The engine thermostat: what it does, how it fails, and why a cold engine is a problem

A thermostat is a valve that stays shut until the coolant around it reaches a set temperature, then opens. That is the whole of the mechanism. What makes it worth an article is that almost everything people believe about it is the wrong way round: it is not there to keep the engine cool, and fitting one that opens sooner does not fix an engine that runs hot.

What it is actually for

An engine has one temperature it was designed to run at, and it is hotter than most people expect — typically 85 to 95 °C for a modern petrol engine. Everything else was chosen around that figure: the oil's viscosity, the clearances the designer left between parts that expand at different rates, the fuelling map, the point at which the ECU stops guessing and starts trusting the oxygen sensor.

The thermostat's job is to get the engine to that temperature quickly and then hold it there. When the engine is cold the valve is shut, so coolant circulates only around the block and head and through the heater matrix. None of it goes to the radiator, because sending it to a radiator is how you refuse to warm up.

Why a cold engine is the problem, not the safe state

This is the part worth internalising, because it inverts the intuition that cooler is gentler.

Most engine wear happens while the engine is cold. Cold oil is thick and slow to reach the top of the head. Fuel condenses on cold cylinder walls instead of burning, washes the oil film off the bores, and drains into the sump where it thins the oil further. Cold metal has not expanded to its designed clearances yet. A warm engine in good condition wears remarkably slowly; the same engine spends its short life in the first two minutes after a cold start, repeated daily.

A cold engine also uses noticeably more fuel, and the reason is worth being precise about, because the original version of this article got it wrong. Warm-up enrichment is commanded from the engine coolant temperature sensor, not the intake air temperature sensor. The ECU reads coolant temperature, adds a warm-up enrichment factor that is largest when the engine is coldest, and winds that factor down to zero as the engine approaches operating temperature — around 80 °C on most calibrations. The intake air temperature sensor matters too, but it is measuring the density of the air going in, and its role in warm-up is secondary and short-lived. If the engine never reaches temperature, the enrichment never fully goes away.

What it does not do

A thermostat does not determine how cool your engine runs. It sets a floor, not a ceiling.

Once the valve is fully open, the thermostat has done everything it can do and is simply a hole. From that point the temperature is decided by the rest of the system: radiator area, whether the core is blocked internally or packed with flies externally, whether the fan comes on when it should, whether the water pump impeller is still moving water, and how hard the engine is working.

So under light load an engine will often sit only a little above its thermostat's opening temperature — that is the thermostat controlling things. Under sustained load on a hot day it will sit wherever the cooling system's capacity puts it, and the thermostat's rating is irrelevant to that number, because the valve was wide open long before.

The cooler-thermostat false fix

This follows directly from the previous section and it is the most useful thing in this article.

A car runs hot. Somebody fits a thermostat that opens at 75 °C instead of the standard 83 or 88, or removes the thermostat altogether. Sometimes the gauge does look better afterwards, in traffic, on a cool day.

It has fixed nothing. If the engine was overheating under load, the thermostat was already fully open when it overheated, so changing the temperature at which it opens changes nothing about the condition that caused it. The fault is a blocked radiator, a failed fan, a slipping or eroded water pump, air trapped in the system, or a head gasket pushing combustion gas into the coolant — and all of those are still there. What has changed is that the problem is now hidden at low load and still present at high load, which is worse than a fault that is honest about itself.

Meanwhile you have made three things worse permanently:

• Fuel consumption rises, because the coolant sensor now reports a lower temperature and the ECU trims less enrichment out.
• Wear rises, because the engine spends more of its life below its design temperature.
• The heater gets worse, which in winter is the complaint that brings the car back.

Removing the thermostat entirely adds a fourth: coolant can circulate so freely that the block and head heat unevenly, and uneven heating across a head is how a head warps.

If the engine runs hot and the thermostat is known good, the fault is somewhere else in the cooling system. Fitting a cooler thermostat erases the symptom, not the fault.

How it works inside

The valve is opened by a wax pellet — a sealed brass capsule packed with a wax blended to melt at a chosen temperature. Wax expands substantially as it melts, and that expansion is used to drive a pin out of the capsule; the pin pushes the valve plate off its seat against a return spring. As the coolant cools, the wax solidifies and shrinks, and the spring closes the valve.

It is worth being clear that there is no mercury in a thermostat, and no electrical component either. The blend of the wax is the entire calibration, which is why a thermostat is stamped with its opening temperature and why that number is the only specification that matters when buying one.

Older thermostats used a different principle: a sealed bellows containing alcohol or ether, which boiled and expanded to open the valve. Bellows types are pressure-sensitive in a way wax pellets are not — as cooling systems became pressurised, a bellows thermostat's opening temperature drifted with system pressure, and the wax pellet replaced it for that reason.

How it fails, and what each failure looks like

A thermostat has two failure modes and they present as opposite complaints.

Stuck closed. Coolant never reaches the radiator. Temperature climbs fast and keeps climbing, often within a few minutes of starting, and the top radiator hose stays cold while the engine gets hot — which is the single most useful check you can make by hand. This one can destroy an engine quickly.

Stuck open, or opening too early. The engine warms slowly or never quite gets there. The gauge sits below normal, the heater is lukewarm, fuel consumption is up, and on many cars the ECU will eventually log a coolant-temperature-below-thermostat-regulating-temperature fault because it can see the engine failing to reach the temperature its own model expects. It does no immediate damage and quietly costs fuel and engine life.

A third case is worth naming because it is missed: a thermostat that opens at the right temperature but too slowly, or does not open fully. This gives an engine that is fine around town and overheats on a long climb, which reads like a radiator or fan problem and often gets diagnosed as one.

Checking one

On the car, the hand test above answers most cases: start from cold, let it idle, and feel the top hose. It should stay cool while the gauge rises, then warm noticeably and fairly suddenly as the valve opens. A hose that gets hot straight away suggests a thermostat already open; one that is still cold when the gauge is high suggests one stuck shut.

Off the car, the classic bench test still works and is demonstrated in the first of the videos at the end of this article: suspend the thermostat in a pan of water with a thermometer, heat it, and watch. It should begin to open within a few degrees of its stamped temperature and be fully open perhaps 10–15 °C above that. One that is already open at room temperature, or that never opens, has answered the question.

One caution on replacement: fit the rating the manufacturer specified, not a cooler one, and bleed the system properly afterwards. Air trapped against a thermostat can hold it shut while the engine boils around it, which produces an overheat that looks exactly like a faulty new part.

Video guides

Video, for the parts of this that are easier watched than read. The English-language ones come first.