Mercury Tracker 25EL Lower Unit Pressure Test

A pressure test serves to find out where the lower unit of an outboard is leaking, if there's even a leak at all. The only special tool needed for this simple task is a gearcase pressure tester

Watch the video above or follow the steps below perform a gearcase pressure test on a Mercury outboard motor.

NOTE: We pressure-tested a Mercury Tracker 25EL outboard, but the steps are the same or similar for all outboard motor makes and modelsAlso, you don't need to remove the lower unit to perform a pressure test. Ours was removed as part of a larger project.

Stevens Instrument gearcase pressure tester

Buy Mercury 25HP outboard parts

How to Pressure Test an Outboard Motor 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.

Mercury Tracker 25 outboard gearcase pressure test

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.

Mercury 25 outboard engine lower unit pressure test

Mercury 25 outboard motor gearcase pressure test

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




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