Boat motor overheating? Ten causes, in the order to check them

When the temperature gauge climbs or the overheat alarm sounds, the cause is usually water that is not getting through, not a failed engine. Most of the checks below take minutes and need no tools. Work through them in order and stop at the first one you find.

First, what to do on the water

• Throttle back. If the temperature keeps rising or the alarm sounds, stop the engine as soon as it is safe to, and anchor or call for help if you need to.
• Let it cool. Never open the coolant cap on a hot engine: the system is under pressure and the coolant can scald.
• Look over the stern. Is water coming out with the exhaust on an inboard, or from the telltale on an outboard?

Ten causes, in the order to check them

1. Something over the intake. A plastic bag, weed or mud over the hull intake or the water inlets on an outboard's gearcase. Look, and clear it. If the intake is out of reach, a short burst in reverse can sometimes shake a bag loose. Stop the engine before anyone goes in the water to clear it.
2. A closed seacock, a blocked strainer or an air leak. On an inboard, check the seacock is open. To clean the strainer basket, stop the engine and close the seacock first, or water comes in when you lift the lid. A strainer lid that does not seal lets the pump draw air instead of water.
3. A slipping or broken belt. On many inboards a belt drives the freshwater circulating pump, and on some the raw water pump too. With the engine off, look and press on it.
4. A worn or broken impeller. Flow drops, often after a season or a dry start. See our guide to replacing an outboard or boat impeller.
5. Impeller pieces or debris at the heat exchanger. Lost vanes and scale collect at the heat exchanger inlet and block the water.
6. A scaled or clogged heat exchanger or oil cooler. Salt scale builds up over the years. Clean it at the interval in your manual. With the end caps off, you can see whether the tubes are open; a tube bundle half blocked with white scale or old impeller pieces explains a slow climb in temperature that no other check finds.
7. A clogged exhaust mixing elbow. Where the cooling water joins the exhaust, carbon and scale narrow the passage until the water cannot get out. Signs are a rising temperature with a good raw water flow at the pump, black soot at the exhaust outlet, or an elbow that looks rusted or weeps at its joints. Many makers treat the elbow as a part that wears out, with a check or replacement interval in the manual. Taking it off is usually a few bolts and a gasket, and the inside tells you at once whether it is the cause.
8. Low coolant or a leak, on a freshwater cooled engine. Check the level only when the engine is cold, and look for drips at the hoses, the circulating pump and the heat exchanger end caps.
9. A stuck thermostat. It stays closed, and the coolant never reaches the heat exchanger. On an outboard, a thermostat or pressure relief valve stuck with salt does the same.
10. An engine working too hard. A fouled bottom or propeller, the wrong propeller, or too much weight aboard.

Outboard overheating: what is different

Most outboards have no seacock, strainer or heat exchanger. They draw water through screens on the gearcase, pump it with an impeller in the lower unit, and send it through the block and out with the exhaust and the telltale. So the checks are shorter.

• The telltale is your first gauge. If it is weak, stop the engine and push a thin wire into the outlet to clear salt or an insect nest. If it stays weak once clear, the impeller is the likely cause.
• The water inlet screens on the gearcase clog with weed, mud or barnacles. Clear them by hand or with a soft brush.
• The thermostat and the pressure relief valve sit at the top of the block on many outboards. Salt can hold either closed. Your service manual shows where they are.
• A warning horn or reduced power. Many modern outboards sound an alarm and cut engine speed when they run hot. Treat it as a stop, not a nuisance.
• On a flushing attachment, turn the water on as your manual specifies before you start the engine, keep the attachment seated over the inlets, and never run the engine without water.

After running in salt water, flush the engine with fresh water as its manual describes. Salt left in the passages is how the thermostat and the relief valve stick.

When to stop and call a mechanic

If it still runs hot after these checks, or you see white smoke, milky oil or a loss of power, stop running it. Running a hot engine can warp the cylinder head, blow the head gasket and seize the engine. Tell the mechanic what you saw: the temperature, the engine speed when it started, and which of the checks above you already did. It saves time on the bill.

Keeping it from happening

Change the impeller on schedule, clean the strainer, service the heat exchanger and any anodes in the cooling system at the intervals in your manual, and change the coolant when it is due. On an outboard, flush after every saltwater trip and look at the telltale each time you start. Write each job down with the date and engine hours, so the next one is a plan and not a guess.