Mercury 40HP Outboard Gearcase Oil Change & Pressure Test

Mercury recommends changing the oil in your outboard’s lower unit after every 100 hours of use or once a year at the very minimum. 

Watch the video above and follow the steps below to replace the gearcase oil in and pressure test the lower unit of a Mercury 40HP outboard engine.

NOTE: We worked on a Mercury 40HP motor, but the process is the same or similar for most small-to-medium sized outboard makes and models.

Tools and Parts – Mercury 40 Gearcase Oil and Pressure Test

  • Flathead screwdriver
  • Contact/brake cleaner
  • Drain pan

Buy Mercury SAE 90 gear lube

Buy Mercury gearcase lube pump

How to Change Mercury 40HP Outboard Gearcase Lube

Step 1. Remove the vent screw and its nylon washer from the upper right side of the lower unit.

Mercury outboard 40 gearcase oil drain screw

Step 2. Put a drain pan under the lower unit, and remove the drain/fill screw and its nylon washer from the bottom right side of the lower unit. Wait for the gearcase oil to drain out.

Mercury 40 gearcase oil change

Step 3. Inspect the vent screw and the drain/fill screw washers for wear, and replace them if necessary. 

NOTE: To make sure there are no oil leaks, the washers should be replaced every couple of times you change the lower unit oil, regardless of their condition. 

Draining the lower unit oil presents the perfect opportunity to pressure test the gearcase. If you don’t need to pressure test it, skip ahead to Step 7. However, if the gearcase is losing oil, or there’s evidence that water is mixing with it (oil appears milky), then a pressure test is necessary. 

Mercury outboard 40HP gearcase pressure test

How to Pressure Test Mercury 40HP Outboard Lower Unit

Step 4. Reinstall the drain/fill screw and its washer.

Step 5. Connect a pressure/vacuum tester to the vent hole, then pressurize the gearcase to about 12-pounds and see if the pressure remains stable for a couple of minutes.

Mercury 40 outboard gearcase pressure tester

Step 6. Refill the lower unit with oil if the pressure remains constant, then disconnect the pressure/vacuum tester. However, if the pressure drops, there’s a leak somewhere (usually an input or prop shaft seal) and you’ll have to find and repair it first. 

How to Change Mercury 40HP Outboard Gearcase Lube (Cont’d)

Step 7. Put a Mercury gear lube pump into a quart of Mercury SAE90 gear lube, and attach the other end of the pump to the drain/fill hole in the lower unit. 

NOTE: If you get the Mercury brand pump, you won’t need to buy a separate adapter to fit it to your Mercury outboard.

Mercury outboard gearcase lube pump

Step 8. Gently pump the new gear lube into the lower unit until it starts to flow out from the vent hole.

Mercury 40HP lower unit gearcase oil change

Step 9. Reinstall the vent screw and its washer.

Step 10. Remove the gear lube pump, and quickly reinstall the drain/fill screw and its washer.

Ready to change the engine oil on your Mercury engine? Watch the video above to see how to change the oil in a Mercury 40HP 4-stroke outboard



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