Mercury Tracker 25EL Lower Unit Pressure Test

A pressure test serves to find out where your lower unit is leaking, or whether it’s even leaking at all!

The only special tool you’ll need for this simple task is a gearcase pressure tester. Watch our lower unit pressure test video above and follow the steps below to test the pressure on an outboard engine’s lower unit. 

NOTE: We pressure-tested a Mercury Tracker 25EL outboard, but the steps are the same or similar for all outboard motor makes and models.

How to Pressure Test a Gearcase

Step 1. Remove the lower unit vent bolt and washer from the top of the gearcase.

Mercury Tracker 25 outboard lower unit pressure test

Step 2. Remove the drain/fill screw and washer from the bottom of the gearcase and allow the oil to drain out, then reinstall the washer and screw.

Step 3. Attach a gearcase pressure tester to the vent hole.

Mercury outboard lower unit pressure test

Step 4. Use the tester to increase the pressure within the lower unit to about 12 PSI, and let it sit for about 2 minutes. 

  • If the lower unit is properly sealed, the pressure will remain constant
  • If there’s a leak somewhere, the pressure will drop

PRO TIP: To find a leak, spray a solution of dishwashing detergent and water onto the prop shaft seal, the shift shaft seal and the water pump housing. Bubbles will appear wherever there’s a leak.

Step 5. Release the pressure, then remove the pressure tester, reinstall the lower unit vent bolt and you’re done.

As mentioned earlier, the lower unit or gear case pressure test is the same or similar for all outboard makes and models. However, if you want to see the lower unit pressure test done on a different outboard engine, watch the video above to see how to do a Yamaha outboard gearcase pressure test



Commercial Discounts

Special discounts for companies in the marine industry


Government Sales

Discounts for federal and most state and municipal agencies