How to Change the Lower Oil Unit on a Yamaha 250 HPDI

With multiple plugs in its lower unit, the 250-horsepower Yamaha "High Pressure Direct Injection" outboard motor may seem intimidating at first. Changing the gear lubricant, the oil in the lower unit of the motor, also means getting your hands dirty as you inspect the oil, change the oil-tight fittings and add new gear lubricant from a tube. It's a once-a-year job, undertaken when you lay the motor up for the winter, that makes the next year's boating a bit more trouble free.

Place a suitable receptacle for receiving the used oil beneath the bottom of the motor's lower unit. Remove the fill/drain plug, located on the bottom of the motor's lower unit, with a screwdriver and do not confuse one for the other. Remove and discard the grommet from the plug. Feel the magnetic tip of the plug for bits of metal.

Remove the front and rear vent plugs, located on the starboard side of the lower unit. Remove and discard the grommets from the vent plugs and the fill/drain plug. Replace the grommets with new grommets.

Allow the gear lubricant to drain completely. While the lubricant is draining from the gearcase, note its color; if it has a creamy or milky appearance, water has infiltrated the lower unit.

Push the tip of the tube of gear lubricant into the the fill/drain hole as far as it will go. Squeeze the tube from the bottom until the lubricant appears at the forward vent hole. Screw the forward vent plug back into place and continue squeezing lubricant into the gearcase until lubricant appears at the upper vent plug.

Thread the upper vent plug into place and tighten it with the screwdriver. Pull the lubricant tube out of the fill/drain hole and, as quickly as possible, thread the fill/drain plug in place. Set the lubricant tube aside and tighten the fill/drain plug. Wash the lower unit's surface with liquid dish detergent and rinse with water.


  • Do not confuse the forward and rear vent plugs with each other, or with the fill/drain plug. The vent plugs have "worn into" their separate bores and will not seat properly into a different bore. The fill/drain plug has a magnet in it; not only will it not seat properly in the vent bores, it won't act as a telltale for larger problems in the lower unit.


  • "Yamaha/Mercury/Mariner Outboard Repair Manual 2 - 250 HP 1995-2004"; Seloc Marine; 2007

About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.