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How to Change a Boat Engine's Oil

Using an Oil Extractor to Make the Job Easy


Oil Extractor Pump

It's generally easy to change your engine oil, transmission fluid, and other engine fluids on a marine gasoline or diesel engine - if you have the right equipment. Follow your engine manual's recommendation for how often to change the oil or other fluids, such as every 100 hours. On many sailboats, this means once a season, best done at the end of the season when winterizing or before the boat will be laid up for time. Be sure to do it before shrinkwrapping or covering your boat for the winter.

The problem in many sailboats is that because the engine sits so low in the boat, it is impossible to drain the oil from a bottom plug as you would do with a car. Instead, the old oil must be extracted out the dipstick hole.

Fortunately, inexpensive equipment is available for just this purpose. Use it to do your own oil changes and engine winterizing for a big savings in yard bills.

A Fluid Extractor - The Right Tool for the Job

Several different kinds of pumps are available that can suck engine oil out a tube long and thin enough to insert down into the crankcase through the dipstick hole, including a range of manual and powered pumps. Unless price is no object and you can't waste even a minute longer than necessary (in which case you're probably not doing your own maintenance), the vacuum type of oil extractor is far away the best inexpensive tool for removing old engine oil.

Available Models

Boating big box stores and chandleries handle several different models that, except for size of the collection container, are virtually the same. At the time of this writing, the most available models are:

  • Marpac Fluid Extractor (4 liter) - available online from Defender Marine for about $41
  • Several sizes of West Marine Manual Oil Changers - in stores or online from about $50
  • Mityvac 7400 7.3 Liter Fluid Evacuator - available at Amazon.com or elsewhere for about $60
  • Tempo "Oil Boy" Fluid Extractor (4 liter) - at present no longer available (virtually identical to the Marpac - and the model I've used myself for many years)

These fluid extractors all work in the same manner. Each comes with tubes of different diameters that connect to the pump container; you use the largest that will fit down the dipstick hole. On top of the pump housing is a plunger-type pump handle. Once the tube is connected and the free end inserted into the fluid, pumping the handle up and down, usually only 10-15 times, creates a vacuum inside the container that then sucks the oil or other fluid up through the tube into the container.

Tips for Draining Old Oil with a Fluid Extractor

The process may seem slightly intimidating before you try it the first time, but it's amazingly simple (and much, much easier than other kinds of manual pumps). As noted above, just choose the right size tube, insert it in the dipstick hole, pump the extractor's handle, and wait for the oil to flow out. Some tips to ensure the process goes smoothly:

  • Always change oil while the engine is warm. Cold oil is thicker and harder to suck out, and more of the old oil is left inside the engine, which means the fresh oil added already has one strike against it.
  • If the old oil is really dirty, thick, or sludgy, adding a pint or even a quart of Marvel Mystery Oil will thin the oil and make extraction easier and leave the engine cleaner. Warm the engine up first, then remove as much of the old oil as you'll be adding now in Mystery Oil, and then add the Marvel and run the engine again to thoroughly mix it and circulate it throughout the engine.
  • Have some rags ready before you begin - there's always a chance of spilling oil even when you're careful. If you're also changing your oil filter, have a gallon-size ziplock bag to catch the old filter in before the oil drips all over.
  • Have your new oil ready to add after extracting the old.
  • Depending on the size of the container of the extractor model you're using, you may need some empty plastic jugs for the old oil (if more is to be extracted than what the container holds). The extractor has a vacuum relief valve that allows you to stop sucking out oil at any time, so you can pour it from the extractor into jugs for the recycling center.
  • Be patient. Even hot oil still flow slowly, and it may take a while to suck it all out. Pump some more vacuum if the process seems to be slowing. You'll hear a gurgling sound when the bottom is reached and air starts flowing through the tube with the last of the oil. Keep the pump going as long as you're getting any out.


With care, an oil change is an efficient and clean process using an oil extractor. Then simply remove the old oil from the extractor container through its pour spout.

The problem then is cleaning the extractor itself. I wouldn't want to leave the grungy old oil inside the pump for a year until used again, so I simply flush the tube and pump by sucking a small amount of solvent (like gasoline) through the tube and into the container. Swish it around and pour it out, and put it all away until next year!

Here are some other boat maintenance tasks you can easily do yourself.

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