How to Change the Oil & Filter on a Mercury 115HP Outboard

Changing the oil and oil filter on your Mercury 115HP EFI outboard is a quick do-it-yourself type of job. 

Besides, when you do the work, you can make sure you're fitting a genuine OEM Mercury oil filter and using quality engine oil. Mercury recommends changing the oil at least once every 100 hours, and doing it yourself saves the trouble of having to pay a mechanic repeatedly to do it for you. 

Tools and Parts Needed - Mercury 115 Outboard Oil Change


  • 18mm socket
  • 3/8" ratchet
  • Pick tool
  • 65mm oil filter wrench

Mercury 115hp outboard engine oil change

The simplest and cheapest method of changing your Mercury outboard's oil filter and oil is to purchase a Mercury oil change kit. The kit includes a genuine Mercury oil filter, 5 quarts of Mercury oil and a replacement crush washer. You can also purchase the parts separately. 

Mercury 115HP EFI Outboard Oil and Filter Change

PRO TIP: Run your outboard for a minute or two before changing the engine oil. This warms the old oil, and makes it flow and drain out better.

Step 1. Remove the top engine cowling, raise the outboard fully up to the trailer position and tilt the engine so the drain plug (on the left side of the motor) is leaning toward the ground.

Mercury outboard oil change

Step 2. Remove the 18mm oil drain bolt and allow the engine oil to drain out completely into a drain pan.

mercury outboard engine oil and filter change

Step 3. Check to see whether the old crush washer has stuck to the side of the engine block around the drain hole. If it has, remove it and discard it. Do not reuse old crush washers because they no longer provide a reliable seal.

Step 4. Put a new crush washer on the oil drain bolt and reinstall the bolt into the drain hole. Tighten the drain bolt until it bottoms out on the engine block, and then give it an additional quarter of a turn to compress the crush washer and get a good seal. Don't overtighten the drain bolt.

Mercury outboard oil drain plug crush washer

Step 5. Lower the outboard back down to the running position, and place some rags or shop towels beneath the oil filter (on the right side of the outboard) to catch any oil that may spill out when you remove it.

outboard motor mercury oil change

Step 6. Remove the old oil filter with a 65mm oil filter wrench. If the filter's gasket sticks to the engine block, remove it with a pick tool, being careful not to scratch the surface of the engine.

oil and filter change Mercury outboard motor

Step 7. Smear a thin coat of oil onto the new oil filter's rubber gasket and install the new filter, tightening it by hand until you feel it make contact with the engine block. Then, tighten it down with a wrench by ¾ of a turn.

Mercury outboard motor filter and oil change

Step 8. Remove the oil filler cap (at the upper rear of the outboard) and fill the engine with the type and quantity of oil specified in your Mercury owner's manual. Replace the filler cap.

mercury oil and filter change outboard engine

Step 9. Run the motor for a couple of minutes to distribute the new oil throughout the motor, then let it sit for a minute. Next, check the oil level using the dipstick (just below the oil filter), and top off the oil level if necessary. 

Step 10. Check the oil filter and oil drain bolt for any leaks and tighten them if necessary. Replace the top engine cowling, and then enjoy another 100 hours of outboard use knowing it has a quality oil and oil filter protecting it!





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