Marine Thermostats: Running Cold, Running Hot and How to Test One

The thermostat is a wax pellet in a brass cup, and it is the part most often blamed for an overheating marine engine and most often removed as a cure. Both are usually wrong. This guide explains what the thermostat is actually doing in an outboard, a sterndrive and a diesel, how to tell a thermostat fault from the other things that look like one, how to test the old one in a saucepan, and why running without one is a slow way to wreck an engine.

What it controls

A thermostat holds cooling water back until the engine is warm, then opens by just enough to hold it at its design temperature. The opening temperature is stamped on the flange, and it varies more than people expect. Raw-water-cooled outboards and petrol sterndrives run cool, with thermostats that start to open in the low fifties to around sixty degrees Celsius, because seawater left to sit in a hot engine drops its salt out as scale and the makers want the metal below the temperature where that happens quickly. A closed-circuit sterndrive or a marine diesel with a heat exchanger has fresh coolant on the engine side and runs hotter, usually in the seventies to low eighties, which is where a diesel burns cleanly and the oil stays dry. Fitting the wrong temperature is as much a fault as fitting a broken one.

Running cold

A thermostat stuck open, or missing, lets the engine run twenty or thirty degrees below design. On a two-stroke outboard the first signs are poor idle, plug fouling and a smoky, rich-running engine, because the engine computer or carburettor is still feeding it a cold-start mixture. On a four-stroke the oil never gets hot enough to boil off the water and fuel that condense into it, so the dipstick shows a milky oil or a rising level, and wear in the bores goes up. A diesel run cold builds carbon on the injectors and in the exhaust and will often start to show white smoke under light load, which is the subject of the exhaust smoke guide. The gauge tells the story: a needle that never reaches the normal band, or takes half an hour to get there, is a thermostat that is not closing.

Running hot

A thermostat stuck shut gives an overheat that comes on quickly from cold, within the first few minutes of running, with the engine hot but the outlet hose or the raw water discharge still cool, because the water is not getting through. That pattern, hot engine and cold outlet, is the one that points at the thermostat. An overheat that arrives after a run at speed, or only under load, with a hot outlet as well, is a flow problem and the thermostat is almost never it: a tired impeller, a blocked strainer, a scaled exhaust manifold or a collapsed hose. Our impeller guide covers the commonest cause, and the raw water cooling guide works through the rest. A gauge that swings up and down while cruising is a thermostat that is sticking and freeing, or air in a closed circuit after a coolant change.

The saucepan test

Thermostats are cheap, but if you want to know rather than guess, test the old one. Drop it in a saucepan of water with a cooking thermometer, heat it slowly, and watch. It should start to open a few degrees either side of the stamped figure, be fully open ten to fifteen degrees above it, and close again as the water cools. One that opens late, only partly, or stays open when cold is finished. Look at the old one as well: a build-up of salt crystals around the pellet on an outboard thermostat is a sign the engine has not been flushed, and the new one will go the same way unless it is, which is what the flushing guide is for.

Why removing it is not a fix

Pulling the thermostat out makes the overheat go away, which is why people do it. It also removes the engine's ability to warm up, so you get every one of the running-cold problems above, and on many outboards and sterndrives it does not even cure the overheat at cruise, because the thermostat housing is part of how the pressure in the cooling system is managed and without it the water takes the easy path and bypasses the hottest parts of the block. An engine that only runs at temperature without its thermostat has a flow restriction somewhere else, and the thermostat was the messenger.

Fitting a new one

Match the opening temperature to the engine, not to what was in it, since the last owner may have fitted the wrong one. Replace the gasket or O-ring with it, clean the housing faces, and on many outboards there is a pressure relief valve or poppet behind the thermostat cover that should be checked and its spring and seat cleaned while it is open. On a closed circuit, refill with the bleed screws open and run the engine with the cap off to purge air. Genuine thermostats and housings for Volvo engines are in the Volvo Penta cooling system range, and the right one depends on the engine serial, because the same model has used more than one temperature over its life. Ring 0412 532 984 with the engine and serial number and we will confirm the figure before you order.