sterndrive pressure test

Hi all, just replaced the intake impeller and seals , installed the lower gear housing , hooked up a pressure tester to the oil vent , put in 15 psi , after 6 hours it has gone
down to 13 psi , any thoughts why ? , I will let it go overnight to see if it keeps going down , maybe my homemade tester has a leak , used soapy water at the inlet ,
but dont see any bubbles.

If it only lost 2 lbs after 6 hours then you are all set..............

Not to worry. there are many reasons for aq 2 lb loss........

you are only suppose to keep the pressure for 2-3 minutes.........5 minutes at max......

you are good to go.

Omc if I remember correctly said to pressurize to 17 lbs for two minutes,

I always go to 15 lbs but typically if it holds for a minute I consider it good!

Thanks for the input, after about 20 hours went down to 9 psi , so I think things are ok .
Question tho, when I drained the oil in a glass jar it was a puky green on the bottom and top halph was darker, dont see any sighns of water, the new oil is a blue color , but I wont put it in just yet.
Not sure of the previous owner maintenace ways of doing things, he had told me he took it in for servicing before winter.

A Pressure/Vacuum leak down test involves a time frame..... that's why it's called a "leak down" test.

For pressure testing, the drive must be absent of oil. If oil is present, the oil will hydraulically aid in sealing lip function.
We don't want this to occur during the testing or the reading may not be accurate.

The vacuum test doesn't care, because under vacuum there is no hydraulically aided sealing function.


While under either pressure or vacuum, you'll want to rotate any seal surface that glides under the seal lips.
IOW, rotate the input shaft, the propeller shaft and any shift shaft that penetrates the oil cavitiy.

Due to the nature of the seal's lip orientation, drives will generally hold pressure better than they will vacuum, so don't be surprised if the Positive pressure testing holds a greater value than the Negative pressure testing does.

Watch as the pressure or vacuum either holds during the duration, or reduces in value during the duration.
Generally these values will drop some and then hold.
If they hold within specs for duration, you should be OK.



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Over a great deal of time......20 hours or so is a good example, permeation can occur

that is the air will naturally leak past even through certain materials such as rubber and plastic over time. Most rubber is not solid it is more like a very very dense sponge material that is why it is somewhat flexible.

The odds are you could have a weak seal or more than likely the attachment location of your pressure checker is leaking. this is the most common point of loss if the drive is tight. the air simply leaks back thru the checker............

So in the end if it holds 15 psi for 2-3 minutes with NO loss you are good to go.

kghost said:

.......................
So in the end if it holds 15 psi for 2-3 minutes with NO loss you are good to go.

Click to expand...

Kghost, you are very likely correct if you are refering to a drive with all new seals.
And I agree, the OP's tester itself could be leaking off pressure.

However, as I'm reading in this thread, the OP installed one new seal, but also wants to test the existing seals (of which there is no choice).

As you know, I do the AQ series Volvo Penta, not Mercs, but the drive seals in general are not all that different from one another.


Here's what I believe the OEM would suggest:

New drive or new seals installed:
They want the drive to hold what their specs suggest, and I can only imagine that they want it to hold for a duration, and most likely longer than 2 to 3 minutes. (but I'm not up to speed on the Mercs)

Testing a drive with existing seals:
As seals age, they will no longer hold to new specs, and will likely be below that of new seal specs, but again for a duration.


I may see a re-sealed V/P drive hold 10 to 12 psi for 8 to 10 hours or so.
I may see existing but good V/P seals hold only 8 psi for that same duration.

Now, normally I'd not leave the tester connected for even close to 8 to 10 hours, but I'll often walk away over night with the tester still connected.
Come morning, if it is still holding, I'm happy.


Other than the new seal specs not being quite the same as would be for used seals, I agree with Kghost.



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searay91, here's my take on what occurs:

When we cap off the gear oil fill plug, ambient pressure is now captive within the drive.
Even though the drive is submerged in cool water, once the work load begins, heat is produced.
This heat causes mild expansion, so now the seals must hold against this mild expansion pressure (probably well below the specified testing pressure).
If one was to leak down to ambient pressure, some oil may be lost.

