Overheating is one of the few faults that turns a cheap repair into a new engine in the time it takes to finish your trip. What to do at the roadside, and what caused it.
Most car problems get worse slowly. Overheating is not one of them.
A thermostat is a modest repair. A warped cylinder head or a blown head gasket is not, and the distance between those two outcomes is often about five miles of continuing to drive.
At the roadside, in order
Turn the A/C off. Turn the heater on full, fan on high. This feels absurd in July and it works. The heater core is a small radiator, and running it dumps real heat out of the engine and into the cabin. Open the windows and be uncomfortable for a few minutes.
Get out of stop-and-go if you safely can. Airflow through the radiator is what cools it. At a standstill you are relying entirely on the cooling fan, which may be the thing that failed.
Pull over and shut it off if the gauge keeps climbing or the light comes on. There is no destination worth an engine.
Do not open the radiator cap. A hot cooling system is pressurised well above atmospheric, which holds the coolant above its normal boiling point. Release that pressure and it flashes to steam instantly, straight up at whoever is standing there. Wait until the engine is cold to the touch. Not warm. Cold.
Wait, then look. Once cool, check the coolant level in the overflow tank. If it is empty, you have found a symptom, not the cause.
What overheating usually is
Low coolant, from a leak. The most common cause by far. Look for crusty white or coloured residue at hose joints, a damp radiator seam, or a puddle. Coolant is usually green, orange, pink or yellow, and it feels slippery.
A failed thermostat. It is a valve that opens to let coolant into the radiator once the engine warms up. Stuck closed, coolant never reaches the radiator and the engine overheats quickly and predictably, often within a few miles of a cold start.
A cooling fan not running. Symptom to notice: the car runs fine at highway speed and overheats in traffic. At speed there is enough air through the radiator without the fan. In traffic there is not. Could be the fan motor, a relay, a fuse, or the temperature sensor telling it when to switch on.
A failed water pump. It circulates the coolant. When the impeller corrodes away or the bearing fails, flow stops. Often announced beforehand by a whine from the front of the engine or a coolant drip from the pump's weep hole.
A blocked radiator. Internally from corrosion and old coolant, or externally from a decade of leaves and road debris packed into the fins.
A collapsed lower hose. It softens with age and gets sucked shut by the pump at higher RPM. Cools fine at idle, overheats under load, and looks perfectly normal when the engine is off.
The one that changes the conversation
A head gasket failure can cause overheating, and overheating can cause a head gasket failure. That circularity is why continuing to drive is so expensive.
Signs the gasket is already involved:
- White sweet-smelling smoke from the exhaust that does not clear once warm
- Bubbles in the coolant reservoir with the engine running
- Coolant level dropping with no external leak anywhere
- A milky, coffee-coloured film on the oil cap or dipstick
- The overflow tank pushing coolant out and overflowing
A block test checks for combustion gases in the coolant and settles it in about ten minutes.
Use the right coolant
Coolant is not one product. There are several incompatible chemistries — IAT, OAT, HOAT and their variations — and the colours are not reliable identifiers, because different manufacturers use the same dye for different formulations.
Mixing incompatible types can drop the additive package out as a gel that blocks the radiator and heater core. If you are topping up in an emergency and do not know what is in there, distilled water alone is safer than the wrong coolant. Get it flushed and refilled properly afterwards.
Also worth knowing: coolant is not just antifreeze. It raises the boiling point and carries corrosion inhibitors that protect the water pump, the radiator and the aluminium in the engine. Those inhibitors deplete on a schedule, which is why coolant has a replacement interval even when it looks clean.
The habit that prevents most of this
Check the coolant level in the overflow tank once a month, cold, when you check the oil. Ten seconds, no tools.
Coolant does not get consumed the way oil does. If the level is dropping, it is going somewhere, and finding a weeping hose in the driveway is a great deal cheaper than finding it on the shoulder of I-270 in August.
If your gauge has been sitting higher than it used to, or the level keeps needing topping up, bring it in before summer. We pressure-test the cooling system to find leaks that are invisible when the engine is cold and sitting still.
Filed under AC & Cooling
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