Engine overheating: seven causes, in the order to check them

The temperature gauge starts climbing. What happens next depends a lot on what you do in the following five minutes — and in what order you look for the cause.

The order here isn't arbitrary. It goes from the most common and cheapest cause to the rarest and most expensive, and correspondingly from what can be fixed in a minute to what sends the boat to the yard.

First — shut down or not

Shut down. If water stops coming out of the exhaust, steam appears, you smell burning, or the temperature goes into the red. Every minute an overheated engine keeps running eats into the life of the head gasket, and replacing it costs more than everything else in this article combined.

You can throttle back and continue, if the gauge has risen above normal but not into the red, and water is still coming from the exhaust. Proceed at low speed and keep watching: if the temperature keeps rising, shut down.

If you restart the engine after it cools down and it overheats again within ten minutes, don't try again. The cause hasn't gone anywhere.

1. Closed seacock

The most common cause, and the most annoying one. The through-hull valve was left closed after being laid up or over winter storage. Takes ten seconds to check.

2. Clogged raw-water strainer

A plastic bag, weed, or a sheet of polyethylene on the grille — common after lying in murky water or passing close to a beach. Close the seacock, open the strainer bowl, and rinse the mesh.

Also check the through-hull intake from the outside: it fouls over with growth, and from the water you can only spot this with a mask on.

3. Raw-water pump impeller

The rubber impeller lasts two to three seasons and can fall apart without warning. If the seacock is open and the strainer is clean but there's no water from the exhaust, this is almost certainly it. A detailed step-by-step replacement guide.

Keep the main thing in mind: the broken-off vanes have gone further into the circuit, into the heat exchanger, and need to be retrieved from there.

4. Debris in the heat exchanger

If the impeller was ever replaced without counting the vanes afterward, pieces of rubber are sitting in the heat exchanger tubes, blocking part of the cross-section. The engine will hold temperature at idle and overheat under load — a telltale sign.

The fix is opening the heat exchanger end cap and cleaning it out. While you're at it, replace the end cap gaskets: the old ones almost always tear on removal.

5. Thermostat

Stuck closed, and the engine overheats quickly and badly. Stuck open, and conversely it takes a long time to warm up, which is also harmful.

Checking it is simple: remove it and drop it into a pot of hot water, watching whether it opens at around its rated temperature. It's a cheap part, worth carrying a spare.

6. Freshwater pump drive belt

On engines with a closed cooling circuit, the freshwater pump is driven by a belt, usually the same one that drives the alternator. If it's loose or broken, there's no circulation. A telltale clue: charging output drops out along with the overheating.

7. Coolant level and condition

Low antifreeze in a closed circuit, an air lock after topping up, or old coolant that has lost its properties. Check the level with the engine cold: opening the cap of a hot expansion tank is dangerous.

If everything above checks out

What comes next is what actually sends a boat to the yard: a worn pump housing with scoring, a clogged freshwater circuit, a blown head gasket, or warping. An indirect sign of a gasket problem is emulsion in the oil, bubbles in the expansion tank, or white smoke.

At this point it's not worth guessing on your own. But even here, half the job is finding the right part, not fitting it.

What to photograph if you need a part

• A close-up of the engine nameplate — manufacturer, model, serial number. How to read and photograph it.
• The tag or stamped marking on the component itself — the pump, heat exchanger, and thermostat each have their own part numbers.
• The removed part next to a ruler, from both sides.

With this set of photos, the part can be tracked down in a day, even in cases where the dealer says the item has been discontinued.

In short

Check in this order: seacock, strainer, impeller, heat exchanger, thermostat, belt, coolant. Six causes out of seven can be fixed by hand in an hour and cost less than a night in a marina. The seventh gets expensive precisely because someone didn't shut the engine down in time.