Car overheating: what to do in the first five minutes
Most faults get slightly worse if you keep driving. This one can go from a small part to a new engine in the space of a few miles.
Before you drive itIf the temperature gauge is in the red or a warning light is on, pull over as soon as it is safe and switch the engine off. Never open the radiator cap or coolant reservoir on a hot engine: the system is pressurised and it will spray scalding coolant.
This is background, not a diagnosis. Symptoms overlap, and the same noise can come from more than one thing. Use it to understand what your vehicle is doing and to describe it accurately. A shop confirms the cause with the vehicle in front of them.
What to do right now
Turn the air conditioning off and the cabin heater on, full heat and full fan. It is deeply unpleasant in summer and it genuinely helps, because the heater is a small radiator and running it pulls heat out of the engine.
Get off the road as soon as it is safe and switch the engine off. If you are stuck in traffic and cannot pull over, put it in neutral and raise the idle slightly rather than sitting in gear, since airflow and coolant circulation both improve with engine speed.
Then wait. An engine needs a good while to cool enough to be safe to open, and there is no way to hurry it. Opening a hot system is one of the more common ways people get seriously burned.
Why the urgency is real
Engines are built to a tight tolerance and rely on staying within a fairly narrow temperature band. Run hot enough for long enough and aluminium components distort.
What starts as a failed hose or a low coolant level can become a warped cylinder head or a blown head gasket, and in the worst outcomes a damaged engine. Those are different orders of repair entirely.
This is the one case where stopping and arranging a tow is usually the right call, even though pressing on feels like the practical option at the time.
Common causes
Roughly in order of how often they turn out to be the answer.
- Low coolant , usually because of a leak. Coolant is not consumed in normal use, so a low level means it went somewhere.
- A failed thermostat stuck closed, which stops coolant circulating to the radiator at all.
- A cooling fan not running. Classic symptom is overheating in traffic but running fine at speed, because airflow does the job once you are moving.
- A failed water pump , sometimes with a whine or a leak at the front of the engine.
- A blocked radiator , either internally with debris or externally packed with leaves and dirt.
- A head gasket failure , which can be both a cause and a consequence, and often comes with white exhaust smoke or a mayonnaise-like residue under the oil cap.
Overheats in traffic but fine on the highway
This pattern is specific enough to be worth calling out. At speed, air is forced through the radiator whether the fan works or not. In slow traffic, the fan is doing all the work.
So a car that runs at normal temperature on an open road and climbs in stop-start traffic is pointing fairly directly at the cooling fan or its control circuit rather than the coolant itself.
Once it has cooled down
Check the coolant level in the expansion tank, which has minimum and maximum marks on the side and can be read without opening anything. Look under the car and around the engine bay for wet patches or dried coloured residue, which is usually a bright colour and slightly sticky.
If the level is low and you have the correct coolant, topping it up is reasonable to get to a shop. Water alone is acceptable in a genuine emergency to move the car a short distance, but it dilutes the corrosion protection and lowers the boiling point, so it is a get-me-there measure, not a repair.
If it emptied quickly, expect it to empty again. A large leak will not get you far regardless of what you put in.
What not to do
Do not open the cap while hot. This is worth repeating because it is the most common serious injury associated with this fault.
Do not pour cold water into a hot engine. The thermal shock can crack a head or a block, turning a moderate problem into a severe one.
Do not just keep topping it up and carrying on for weeks. That is a leak getting worse, and the day it fails properly it may not be somewhere convenient.
Do not ignore it because the gauge came back down. A gauge returning to normal after the engine cools tells you nothing about whether the cause is fixed.
What to tell your service advisor
Overheating has several possible causes that need completely different checks, so what you observed genuinely narrows it down before anyone opens the hood.
- Whether it overheats in traffic or at speed. This one alone separates a fan problem from most others.
- How fast the gauge climbed , and how far it got.
- Whether you have been topping up coolant , and how often.
- Whether the cabin heater still blows hot. More informative than it sounds: a heater gone cold while the engine runs hot often points at low coolant or circulation rather than a fan or radiator.
- Any steam, smoke or sweet smell , and where it seemed to come from.