Car overheating: what to do and why it happens

When it overheats, and what that narrows it to

Whether it overheats in traffic or on the highway is the most useful single observation. Traffic overheating points at the fan. Highway overheating points at flow.

What to do in the first two minutes

Turn the air conditioning off and the heater on full, with the fan at maximum. It is deeply unpleasant in summer and it works, because the heater core is a small radiator and running it pulls real heat out of the engine.

Get off the road and switch the engine off. An engine sitting at temperature with no coolant flow is still better than one being driven while overheating. If you are stuck in traffic and cannot pull over, shift to Neutral and raise the idle slightly, which moves more coolant and spins the fan.

Do not open the radiator cap or the expansion tank. A hot cooling system is pressurised well above atmospheric, which raises the boiling point of the coolant above 240 degrees. Releasing that pressure makes the whole system flash to steam instantly, and it comes out of the opening you just made. This causes serious burns every summer and it is entirely avoidable.

Wait until the engine is cool enough to rest a hand on the upper radiator hose, which takes at least thirty minutes and often longer. Then, if you must, open it slowly with a cloth over it.

If steam is coming from under the hood, or the gauge is in the red, or you can smell something sweet and hot, do not restart it to move it a few blocks. That is the drive that turns a thermostat into a head gasket.

Overheating in traffic but fine on the highway

This pattern is close to diagnostic on its own. At 50 mph the radiator has a large volume of air being forced through it whether the fan runs or not. At a standstill the only airflow is the fan.

So a car that runs happily on the highway and climbs in stop-start traffic is telling you the fan is not doing its job. The fan itself may have failed, or the relay, or the fuse, or the temperature sensor that decides when to switch it on, or the wiring in between. On many cars the fan is also commanded by the engine computer, so a sensor reading low means the computer never asks for it.

There is a quick check you can do. With the engine warmed up and idling, watch the fan. It should cut in before the gauge reaches the halfway mark and stay on until the temperature drops. Switching the air conditioning on should also force it to run on most cars, which is a useful way to test the fan circuit independently of the temperature sensor.

The reverse pattern, fine in traffic and overheating on the highway, means the fan is working and the problem is flow. A partially blocked radiator, a failing water pump impeller, or a thermostat not opening fully all restrict circulation, and circulation matters most when the engine is producing the most heat.

Coolant that keeps disappearing

A cooling system is sealed. Coolant does not get used up, so if you are topping it up regularly it is going somewhere, and finding where is the whole job.

External leaks announce themselves eventually: a puddle under the car, crust around a hose clamp, a wet radiator seam, a stain down the timing cover from the water pump weep hole. Coolant leaves a distinctive residue as it dries, so a leak that only weeps while hot still leaves evidence.

Internal leaks are the ones that matter. A failed head gasket can let coolant into a cylinder, where it gets burned and leaves through the exhaust, or into the oil, where it turns the dipstick into a pale sludge. The tells are white sweet-smelling smoke from the exhaust well after the engine is warm, a coolant level that drops with no puddle anywhere, bubbles in the expansion tank while it idles, and oil that has gone the colour of coffee with milk.

We pressure test the cooling system cold, which holds it at operating pressure without the engine running and shows where it goes. A combustion gas test on the coolant answers the head gasket question directly, by detecting exhaust gases in a system that should never contain any.

Being straight about this: a head gasket on most engines is a significant job, and it is worth knowing that before spending money on hoses and thermostats hoping. The test costs a fraction of the guess.

Thermostats, airlocks and the gauge that swings

A thermostat is a valve that stays shut until the engine reaches operating temperature, then opens to let coolant into the radiator. Stuck shut, the engine overheats quickly and predictably while the radiator stays cold, which is a fairly easy thing to feel with a hand on the upper hose.

Stuck open is the opposite problem. The engine never warms up properly, the heater is lukewarm, and fuel economy suffers, though it will not overheat. Worth mentioning because it is the same part with the opposite symptom.

An air pocket produces the most confusing version of all: a temperature gauge that climbs, drops, climbs again, and a heater that blows cold at exactly the wrong moment. Air in the system means the sensor is sometimes reading air rather than liquid, so the gauge stops telling the truth. This almost always follows recent work where the system was opened and not bled properly, so if the overheating started within a week of a hose, radiator or water pump job, say so, because it changes where we look first.

What we do about it, and what it costs

We come to the car and start with a cold pressure test, which finds external leaks without running the engine and without guessing. Then the thermostat operation, the fan circuit including relay and sensor, the radiator for blockage and the coolant itself for combustion gases where the symptoms warrant it.

The scan matters here too, because the computer stores coolant temperature history and fan commands, and it can tell us whether the fan was asked to run and refused, or never asked at all. Those are different faults with different fixes.

Diagnosis starts at $95 and includes the pressure test. The repair gets quoted in writing once we know which component it is, because a thermostat, a fan relay, a radiator and a head gasket are four very different numbers and quoting a range before testing would be a guess dressed up as a price.

One honest limit: a head gasket or a cracked head is generally not a driveway job, and we will tell you plainly if that is where the testing lands rather than starting work we cannot finish well. Knowing that for $95 is considerably better than finding out after buying parts.

We cover Flushing and the rest of Queens, Brooklyn, Manhattan, the Bronx, Staten Island and Long Island. If the car is sitting somewhere hot right now, leave it off and text us the location.

Common questions