How To Drain Antifreeze and Coolant From Your Boat Engine

Written by William Porter in Maintenance

Dealing with antifreeze and engine coolant are routine maintenance tasks, but you don't get to do them that often since at most it's once a year. You can pay someone, but is it necessary?

How do you drain antifreeze and coolant from your boat engine? For engine coolant, there should be a plug or valve low on the engine which you can open to drain the coolant for disposal. In the spring, when it's time to get the antifreeze from winterizing out of your engine, it's much easier - just start the engine and it will come out with the raw water exhaust.

Though that is not always the best way. While antifreeze is non-toxic, it's still better to collect it. And engine coolant is nasty stuff and very toxic. But don't worry, we'll help you get it out safely so you can dispose of it.

On this page:

  1. Draining the Engine Coolant
  2. Antifreeze vs. Coolant - What's the Difference?
  3. Cooling Systems on Boats
  4. Draining the Antifreeze
  5. Proper Disposal

Draining the Engine Coolant

The major reason to drain engine coolant is to replace or refresh the coolant. The most important part of draining the engine coolant is capturing the waste coolant. Working in cramped engine spaces this can be tricky, but you don't want this stuff going in the bilge or overboard. Though the specifics of your boat may vary, you will need a few tools to help you, including:

  • A flat catch pan, like an oil change pan.
  • A sealable container to hold the waste coolant; old coolant jugs are perfect for this.
  • A funnel.
  • Lots of rags.
  • Optional - a shop vacuum.

Somewhere near the bottom of the engine, you will find an engine drain plug. It may be tricky to find, as it usually looks like another knobby thing on the engine. Refer to the engine manual in your owner's manual. Engine coolant drains are usually identified in the engine diagram. It may be labeled "Block Drain" or "Coolant Drain" or something similar.

If you are lucky, you'll have an easy-open valve or petcock in an accessible place like we do on our generator. But on our primary engine, the drain plug is a more awkward place and harder to get something under.

Steps to drain the coolant:

  1. Place a pan under the plug opening. Make sure it's positioned to catch any overshoot, and the pan is big enough to catch all the coolant. If it isn’t, stop midway and drain the pan.
  2. Slowly open the drain plug or petcock. Coolant will flow. Open the coolant fill cap at the top of the engine if the flow is low.
  3. Drain the engine.
  4. If the drain pan gets full, close the plug, empty the pan into the waste storage jugs, then replace it under the drain and continue.
  5. Let the engine drain for a while; there are a lot of jackets and crevices and that heat exchanger to empty. A lot may dribble out.
  6. OPTIONAL: Using the exhaust/blower side of a shop vacuum, put the hose on the fill cap hole to blow out any remaining coolant. You may need to seal the gap between the hose and the fill cap hole edge with rags or your hand to get more effective pressure.
  7. Transfer the waste coolant to the storage bottles.
  8. Wipe up all spills.
  9. Dispose of waste coolant properly.

If you are replacing the coolant with fresh coolant, it is recommended that after step 6 you fill the engine with fresh water and run it for a few minutes to circulate and rinse it. Then when the engine has cooled enough, repeat steps 2 through 6 before you refill with fresh coolant.

Some recommend running the engine, so it warms enough to open the thermostats for full draining. This may not be the best idea, as hot coolant is different to work with, as are hot engines.

When replacing your engine coolant, check your boat's engine manual for guidelines on which coolant to use. There are different types of coolant, and you will need to mix the coolant with the correct proportion of water. Most specify 50%/50% water to coolant for optimal heat removal and freeze protection.

Antifreeze vs. Coolant - What's the Difference?

Referring to "engine coolant" as "antifreeze" has slipped into common usage and muddies the picture of two very different but important boat chemicals. This confusion can be harmful for people working on their boats if they mix them up.

While engine coolant prevents freezing in an engine and this is one of its roles, its primary function is to make engine cooling more efficient by raising the boiling point of the cooling water in the engine. Most engine coolant is made from ethylene glycol and protective additives and is highly toxic. It can cause blindness or death and may have a sweet taste and smell that attracts pets and young children.

You do NOT want to leave puddles or buckets of engine coolant lying in the open, and it must be handled carefully and disposed of properly.

Boat antifreeze is a tasteless and non-toxic blend of propylene glycol and water used for preventing freezing in boat systems. It's safe to use in drinking water systems and is not dangerous like engine coolant.

In your engine, you will use engine coolant in the freshwater side of the cooling system and will last several seasons. We use antifreeze in the winter to prevent freezing in the raw water side of the cooling system.

Cooling Systems on Boats

Most modern marine engines are fresh water cooled, meaning they have a fresh water/coolant mix circulating through the engine block to remove heat from combustion. In a car, heat is removed by circulating the coolant through the engine, then through a radiator. The radiator has a fan blowing on it and is at the front of the car, so a constant wash of fresh, cooler air is forced through it as the car moves down the road.

Deep in the bowels of your boat, there's no airflow, so air cooling does not work. Instead, raw water is pulled in from outside the boat and run through a heat exchanger. Hot engine coolant runs through sealed tubes inside the heat exchanger and to shed the heat from the engine into the cool seawater. The water gushing out with your exhaust is this heated seawater carrying the excess engine heat away.

A raw water cooled engine does not have the freshwater coolant side or the heat exchanger. Newer boats rarely use raw water cooling, as circulating corrosive seawater through the engine block reduces engine life.

Draining the Antifreeze

Getting the antifreeze out of a winterized engine is easy, as whatever fluids are in the seawater side of the cooling system will shoot out the exhaust when the engine starts. While antifreeze is non-toxic, it's not something you'd want to drink over ice, either, since it's not completely benign.
It's not the best practice to eject all of it into your local waterway. While it's not immediately toxic to wildlife like coolant, we do not fully understand the impact of lots and lots of propylene glycol hitting the harbor every spring.

So recapture your antifreeze if you can. There are two ways you can do it - either before you launch or in the water.

Recapture before Launching

If you winterized your boat in the fall, you know about running an engine with the intake hose in a bucket. If you've left this hose disconnected over the winter, it's not that big of a deal to flush the engine with fresh water in the spring.

Have someone stand at the exhaust outlet with a five-gallon bucket underneath it. With the engine intake hose in a full bucket of fresh water, start the engine and let it run, catching the exhaust water as it comes out. Don’t let the intake bucket go dry, as running your engine dry may damage the water pump impeller.

Recapture it After Launching

This is a lot less practical if your exhaust outlet is low near the water or hard to reach from the dock. But if you can do it, catch the first buckets full of pink water that come out.

Proper Disposal

Collecting the waste coolant and antifreeze makes this process more work, so it's important it gets to the right place for disposal. You can't just dump it down the drain or throw it in the trash. Though pickups may be available in your town, contact your local sanitation department or trash hauler for more information.

Most marinas have a disposal area for waste oil, coolant, and other toxic waste products. Ask in the office at your marina for disposal instructions. Well set up yards will have a separate shed or fenced off area with containers for each type of waste (oil, gas/petrol, diesel, coolant and antifreeze, batteries, etc.). Some will just have marked areas to leave each type of waste; make sure dump or leave your jugs in the right place.

If you are not working in a marina, take the coolant to an appropriate disposal facility. Some service stations may accept waste coolant. Most municipal or regional household hazardous waste handling centers accept waste antifreeze and coolant.

Propylene Glycol can be recycled, but they typically do this in industrial volumes, not the small amounts we use in boats. Collection centers may recycle, but you won't have much luck trying to recycle a gallon or two yourself.

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