Re: pressure testing outdrive
the drive wont create a vacum while running. if anything it will create pressure due to the heat expansion of the oil. the vacum may come during a cool down. the factory input drive shaft has grooves in it to assist oil control. water in the bellows will force water past the seal. will also cause a yoke failure. so a teardown and inspection is always in order. could be water in the bellows,if so you have a lot more problems. could be the lower seals in the upper case. could be shift shaft seals,prop shaft, drive shaft or occasionally the carrier oring will fail. could be something as simple as someone reused the drain/fill gaskets to many times or left on off.<br /> so follow the service manual and carefully inspect all parts. watch for grooved seal surfaces and damaged parts. remember this will involve some special tool purchases.<br /> ypi will need a torque wrench. possibly a dail type inch pounds torque wrench. you will need the upper spanner and lower unit cover nut tool.<br /> if you wish to remove the lower drive shaft it will require some extra tools. some are availible from aftermarket some are merc only . the tool from merc to remove the propshaft bearing carrier usually only works at the merc school. if the unit has been is saltwater you will also need oxy/acetylene.
<br /><br />If you know there's a leak, finding it is only of passing concern. If it's a damaged surface, then you replace the part. But if it's a seal (most common) then it's time to replace all of them anyhow. They're all worn and/or weathered.<br /><br />On a MerCruiser, a vacuum test is more accurate. The shift shaft seal is pointed in a direction that air pressure will leak past it because it's faced the opposite way to keep water out. That's what seals need to do, seal water out as well as seal the lube in. In most places, there are two seals. One faces the water and one faces the lube. The shiftshaft is one exception.
