, but picking a decent shop is a challenge. I figure the leak test will at least narrow it down to which seal is leaking. From there I can decide whether to try a repair myself or farm it out.

I've seen some real nightmare tales online about dismantling these units, both by owners and by shops, where the unit was turned to scrap. In my lifetime I've certainly turned a few things to scrap but I think I stand a fair chance of avoiding that if I end up doing the job myself. I've gained a lot of insight on this and other forums, and the motor is a fresh water unit so maybe the carrier won't be corroded into place. One way or another, it's gotta get done.

Yes Best to drain the unit. I usually screw the tester in the drain hole. The unit should hold 9# of pressure for 15 min.Should also hold 6 in. of vacuum for 15 min. While under pressure you can submerge it to look for bubbles from the leaky seal.I have run across many units over the years that the only problem was the gaskets around the drain & fill plugs.
Replacing seals does require special tools to disassenble & reassemble the unit.
Pressure testing IS the first step.

Good info, thanks Tim. I scrounged around the shop and I have what I need to make up the test fixture. I'm pretty sure the problem is at the prop shaft seal but I'll check everything including those fill plug screws and the shift shaft and impeller area.

You can make one easy..fixture. Get another drain plug, drill a hole in it, and braze on a quick disconnect air fitting. Easy on the air pressure though..I never went over 5/10lbs.

I've started a new thread on this lower unit. Seems I've stumbled into a new mystery regarding the prop seals. See this post:
www.fiberglassics.com/glassic-forums/home/mercury-outpost/64-650-prop-seals-what-the

Joe, hope you got your pressure test sorted out ok. I ended up using a MityVac hand pump that does vacuum and pressure tests, rigged with a tee, one line to the pump, one line going to a threaded filler plug from a WalMart pump kit, and the third line going to an old pressure/vacuum meter that was laying around the shop. I tested after replacing my prop shaft seals which had already blown out. I got a steady 9 lbs of pressure for 15 minutes and 6 lbs of vacuum for 15 minutes. I tested with the oil drained out of the lower unit. If I can help with more info let me know.

bonesdc wrote:

Hi Norm,

I am interested in this topic also. I had posted a few weeks ago about replacing gaskets in my 1964 500 lower unit. I had some water in it last fall when I winterized. Some suggested a pressure test, before haphazardly replacing gaskets. Nobody elaborated on specifics of such a test. I just talked to my buddy / boat motor helper last night. He said we will put low pressure air to the vent hole. He said it will not really use the threads of the vent hole but just a make shift rubber gasket around the air supply tip, enough to get a reasonable seal for low pressure air. If there is a leak, lower unit oil will ooze out from one of the potential leak sites. He claims it will be obvious where the leak is at.

I have never done this and have no real specific info to offer. It sounds pretty straight forward, but with antique motors, what sounds simple and quickly turn into a nightmare.

I am not recommending others do this. I post this to help stimulate the conversation. I am hoping others with experience at this will post more here and say this is wrong or here is how to do it.

Joe

Don't overdo it on the pressure, you can get a bad reading. A couple of pounds is enough, any lip seal will deform with high pressure and grip the shaft tighter leading you to believe the damn thing is fine. Keep the pressure under 4 psi and spin the shaft as you test it.

I can see where that could happen. Thanks for the tip.