Coolant for Marine Engines: Which Type, How Often, and Why Mixing Is a Mistake

Most marine diesels and a good share of petrol sterndrives have two cooling circuits. Seawater comes in through the hull, passes through a heat exchanger and goes out with the exhaust, and that side is covered in our raw water cooling guide. The other side is the closed circuit: a fixed charge of coolant that circulates through the block, the head and the exhaust manifold and gives its heat up to the seawater in the exchanger. That coolant is the part owners forget, because it is sealed in and does not visibly wear. It does, and the failure mode is not freezing. On the Yorke Peninsula it is corrosion, and it costs heat exchangers, manifolds and, eventually, blocks.

What the coolant is doing

Antifreeze is a misleading name for it in South Australia. The glycol in the mix raises the boiling point, which matters in a hot engine room, and lowers the freezing point, which matters to almost nobody here. The work is done by the inhibitor package: a few per cent of additives that coat the metal, buffer the acidity, stop the different metals in the circuit from eating each other, and stop the cavitation pitting that a vibrating cylinder liner causes in the coolant alongside it. The inhibitors are consumed as they do that work. Once they are used up, the glycol is still there, the coolant still looks like coolant, and the circuit is quietly corroding. That is why coolant has a change interval that has nothing to do with how it looks.

Colour is a label, not a specification

The most expensive mistake with coolant is treating colour as a type. There is no standard that says green is one chemistry and yellow is another; each maker picks a dye. Volvo Penta's own range makes the point: the older Volvo Penta green coolant is a conventional ethylene glycol mix supplied ready to use, and its later long-life coolant is a different formulation for the later engines, and the two are not to be mixed. Penrite sells coolants in red, blue, green, purple and pink, and the colours follow what the vehicle makers factory-fill, not a chemistry scale. The one in the Penrite range that is sold as a green is a hybrid organic acid type with a long service life, which is a very different product from a conventional green that wants changing every couple of years. Read the engine's operator manual for the specification, buy a coolant that states it meets that specification, and ignore the colour of what came out.

Why mixing is a mistake

Two coolants of different inhibitor chemistry do not average out. Some combinations react and drop a gel or a sludge that blocks the fine tubes of a heat exchanger and the small passages in a cylinder head, and the first symptom is an engine that runs hotter than it used to with nothing obviously wrong. Even where they do not react, each inhibitor package is diluted below the level it needs to protect the metal, so you end up with the corrosion protection of neither. If you have to top up on the water with something other than the right coolant, use clean fresh water, write down that you did it, and get the whole system drained, flushed and refilled when you are home. The Volvo Penta oils and chemicals range carries the factory coolants in five litre packs for a top-up and in larger drums for a full change.

Ready-mixed or concentrate

Ready-mixed coolant is diluted at the factory with deionised water to the stated protection level, and the Volvo mixes are sold that way. Do not dilute it further; adding water weakens both the inhibitor concentration and the boiling protection. Concentrate is mixed by you, usually one to one, and the water matters: tap water in much of South Australia is hard, and the calcium and magnesium in it drop out as scale on the hottest metal, which is the cylinder head and the exhaust manifold. Use demineralised water from the supermarket for concentrate, every time.

How often

The interval is in the engine's service schedule and it is set by chemistry, not hours alone. Conventional coolants are typically a two year item regardless of how little the boat ran. Long-life hybrid and organic acid coolants run considerably longer, but only if the system was clean when they went in and nothing else was added since. If you do not know what is in the engine or when it went in, that is the answer: change it, and start the clock. Between changes, look at it. Coolant in the header tank should be clear and the colour it was. Cloudy, rusty or brown means the inhibitors are gone and corrosion has started; an oily film or a mayonnaise emulsion on the surface means oil is getting in, which is a cooler or a head gasket, and that is a workshop job rather than a coolant change.

Doing the change properly

Drain from the lowest point, which on most engines is a tap or plug on the block as well as the heat exchanger, because draining the exchanger alone leaves half the old coolant in the engine. Flush with fresh water until it runs clear, and if the old coolant was dirty, run a proper cooling system flush through it first. Refill slowly through the header tank with the engine's bleed screws open so air is not trapped in the head or the thermostat housing; an air lock gives a false level and a hot spot at the top of the engine. Run it up to temperature with the cap off, watch the level, top up, and check again after the first trip. While the system is open, look at the hoses, which is a separate subject but the same twenty minutes, and the heater and coolant hose range has the sizes a closed circuit uses. Volvo engines have their own pressure caps and expansion tanks in the Volvo Penta cooling system range, and a cap that no longer holds pressure lowers the boiling point of the whole circuit.

If you are not sure which coolant your engine was built for, ring 0412 532 984 with the model and serial number. It takes a minute to look up and it is a great deal cheaper than a heat exchanger.