But here's the no-so-good part......
As the drive cools down (once the outing ends), and after a small leak, mild negative pressure is now created due to the lost oil volume... even though minimal.
This cycle will repeat itself each time the drive heats and cools.
If that same seal is exposed to the water side, water may be pulled back in as the pressure returns to ambient.

Main drive seals are within the dry bellows area, so typically we do not see water intrusion from this sealing area that's either failing or has failed.



So, other than environmental concerns, I guess we could say that a very small oil leak poses no issues to the drive......, until it begins to allow water back in.




Again, this is just my take on this.



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New drive or new seals installed:
They want the drive to hold what their specs suggest, and I can only imagine that they want it to hold for a duration, and most likely longer than 2 to 3 minutes. (but I'm not up to speed on the Mercs)

I just looked thru both the service manuals # 6 and #14 for Mercruiser outdrives Alpha I Gen I and Gen II

No information on pressure checking outdrive....Maybe someone else has seen this referenced in Merc manuals?????
If I remember correctly it is not mentioned anywhere in mercs manuals. WHy? I don't know.............


So the information comes from actually working on them for many many years and using OMC spec as a reference.

The OMC spec is if I remember correctly 17 psi for 2-3 minutes.
I always felt that 17 psi was a bit high.

I know that a merc alpha outdrive produces about 7-9 psi when running.

I took the OMC spec and decided that 15 psi was all needed to test the drive.

So that you fully understand............there is no need nor is there time when working in the business to pressure check an outdrive for hours.......you would end up with a shop full of outdrive waiting on leak down results.

My experiences says................2-3 minutes is all that is needed. If water is still getting in and oil out then a vacuum check is needed and you will only get to about 8 in/hg maybe 9 if you keep trying........a leak will show up by 4-6 in/hg.................

this is not rocket science so no need to go deeper into theories of any kind............

If there is a bad seal it will show up under 15 psi within 2-3 minutes.............in some rare cases it may take more than that but it is rare.

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kghost said:

So the information comes from actually working on them for many many years and using OMC spec as a reference.

The OMC spec is if I remember correctly 17 psi for 2-3 minutes.
I always felt that 17 psi was a bit high.
I do also. That seems high to me.

I know that a merc alpha outdrive produces about 7-9 psi when running.
Interesting. I"ve not checked this before.
The one nice aspect of Merc's drive oil reservoir, is that ambient pressure is maintained.

I took the OMC spec and decided that 15 psi was all needed to test the drive.

So that you fully understand............ there is no need nor is there time when working in the business to pressure check an outdrive for hours.......you would end up with a shop full of outdrive waiting on leak down results.
Just to be clear... I'm NOT suggesting that the test duration be hours and hours.
While being tested, we move on to another task and return later. 30 minutes should do just fine.
I was simply saying that upon occasion, I have left my tester installed over-night.


My experiences says................2-3 minutes is all that is needed. If water is still getting in and oil out then a vacuum check is needed and you will only get to about 8 in/hg maybe 9 if you keep trying........a leak will show up by 4-6 in/hg.................
I always perfrom both Pressure and Vacuum leak down tests.


this is not rocket science so no need to go deeper into theories of any kind............
Understood....... but average Joe may not understand the resulting dynamics of a leak.

If there is a bad seal it will show up under 15 psi within 2-3 minutes............. in some rare cases it may take more than that but it is rare.
I see seals that will not hold XX psi, but will hold a lessor and within specs YY psi for the duration.
IMO, that is the other aspect of "Leak Down".

Click to expand...

Thanks for understanding that I'm not attempting to start an arguement... I'm just sharing thoughts.


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Ok , all undersstud , I will still try the vaccum test to make sure , still the oil came out ugly green , and the upper halph was darker , in a glass jar, no water at the bottom as this should be as water is
heavier than oil.
Should I flush the system before I put in the new oil? , is this possible ? thanks to all for the input.

