Your Car Is Overheating: What to Do (and What Not To)

Cooling system failure is the most expensive cheap repair in motoring. The parts that fail are usually inexpensive — a hose, a thermostat, a radiator cap. The damage they cause when ignored is not.

An engine that gets hot enough for long enough will warp its cylinder head or crack its block. That is the difference between a modest bill and a four-figure one, and it is often decided in the few minutes after the gauge starts climbing.

What to do right now

If your temperature gauge is heading for the red, or you see steam:

1. Turn the air conditioning off and the cabin heater on full. It is deeply unpleasant on a hot day, but the heater core is a small radiator, and running it pulls real heat out of the engine.
2. Pull over as soon as it is safe and switch the engine off. Do not try to reach somewhere more convenient. Distance is what does the damage.
3. Do not open the bonnet immediately if steam is pouring out. Give it a few minutes.
4. Never open the radiator cap or expansion tank while hot. The system is pressurised well above atmospheric, which raises the coolant’s boiling point. Releasing that pressure makes superheated coolant flash to steam instantly, and people are seriously scalded doing this every year.
5. Wait until it is completely cool before checking the coolant level.
6. Call us on 02 9667 4008. If it will not cool down or the level drops again straight away, get it towed. Towing costs a fraction of a cylinder head.

The single worst thing you can do is keep driving “just a few more minutes”.

What usually causes it

Low coolant. The most common cause, and it always has a reason. Coolant does not get consumed in normal use, so if the level is down there is a leak somewhere — a hose, the radiator, the water pump seal, or internally through a failing head gasket.

A failed thermostat. Stuck closed, coolant cannot circulate to the radiator at all and the temperature climbs fast. Stuck open, the engine runs too cool, which hurts economy and is a common cause of a heater that never gets properly warm.

Water pump failure. The pump circulates coolant. When the bearing or impeller fails, circulation stops. Often preceded by a whine or a weep of coolant from the pump housing.

A blocked radiator. Internally, from old coolant that has lost its corrosion inhibitors and allowed sediment to build. Externally, from leaves, dirt and road debris blocking airflow through the fins.

Cooling fan not running. At highway speed there is enough airflow through the radiator without it. In stationary traffic there is not — which is why some cars only overheat in queues and seem perfectly fine on the motorway.

Head gasket failure. Combustion gases forced into the cooling system create air pockets and pressure that coolant cannot overcome. This can be both a cause and a consequence of overheating, which is why prompt action matters so much.

Warning signs before the gauge moves

• The temperature needle sitting higher than its usual position
• Sweet-smelling coolant, or puddles under the parked car
• The heater blowing cold when the engine is warm
• Coolant level dropping with no visible external leak
• White smoke from the exhaust that does not clear as the engine warms

That third one is a genuinely useful early warning. Because the cabin heater runs off engine coolant, it frequently tells you something is wrong before the temperature gauge does.

Coolant is not just coloured water

This is the most common misunderstanding we encounter. Coolant contains corrosion inhibitors that protect the radiator, water pump, heater core and the alloy passages inside the engine itself. Those additives deplete over time — typically every two to five years depending on the type.

Old coolant allows internal corrosion, and that corrosion produces the sediment that eventually blocks radiators and heater cores. Coolant type also matters: modern engines specify particular chemistries, and mixing incompatible types can cause them to gel.

Topping up with plain water is fine to get you off the road safely. As a permanent fix it offers no corrosion protection and boils sooner.

Sydney summers make this worse

Stop-start traffic on a 38-degree day is the hardest work a cooling system ever does. There is little airflow, high ambient temperature, and the engine is producing heat while barely moving.

A marginal cooling system copes perfectly well through winter and finds its limit on the first properly hot day. If your car sat a little warm last summer, do not assume it was nothing.

What we do

We pressure-test the system to find leaks that are invisible when cold, test thermostat operation and fan function, check coolant condition and strength, and inspect hoses for softening and perishing. If we suspect a head gasket, we use a combustion gas detector to confirm it rather than assuming.

Plenty of cars arrive here after being told they need a head gasket, when the actual fault was a cap, a hose or a thermostat.

Call 02 9667 4008 or book a service.

Related: Radiator, water pump and cooling system repairs · Engine repairs and head gaskets