Coolant and overheating: what the gauge tells you

A combustion engine turns only part of the energy in its fuel into motion; a large share becomes heat, and it has to go. Coolant picks it up in the cylinder head and block, a pump drives it to the radiator at the front of the car, and there it gives the heat to the air. A thermostat decides whether it takes the route through the radiator or stays in the small circuit so the engine warms up quickly. Through the heater matrix, the same circuit warms the cabin.

Coolant is more than water. The concentrate, usually glycol-based, lowers the freezing point and raises the boiling point. Additives protect aluminium, cast iron, copper and seals against corrosion and deposits, and lubricate the water pump seal. When warm, the circuit is under pressure, which raises the boiling point further; the expansion tank cap is therefore a valve, not just a lid.

Electric cars and hybrids have cooling circuits too — for the battery, the power electronics and the motor. They are serviced according to the manufacturer's schedule. This article stays with the combustion engine.

Coolants differ less in their glycol than in their corrosion inhibitors. Older types work with inorganic substances such as silicates, newer ones with organic acids, and there are hybrids in between. Each type is matched to the materials of particular engines, and manufacturers approve it under their own specification. Which one applies is in the handbook.

Colour does not help. It is a dye each maker chooses at will; two liquids of the same colour can be chemically different and two of different colours the same. Topping up by colour is guessing.

Mixing is bad for two reasons. Some additives are incompatible and precipitate or form a gel that clogs the radiator tubes and the heater matrix. And even where nothing precipitates, the protective agents dilute one another until none is left in the quantity it was designed for. In an emergency, plain water gets the car to a workshop; afterwards the frost protection is measured and the circuit is brought back to the right type and ratio.

Coolant also ages, because the additives get used up. Some manufacturers specify a change by time or mileage, others none at all; the car's service schedule is what counts.

A cooling circuit is sealed. The level in the expansion tank moves with temperature, but it does not fall over weeks. If it does, coolant is going somewhere, and the only question is whether out or in.

Out is the milder case, because you can see it: drips under the car, white or coloured crusts on hoses, clips, the radiator or the water pump, a sweetish smell after driving. A leaking heater matrix shows inside — damp carpet in the footwell, windows that mist with a greasy film. Coolant is toxic and, because it tastes sweet, dangerous to animals; puddles get wiped up.

In is the more serious case. A leaking head gasket or a cracked head lets coolant into the combustion chamber or the oil, and on many diesels the exhaust gas recirculation cooler can leak internally as well. The signs are white, sweet-smelling smoke from the exhaust even with a warm engine, bubbles in the expansion tank with the engine running, a circuit under pressure although the engine is cold, and at worst a light brown, creamy emulsion in the oil. That is a workshop finding; a workshop can detect exhaust gas in the coolant with a simple test.

Sealants poured into the radiator are an emergency fix for a small external leak, not a repair. They can clog the same fine passages as precipitated coolant.

On many engines the water pump is driven by the timing belt, on others by the auxiliary belt or electrically. It wears at its bearing and seal: a pump that weeps from its vent hole or makes noise is announcing its end. Where it runs off the timing belt it is usually replaced along with the belt, because the labour is the same and a leaking pump ruins the new belt.

The thermostat can fail in two ways. Stuck open, the engine warms up slowly or never properly: the gauge stays low, the heater lukewarm, consumption rises. Annoying, but not acute. Stuck shut, the coolant never reaches the radiator and the engine overheats quickly, often within the first few kilometres. On some engines the thermostat is electronically controlled and reports faults to the fault memory.

The fan has a classic failure pattern. On the motorway enough air flows through the radiator and all seems well. In a queue or at the lights that airflow is missing, and the electric fan has to take over. If it does not start, the temperature rises exactly when you are driving slowly and falls again as soon as traffic moves. The cause is the fan motor, a relay, a fuse or the control. A radiator whose fins are clogged on the outside with insects and leaves has a similar effect.

After any work on the circuit it has to be bled. An air pocket in the cylinder head or the heater circuit leads to overheating at a spot the sensor does not notice, and to a heater that stays cold.

On many cars the temperature gauge is deliberately damped: the needle sits in the middle across a wide range so normal fluctuations do not worry anyone. Conversely, a needle that visibly climbs past the middle is already showing a clear deviation. Many cars also show a red warning light or a message in the display.

If the gauge rises without yet being in the red, take the load off. Air-conditioning off, heater to maximum and the fan on full: the heater matrix is a second, small radiator and this way draws heat out of the engine, uncomfortable as the cabin gets. No full throttle, no climbing with a trailer, and in a queue leave a gap rather than constantly pulling away.

If the gauge reaches the red or the red warning light comes on, stop as soon as it is safe and switch off the engine. Driving on risks a warped cylinder head and a blown gasket — damage that turns a hose into an engine. And even if everything feels normal again afterwards, an engine that has overheated belongs in a workshop: the cause has not gone just because the needle is back in the middle. Until then:

When viewing a car, the cooling circuit is one of the areas that tells a lot and needs little equipment. The condition is a cold engine. Ask the seller not to drive the car before your visit — that also helps with the cold start, and an engine that is already warm when you arrive is worth a question.

Then look at the expansion tank: the level sits between the marks, the fluid is clear and a uniform colour. Brownish sludge, an oily film on the surface or dark rings inside the tank point to oil in the coolant. Glittery particles or crumbs may come from a sealant, and then someone had a leak they did not want to fix.

The oil filler cap shows the other direction: a light brown, creamy deposit on its underside can mean coolant in the oil. But condensation on a car that only does short trips produces it too, so it is a hint, not proof. On the test drive the gauge reaches its normal range and stays there, the heater gets properly hot, and after the drive the engine bay does not smell sweet. White vapour from the exhaust on a cold morning is normal; white smoke from a warm engine is not.

And in the paperwork: a coolant change where the schedule calls for one and, on engines with a timing belt, the water pump at the last belt change. If doubts remain, the test for exhaust gas in the coolant is a small expense against what it rules out.