Method for Changing the Oil on a Mercruiser 7.4MPI

by David Curtis
Changing the engine oil on a Mercruiser 7.4.

Changing the engine oil on a Mercruiser 7.4.

Regular maintenance of your Mercruiser marine engine will help provide years of trouble free service. It is recommended that you change your engine oil after every 100 hours of service or after each boating season. Different oil weights are used for different outdoor temperatures and operating conditions. Consult with your marine supply distributor or check your engine service manual to obtain the proper weight oil for your application. Remember to always use a premium grade of oil.

Connect a Mercruiser engine starting tool to a garden hose. Place the muffs of the tool over the water intake openings on the stern drive just above the propeller. Turn on the water supply. Turn on the blower and allow it to run for a few minutes. Start the engine and allow it to warm up. It is easier to remove the old oil if it is warm. Stop the engine and blower and turn off the water supply.

Open the engine compartment and locate the oil dip stick. Protect your hands with a pair of latex gloves. Remove the dip stick and notice the threads at the base of the opening.

Attach the oil pump hose to the dipstick opening by twisting the end of the hose onto the threaded portion of the base. Place the other end of the pump into an empty plastic container. Pump the oil out by hand if using a manual pump or attach the positive and negative wiring clips to the battery terminals if using a powered pump. Remove the old oil completely and remove the pump.

Remove the oil filter by placing an oil filter wrench over the lower base of the filter and turn it counter clockwise until it becomes loose. Remove the wrench strap and remove the filter with your hands.

Remove the new filter from the box and rub a little engine oil over the seal with your finger. Twist the new filter on the base until the gasket touches and securely tighten it with your hands another three quarters of a turn. Use the filter wrench if necessary but do not over tighten it.

Remove the oil cap located on the valve cover and place a funnel inside the opening. Pour the new oil into the engine until the dipstick reads between "add" and "full".

Turn on the water supply to the starting tool. Turn on the blower and allow it to run for a few minutes. Start the engine and allow it to warm up to circulate the oil throughout the engine and filter.

Turn off the engine and blower, remove the starting tool and turn off the water supply. Check the dipstick and add additional oil as needed. Wipe any excess oil from the engine and tools with a shop towel.

Items you will need

  • Mercruiser starting tool
  • Latex gloves
  • Shop towels
  • Engine oil
  • Oil filter
  • Funnel
  • Oil filter wrench
  • Marine oil pump
  • Container for used oil


  • Properly dispose of the used engine oil.

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