How to Change the Oil in a 5.7L 350 Mag MPI Mercruiser

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Based on the Chevy 350 engine, the Mercruiser 5.7 L 350 Mag MPI has many qualities in common with its landlubber cousin. You may not be able to gain access to the drain plug on the bottom of the oil pan. This means pumping the oil out of the crankcase rather than draining it unceremoniously into a pan. You also need to know the model year, stamped on the information plate on the starboard -- right -- cylinder head.

Determine the Engine's Model Year

  • Look on the center of the top of the engine, at the front of the manifold -- the end where the belts and pulleys are located. You'll see a plate that lists the manufacturer, GM, the engine's size and horsepower and the model year. The model year is important because the oil capacity and the location of the oil filler on engines made from 1998 to 2001 is different from engines made between 2003 and 2009.

Out With the Old

  • As with any oil change, the festivities move more quickly if the oil is warm. Warm oil -- like any fluid -- flows more quickly than cold. This means that you need to start the engine and perhaps take it for a day of boating as a precursor to the oil change. In any case, you have to stop the engine before you begin the process of removing the oil. If you have sufficient room beneath your engine to fit in an oil change pan as well as your arm, you can remove the drain plug from the oil pan with a 7/8-inch box-end wrench and allow the oil to drain. As the oil drains, stick your fingers into the stream of oil and rub them together to feel for bits of metal.

    If your boat's configuration does not allow you access to the drain plug on the bottom of the engine, remove the dipstick. Insert the probe and hose of the crankcase oil pump into the dipstick tube. Push the hose into the tank until it "bumps" on the bottom of the oil pan. Place the pump's discharge container into the oil drain pan or another receptacle suitable for receiving the spent oil. Pump the oil from the crankcase. When the pump no longer discharges oil, stop pumping, remove the probe from the dipstick tube and re-insert the dipstick.

Finishing Touches

  • Move to the starboard side of the engine and kneel. You will see, near the split between the oil pan and the engine block, a canister that in all likelihood bears the name of a filter manufacturer. Remove the oil filter with a strap wrench. Around the top of the filter, you will find a rubber sealing ring. If you do not see it, feel underneath the fitting from which you removed the oil filter. Peel the sealing ring from its place, whether on the filter or the fitting. Discard both the filter and the ring in a metal trash can with a metal lid. Coat the new sealing ring, provided with the new filter, with engine oil and slip it into its groove on the top of the oil filter. Spin the new filter on the fitting gently -- if you feel and resistance, back the filter off and try again to avoid a cross-threaded filter. Once the filter is "started" onto the fitting properly, tighten it as securely as possible with your hand and the strap wrench.

The Oil Capacity Quirk

  • The amount of oil you must add to the engine varies according to the model year. Engines made from 1998 to 2001 have a larger crankcase than those made between 2002 and 2009. The earlier engines require 5 1/2 quarts. Those made between 2002 and 2009 have a smaller crankcase and require only 4 1/2. Remove the oil filler cap from the oil filler, located on the right rocker arm valve cover -- the cap is clearly marked with the word "Oil" in capital letters. Pour the appropriate amount of oil for your model year into the filler. When you've finished, put the filler cap back onto the filler.

Check It and Test It

  • Wait five minutes. Hum, whistle, go get a cup of coffee. When you come back to the engine, bring two clean shop cloths. Pull the dipstick, wipe it on the first shop cloth and re-insert it into the dipstick tube. Pull the dipstick out of the tube again and inspect it. The oil level -- nice, clean, new straw-colored oil -- will appear on the dipstick between the "Add Oil" and "Full" marks. Start the engine and check around the dipstick, the oil filter and the drain plug for leaks.

References

  • Mercury Mercruiser Service Manual Number 24 -- Marine Engines, GM V-8, 305 CID (5.0L)/350 CID (5.7L); Mercury Marine
  • Photo Credit Jupiterimages/Photos.com/Getty Images
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