Install the drive. Fill it with Quick Silver HP Synthetic gear oil (The green Stuff) and run it. Pull the boat and check it. You are over analyzing this.

I agree with Chris your over doing this!!

if it is not milky or has gray looking streaks (metal) then not to worry.

Is this your first gear oil change with this outdrive? If so what was the gear lube used before?

Ok, riddle me this. 2000 alpha gen II. Just did water pump service, pressure tested over night. Lost two or three pounds by morning. I pumped it back up and started poking around. I spun the drive shaft and put it in forward and reverse gears. Every time I put some pressure on the shift shaft with my fingers I get a hiss out of the shift shaft seal.

I assume this is not normal. I went ahead and bought the lower shift bushing 23-815921A21.

Does this sound reasonable or am I replacing this bushing for nothing?

The gen II shift shaft bushing ....SUCKS.........so what you did and found is normal.



Even other areas where you put lateral pressure on a shaft/seal you can induce a leak........

Make sure you grease the seal before installing it over the shaft.

Also get some scotch bright and debur the shaft splines as they can be sharp and can cut the seal lip then grease them also.

Ok, so when I remove the shaft I am planning on putting it in forward gear, then somehow locking the prop shaft to keep it there, then removing the bushing and shaft. Slip the bushing off the shaft, replacing it with the new one. Re-insert the shaft in the same forward gear position. Bolt it down.

Am I missing anything?

Well...that's a problem.

According to kghost this leaking when pressure is applied is normal. That being said does this mean the new bushing will leak as well? Why am I taking the chance replacing it then?

The shaft spines on the lower end are equally spaced. The 90 degree bent upper end when in forward will be pointing straight ahead (12:00)
The lower end has splines the upper end is bent 90 degrees. Carefully watch and look for all washers or anything else as you disassemble.

When removing the lower unit, shift the shift control into forward and spin the prop C'clockwise by hand and this will lock it into forward.
It does not stay "locked" as one may think......it only engages the clutch dog in the lower gears and if pressure is kept on the prop the shift shaft will stay in its position. Any movement and it will change but no big deal and I will explain.

Once you separate the lower from the upper, unscrew mounting bolts for the shift shaft seal, pull up on the shift shaft bushing and the shaft and bushing comes out.
Once removed you can now pull the E ring below the bushing out and then pull STRAIGHT up on the shaft and it will come out of the bushing,

If you pull it out, then debur the splines a bit and grease them and reinstall it thru the new greased bushing assembly remembering to install the E ring back into the shaft. If the top 90 degree bend was facing 12:00 then you need to get it back into that location with the prop "locked" into forward before tightening down on the bushing mounting screws. If you do not have exactly 12:00 on the shift shaft then you must turn it clock wise all the way it will go while turning the prop C'Clockwise until locked and then pull up on the shaft and reinstall with the bend facing 12:00 exactly.



Now spin the shift shaft counter clockwise and then back clockwise while spinning prop c'clockwise and recheck for 12:00 on shift shaft. If it is You are now done and ready to reinstall the lower to the upper.

When installing lower to upper............get the drive shaft and water tube lined up and as you are pushing the lower into the upper you need to keep light pressure on the prop c'clockwise while making sure the shift shaft is pointing at 12:00....this can be a bit tricky so if you F up don't worry and retry.

Dlawrence81 said:

According to kghost this leaking when pressure is applied is normal. That being said does this mean the new bushing will leak as well? Why am I taking the chance replacing it then?

Click to expand...

It may and it may not........if you think its leaking then replace it.........be done with it and don't worry about it............Believe me!!

Holy crap man, thanks!

Ok, split the two housings again. Put it in foward gear and pulled out the shift shaft. That bushing was worn out as it was cracked all around the inner seal ring. I replaced it and reinstalled the shaft with the new bushing. Currently pressure testing but it doesn't leak when I press on the shift shaft now so it appears to be fixed.

Thanks for the help!