How to Check and Change Coolant in a Closed Cooling System

With the engine completely cold, look at the coolant recovery (overflow) bottle. The level should be between the marks, usually "cold" and "hot" or "add" and "full." Don't trust the bottle alone: Northern Lights' daily check for its generators is at the pressure cap, because the bottle can look fine while the engine itself is low (for example, if the hose between them is blocked or the cap seal is bad). Regularly check at the pressure cap on the heat exchanger or manifold too (only when cold). Turn the cap slowly to the first stop, then remove it. The coolant should be up to the level your manual shows.

If the bottle keeps going low, there is a leak somewhere. Low coolant is a warning sign, not a routine top-up.

• Clean and clear (any color): good, but still test it.
• Rusty, brown or like chocolate milk: rust or sediment inside the engine. Time to drain, flush and refill.
• Oily film or sludge: possible oil cooler or head gasket problem. Get a pro.
• Greenish goo or gel: BoatUS explains this can happen when too much antifreeze is added. The system must be fully flushed, not just topped up.

Looks aren't enough. Use a coolant tester:

• Test strips show the mix strength and also whether the anti-corrosion additives are still working. BoatUS notes they are the only simple method that checks the additives.
• A refractometer gives an accurate mix reading from a few drops.
• A floating-ball tester or hydrometer is the cheapest way to check the freeze protection.

Most manufacturers want a 50/50 mix. If the additives are worn out or the mix is off, change the coolant.

If you only need to add a little, use the same coolant premixed 50/50, or concentrate mixed with distilled water. BoatUS explains that tap water adds minerals that weaken the corrosion inhibitors, and softened water adds salts. Don't just pour straight antifreeze in "for extra protection"; too strong a mix cools worse, and above about 80% it can gel.

Plan the job with the engine cold, the boat level and out of the water or with the seacock closed. Find all the coolant drain points in your manual. Common ones are drain plugs or petcocks on each side of the block, a drain on the heat exchanger, drains on the exhaust manifolds, and the lowest coolant hose. Put drain pans and short hoses in place so coolant doesn't go into the bilge.

Remove the pressure cap so air can get in and the coolant drains freely. Open the drains one at a time. Poke a piece of wire gently into each drain hole if it doesn't flow; sediment can block them. Drain the recovery bottle as well, or remove and rinse it. Measure roughly how much comes out so you know how much to put back.

BoatUS recommends flushing the closed system thoroughly before refilling, following the engine manufacturer's directions. A simple flush is to close the drains, fill with distilled water, run the engine to warm it up as your manual allows (afloat with the seacock open, or with raw water supplied through muffs or an approved flush kit; never with the raw-water side dry), let it cool, and drain again. Repeat until the water comes out clean. Some manufacturers specify a cleaning product; use only what your manual says.

Close all drains with new washers or the sealant your manual specifies. Replace any soft, cracked or swollen hoses and rusty clamps while it's empty. Fill slowly through the pressure cap opening with the correct coolant, giving air time to escape. Some engines have bleed screws on the thermostat housing or manifolds; open them until coolant (not air) comes out, then close them.

Open the seacock (or connect the flushing device your manual approves) and start the engine. Follow your manual for whether to leave the cap off until the thermostat opens. Watch the coolant level drop as air works out, and top up. Install the pressure cap, fill the recovery bottle to the cold mark, and watch the temperature gauge. Check every drain and hose for leaks.

After the engine cools completely, check the level again at the cap and the bottle, and top up. Recheck after the first couple of trips; small air pockets often work out and the level drops a bit.

Many heat exchangers have a zinc anode (a "pencil zinc") under a threaded plug near one end, on the raw-water side. BoatUS warns that it protects the copper tubes from corrosion and should be checked at least once a year. Close the seacock first. If it's only lightly corroded, scrape it clean and reinstall it; if it's heavily corroded or gone, replace it. Some exchangers don't use a zinc; your manual will say.

Over time, scale, shells and debris build up inside the raw-water tubes, especially in salt water. The sign is a slow rise in running temperature over months. Discover Boating suggests checking heat exchangers for scale every year. Cleaning means removing the end caps, inspecting and cleaning the tubes, and replacing the gaskets. It's within reach of handy owners on some engines, but tubes are easily damaged, so many owners leave it to a pro.

Pour old coolant into sealed, labeled containers and take it to a hazardous waste site or a shop that recycles antifreeze. Never pour it on the ground, down a drain or into the water.