The cooling system, and why it kills engines

A gasoline engine turns most of the energy in its fuel into heat rather than motion. Some leaves through the exhaust; a great deal has to be carried out of the metal itself, and that job belongs to a closed loop of pressurized liquid moving continuously between the hottest part of the engine and a heat exchanger in the airflow at the front of the car.

Four things the loop is doing at once

Carrying heat. Liquid moves heat far better than air, so the job is to get it from the metal into the liquid and then out of the liquid into the air. Everything else in the system exists to keep that possible.

Holding pressure. The cap is not a lid. It is a pressure-relief valve that lets the system build pressure as it warms, and raising the pressure raises the temperature at which coolant boils. A system holding pressure can run hotter than water would otherwise allow without turning to steam. Lose the pressure — a tired cap, a leak, a cracked tank — and the boiling point falls to meet the engine's operating temperature.

Regulating temperature. The thermostat is a temperature- operated valve that stays shut until the engine reaches its working temperature, so it warms up quickly, then opens to admit the radiator into the loop. Stuck open, the car takes forever to warm, the heater is weak and the engine spends its life colder than designed. Stuck closed, the coolant has nowhere to shed heat and the temperature climbs fast.

Protecting itself. Coolant is not just antifreeze. Its additive package inhibits corrosion between dissimilar metals and protects against cavitation at the pump. Those additives deplete with time and heat. Coolant left in far beyond its life stops protecting and starts attacking — radiators, heater cores and water pumps corrode from the inside, and mixing incompatible coolant types can produce a gel that blocks the narrow passages first.

The failure chain

Catastrophic engine damage from overheating is almost never one event. It is a sequence, and each link is cheap to break until the last two.

Read that backwards and the lesson is unglamorous: the difference between a hose clamp and a scrapped engine is whether somebody opened the hood occasionally, and whether they pulled over the first time the needle moved.

What to check on a used car

All of this happens cold, in the first two minutes, before the seller has warmed the car up for you.

Never open a hot system. A pressurized loop at operating temperature will discharge scalding coolant when the cap comes off. If the car is warm, look but don't open.

Look at the expansion tank: level against its marks, and the color and clarity of what's in it. Coolant should look like coolant — a clear, saturated color. Brown, rusty, murky or oily tells you something. A thin oil sheen on the surface, or a brown emulsion, is worth a professional look. Check the tank itself for stains and crusty white or colored deposits around its seams and around every hose end you can see; dried residue marks where liquid has been escaping and evaporating.

Squeeze the hoses, cold. They should feel firm and springy, not rock hard, not mushy, not crunchy. Look along the radiator's face for bent fins, impact damage and staining. Check the cap's rubber seal for cracking — it is the cheapest part in the system and one of the most consequential.

Then check the oil cap and dipstick. A light tan emulsion under the oil cap alone, in cold weather, is very often condensation from short trips rather than a gasket failure — this is a genuinely common false alarm. The same emulsion throughout the dipstick and on the underside of the oil is a different matter entirely, and warrants a proper diagnosis before any money changes hands.

On the drive: watch the gauge, but do not stop there, because the gauge is damped on many cars to avoid alarming people. Sit in traffic with the air conditioning on, which forces the fans to run, and listen for them cycling. A cooling fan failure is invisible at highway speed — there is enough airflow without it — and only shows itself stopped in traffic, which is precisely where you don't want to discover it. Turn the heat to full and confirm the cabin gets genuinely hot: weak heat with a full system points at air in the loop, a tired thermostat or a blocked heater core. And after the drive, look under the car and smell. Coolant is sweet, and a sweet smell with no visible drip is a leak evaporating on something hot.

General information for used car buyers, not a repair procedure. Cooling systems operate under pressure and at temperatures that cause serious burns; do not open a warm system. This page states no coolant specification, service interval or operating temperature for any vehicle — those differ by manufacturer and must be confirmed in your owner's manual. Diagram is schematic and describes no particular car.