Yanmar 6LY Coolant Loss: Where Is the Coolant Going?

You check the coolant on your Yanmar 6LY marine engine and notice the level is a little low.

You top it up.

A few trips later, you check again and it's dropped.

There's no puddle under the engine. The temperature may still look normal, and the engine seems to be running fine.

So where is the coolant going?

It's easy to start thinking:

“Is it the head gasket?”

Maybe. But I wouldn't start there.

First, have a good look around the engine.

If you're looking for parts rather than troubleshooting information, see our Yanmar Marine Engine Parts page.

Is the Coolant Really Disappearing?

Check the coolant when the engine is cold, and try to check it the same way each time.

This is especially important if the coolant was recently changed or the cooling system was opened for maintenance.

What you really want to know is:

Does the level settle, or do you keep having to top it up?

If you're adding coolant again and again, then you need to find out where it's going.

Have a Good Look Around the Engine

Don't just look underneath for a puddle.

Check the coolant hoses, clamps and connections. Have a look around the freshwater pump and anywhere you can see staining, dried coolant or dampness.

A small leak can be difficult to spot when the engine is cold.

It may only start leaking once the engine warms up and the cooling system is under pressure.

Coolant can also run along a hose, pipe or engine surface before it drips.

So the wet spot you find isn't always where the leak started.

Check the Cap, Tank and Small Hose

This area is easy to overlook.

Look around the coolant tank, pressure cap and recovery tank.

Do you see dried coolant around the cap?

Is there coolant around the small hose or its connections?

Is the level in the recovery tank different from normal?

That small hose is worth checking carefully. If it's loose, damaged or leaking, you can lose coolant without seeing an obvious leak around the rest of the engine.

Have a good look at the pressure cap too.

If it isn't sealing or working properly, the coolant level may not behave the way it should as the engine heats up and cools down.

Could It Be the Heat Exchanger?

It could be, but don't order a heat exchanger just because you can't find a puddle.

The marine cooling system has an engine-coolant side and a seawater side, with the heat exchanger transferring heat between them.

If the coolant keeps disappearing and everything around the outside of the engine looks dry, the heat exchanger is one of the areas worth checking properly.

But remember:

A hidden coolant leak doesn't automatically mean a bad heat exchanger.

It's one possibility, not a diagnosis.

For more information about this part of the cooling system, see our Yanmar Marine Heat Exchanger Maintenance, Cleaning, Gaskets & Parts guide.

Did This Start After Maintenance?

Think about when you first noticed the problem.

Was the coolant changed recently?

Was a hose removed?

Was the heat exchanger opened?

Was anything done around the freshwater pump?

If the coolant started dropping straight after work was done on the cooling system, go back over what was touched.

Sometimes asking:

“What changed?”

gets you closer to the problem than replacing parts.

What If You Still Can't Find the Coolant?

If you've looked around the engine and the coolant keeps disappearing, don't just keep topping it up.

Pay attention to what else the engine is doing.

Is it starting to run hotter than normal?

Is coolant being pushed out?

Have you noticed anything unusual from the exhaust?

Does the engine oil look different from normal?

None of those things by itself proves you have a head-gasket problem.

But the coolant has to be going somewhere.

If there's no obvious external leak, it's time to test the cooling system properly.

Find out where the coolant is going before you start replacing parts.

Make Sure You Know Which 6LY You Have

This matters when you start looking for parts.

The Yanmar 6LY family includes several generations, such as 6LYA, 6LY2 and 6LY3 engines, as well as the newer 6LY common-rail range.

They may use the same basic idea of separate engine-coolant and seawater circuits, but that doesn't mean the pumps, heat-exchanger parts, caps, gaskets or other cooling parts are interchangeable.

So don't order a part just because someone tells you it's “for a 6LY.”

Check the complete engine model and serial number first.

Need Yanmar 6LY Cooling Parts?

Once you know what's wrong, make sure the replacement part matches your exact engine.

If you're unsure which numbers we need, see our Yanmar Marine Engine Parts, Part Numbers & FAQs guide.

For a quote, send us your:

Engine model · Serial number · Existing part number · Quantity · Delivery destination

Don't know the part number?

Send us a clear photo of the complete engine nameplate and the part you need.

Request a Quote

Frequently Asked Questions

Why does my Yanmar 6LY keep losing coolant?

Start by looking around the hoses, clamps, connections, freshwater pump, pressure cap and recovery tank. If the coolant keeps disappearing but everything outside the engine looks dry, the cooling system needs a closer check.

Can the heat exchanger cause coolant loss?

Yes, it can. A leaking heat exchanger is one possibility, but don't assume it's the problem just because you can't see an external leak. Find out where the coolant is going before replacing the heat exchanger.