08 90 hp lower unit service

hi everyone, i would like to know why you fill from the drain plug and not the over flow plug.....

[reelnative]

Hi if you tryed to fill from the vent hole all the fluid would run out and then if you tryed to fill it with the plug in the drain fill hole, then you would make pos pressure because you could not displace the air in there with the fluid, filling the unit really is easy just buy a pump, walmart has them for around 6 bucks I think, first drain all the old fluid by first pulling the drain fill plug, then the vent plug, when that is done screw the pump into the top of your hypoid fluid and screw the end into the drain fill hole and start pumping, when you see some comming out of the vent hole replace the plug in the vent hole first, this will keep the fluid from rushing out the drain fill hole when you take the pump screw out of that hole, when you unscrew the pump tube be ready to put the drain fill plug in, once you have done that tighten and your ready for another 50hrs of runnin, I hope that helped