Temperature alarm: the boat engine is overheating

A temperature alarm going off while under way always comes as a small shock. The good news: in the great majority of cases, the cause is simple and lies along the path of the seawater. The bad news: an engine that keeps running while overheating can be seriously damaged in a short time. So protect it first, then look for the cause.

To understand, a word on cooling. On most inboard diesels there are two circuits. A closed circuit of fresh water and coolant flows through the engine, as in a car. It gives up its heat, in a heat exchanger, to seawater drawn in by an impeller pump through a through-hull, a seacock and a strainer; this seawater is then injected into the exhaust elbow and goes out with the gases. Some older or simpler engines are cooled directly by seawater, with no closed circuit. Your engine manufacturer’s manual always has the last word.

1. What you observe

• The alarm sounds and the warning light comes on, sometimes without the needle having moved.
• The needle rises gradually as you motor along.
• The temperature only rises under load, at high revs or against the current, and drops back at idle.
• The temperature rises suddenly, within a few minutes, often with less water at the exhaust and steam.
• A smell of hot rubber, or the charge warning light coming on at the same time: think of the belt.

2. Safety first

• Throttle back immediately and go into neutral if the situation allows. The temperature often stabilises at idle.
• If the alarm continues or the needle is in the red, stop the engine — provided you are not close to a hazard. If you need the engine to get to safety, keep it at idle for strictly as long as necessary, get the sails up or prepare to anchor, and stop it as soon as possible.
• Never open the cap of the cooling circuit, the expansion tank or the heat exchanger on a hot engine. The coolant is under pressure and above 100 °C: it spurts out and causes severe burns. Wait until the engine is cold to the touch, which often takes more than an hour.
• Do not pour cold water on a very hot engine: the thermal shock can crack parts.
• To touch the belt or the pump, engine stopped and key removed: a pulley that starts turning catches fingers.

3. Likely causes, from most to least common

• Raw-water seacock closed — forgotten after laying up or after cleaning the strainer. More common than you would think.
• Blocked intake or through-hull — a plastic bag, weed, shells after a long stay in harbour. The overheating then often comes suddenly, under way.
• Dirty raw-water strainer, or its lid badly refitted: it lets air in and the pump loses its prime.
• Raw-water pump impeller worn, hardened or with broken blades. An impeller that has run dry for a few moments can be destroyed, and its pieces go off towards the heat exchanger.
• Belt slack, slipping or broken — it drives the closed-circuit pump and often the alternator: the charge warning light comes on too.
• Coolant level too low — leak at a hose, a clip, the pump.
• Heat exchanger scaled up or blocked by pieces of impeller: water gets through but no longer carries away enough heat. Typical of a slow rise, especially under load.
• Exhaust elbow blocked by carbon and corrosion: the seawater gets through badly.
• Thermostat stuck closed.
• Overloaded engine — fouled hull or propeller: it strains and heats up at high revs.
• Less common: closed-circuit pump failure, head gasket; or a false alarm due to a sender or a wire. Until that is proven, treat the alarm as real.

4. What you can safely check yourself

Engine stopped, and cold for anything to do with the closed circuit:

• The raw-water seacock: open?
• The flow at the exhaust on restarting — see the guide on little or no water at the exhaust.
• The raw-water strainer: seacock closed first, then lid open, basket cleaned, seal in place, lid properly tightened, seacock reopened.
• The intake: seacock closed, strainer open, you can sometimes feel the blockage. Clearing it from outside is only done in harbour or at anchor in a calm sea, engine stopped and key removed, if you know how to do it safely.
• The belt: present, no cracks, no black dust around the pulleys, tension as given in the manual.
• The coolant level in the expansion tank, engine cold only; signs of leaks under the hoses and the pump.
• The impeller, if you have done the job before and have a spare impeller: seacock closed, pump cover removed, impeller checked. If pieces of blade are missing, they must be found: they have gone off towards the heat exchanger.
• An infrared thermometer on the manifold and the elbow, after restarting, helps confirm a real overheat — without ever touching the parts.

5. When to stop and call a professional

If the temperature climbs again despite an open seacock, a clean strainer, a sound impeller and a belt in place, the cause lies further on: heat exchanger, exhaust elbow, thermostat, closed-circuit pump. Removing, descaling or replacing them is a job for a marine mechanic. Call one straight away too after severe overheating, or if you see thick white smoke, milky oil or coolant loss with no visible leak: the head gasket may have suffered. Give them the make and model of the engine, its hours, when it started (after antifouling, after laying up, only under load), the temperature reached and the revs, what you have already checked, and the date of the last impeller, the last coolant change and the last check of the elbow.

6. Stopping it from happening again

• Change the impeller every year or on the schedule in the manual, and keep a new impeller and its gasket on board.
• Clean the raw-water strainer regularly, and work the seacock so that it does not seize.
• Check the intake at every haul-out: see the guide to antifouling.
• Replace the coolant and check the belt on schedule; have the heat exchanger descaled and the exhaust elbow checked as the manual says.
• Get into the habit of looking at the water at the exhaust every time you start.

All these due dates are in the guide to maintaining an inboard engine, and recommissioning the raw-water circuit in spring in the one on laying up and recommissioning. Record the fault and its repair in the Hullteam log — cause, parts, date, engine hours: it is this record that will give the boat its value at resale, see which proof really counts.