290DP leak down test vacuum leak

I am getting near closing out my boat restoration project that was planned as a 3 week make over. I am in my 5th month but I am very close to done and ended up doing so much more than I planned.

my last thread is here http://www.marineengine.com/boat-forum/showthread.php?426405-290DP-rebuild-repair-question

I will post a detailed follow up to that with some pictures of some of the things i found to give back a little this forum that gave me so much info. Everyone and especially Rick has been very helpful.

The drive and engine got put back together a couple months ago, now I have finally got around to building a leak down test assembly and used it last night. Results were pretty good for the pressure, it went from 7 to 5.5 psi in 2 hours. For vacuum test, not so great; 15 in. of Hg vacuum bleeds out in about 2 minutes.

I recall reading that there is a specific seal in the 290 DP-A drive that the vacuum test will fail on where the pressure test holds. I can't find that reference anymore. Need some help on this. I probably messed up reassembling something when I replaced all the seals. The drive seemed to work well. it has been wet for an hour when I did some light testing; put the boat in and out of gear a few times in forward and reverse. I haven't run it under much load or for any duration.

Also, is a pressure bleed like I described an issue? Boat is typically slipped in the water all year.

Thanks, Donny

Just saying what i think here so take it for what it's worth.
When i resealed mine i ended up with no leaks pressure or vacuum for 12 hours but mine is a VP aq280 not a dp.
Im guessing here still but i think there is only 3 places a vacuum test could fail.
The 2? seals around the prop shafts or the seal around the upper input shaft or the shifter seal
I would bet you could put pressure to it again and spray it with soapy water to find the pressure leak. Fix that and you may well fix the vacuum leak also.

thanks for response Patin. I figured there is a pretty small amount of air in there so it might be hard to use the soap to find a leak that is 1.5 psi in 2 hours.. I think I might not have a great seal on the oil fill where the test assembly is attached, that is my hope and the easy fix. I figured if it was a typical leak the vacuum would be same rate as pressure and I was hoping that the difference made it easier to diagnose. I will try the soapy water next chance I get. It will be hard to get access to the main drive seal that seals the top side shaft. I hope it is as easy as you suggest to find.

Spray it and watch for a few minutes. You will see it, if not move to the next place and try it

The pressurizing of the drive creates hydraulic or pneumatic pressure against the seal lip, and actually loads the seal lip to some degree, thus allowing the seal lip to hold better.
The drive must be absent of oil during the pressure leak down test.

The vacuum leak down test does not care whether the oil has been drained or not.


The main drive gear seal is a single lip seal. Single lip seals tend to hold in one direction, and tend to not hold as well in the other direction.
This is why some drives will hold pressure, while they may not hold negative pressure (vacuum) quite as well.

The main drive seal surface washer is carbon steel. If a leaking bellows has allowed water to come into contact with this carbon steel washer, it becomes rusty and ruff. This may not only cause the seal to NOT hold, but it also causes future seal wear.

This washer can be polished up by removing no more than .004" to .005" from this surface.
Make certain that you don't end up with any swirl markings that may cause oil wicking.

If you replace this seal surface washer on any pre-A drive transmission, note that these are NOT of the exact same dimensions between old and new... or even between several brand new parts.
The shim control within this washer will affect the bearing box rolling torque value..... which means that the rolling torque procedure must be done again.

The eccentric piston seals are typicaly a cause for Press/Vac leakdown test failures.
Fish line can also damage prop shaft seals.

Be sure to replace ALL O-rings.

Be sure to operate the drive shaft, eccentric piston and prop shaft while doing each test.


Bottom line...... a good re-seal job should hold both Positive and Negative pressure.



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thanks for the info Rick and others. I didn't think about my mistake of pressure testing with oil in the drive until you mentioned it but that makes a lot of sense.

I was able to locate the leak; I did not install the lower O ring that goes on the oil transfer tube in the intermediate, it is located right at the oil strainer. None of the schematic pictures show it. I went back and looked and the shop manual and it does mention it in the text and the parts list shows 2 of them when it points to the top one so I should have caught that but I didn't and was following pictures only when putting it back together. It was with a few extra O rings I had that didn't get used and I put it in, pressure and vacuum tested the drive dry and it held solid without a hint of a leak.

We did do almost a full rebuild on the drive and all seals were replaced so it feels good to have it pass the pressure test. It seems to run nicely and shifts into and out of gear well. Had to overcome a cooling issue that I was able to resolve last night as well as carb issue. We were 3 for 3 with no new issues discovered, shocking, our normal batting average is more in the 300 range. Ready for a full sea trial finally.

Boat is real close to 100% which is 20% more than it has ever been in 10 years I have owned it! All systems have been renovated; electrical, electronics, upholstery,engine, hull, plumbing, steering, new pilot house. Did so much I can't comprehend it.

here are a couple, one middle of the project and the second a couple weeks ago when I put her in for first time for quick check of a few systems. 24'skipjack fly bridge. don't have many pictures of the drive, it is ugly but hopefully very functional.

Yup. We demand pics!!