If your car is overheating or losing coolant, the cause is a fault somewhere in the cooling system, and most of the time it is something simple like a split hose, a loose clamp, a leaking radiator, a stuck thermostat or a failing water pump. A head gasket is the cause people fear most, but it is usually the one to rule out last, after the cheaper and more common faults have been checked.
The important thing is not to keep driving an engine that is running hot. Heat is what turns a small, inexpensive cooling fault into serious engine damage.
What the cooling system does
Your engine makes a huge amount of heat. Coolant, a mix of antifreeze and water, is pumped through passages in the engine block and cylinder head by the water pump, picks up that heat, and carries it to the radiator at the front of the car. Air flowing through the radiator, helped by the cooling fans when you are moving slowly or sitting in traffic, cools it down before it goes round again. The thermostat controls when coolant is allowed to flow to the radiator, so the engine warms up quickly and then stays at a steady temperature.
The whole system runs under pressure, which raises the boiling point of the coolant. That is why a leak, a weak cap or a stuck part anywhere in the loop can show up as overheating, coolant loss, or both.
Common causes of a car overheating or leaking coolant
These are the faults we see most often. They are not in any fixed order, because the likely cause depends on the vehicle, its age and how it has been looked after.
Hoses and clamps. Rubber coolant hoses go hard, crack and swell with age and heat, and clamps can loosen. A split hose can dump coolant quickly, while a weeping joint can lose a little at a time. Look for dried coolant crust, often green, pink, orange or blue, around the hose ends.
The radiator. Radiators can corrode, crack at the plastic end tanks, or get damaged by stones. They can also block up inside with scale and old coolant, or outside with bugs and debris, so they cannot shed heat even when they are full.
The radiator or expansion tank cap. The cap holds the system at pressure. A worn seal lets coolant escape as steam and lowers the boiling point, which can cause overheating with no obvious leak.
The water pump. The pump has a seal and bearing that wear over time. A leaking pump often leaves a trail of coolant down the front of the engine, and a failing bearing can make a whining or grinding noise. On many engines the pump is driven by the cambelt, which is why it is often replaced during a cambelt replacement.
The thermostat. A thermostat that sticks closed stops coolant reaching the radiator, so the engine overheats quickly even with a full system. One that sticks open does the opposite, the engine takes ages to warm up and the heater blows lukewarm.
Cooling fans. If the fan motor, relay, fuse or temperature switch fails, the car may be fine at motorway speeds but overheat in stop-start traffic or at idle, when there is not enough air moving through the radiator on its own.
The heater core. Coolant also flows through a small radiator behind the dashboard to heat the cabin. A leaking heater core can cause a damp passenger footwell, a sweet smell inside the car or a windscreen that fogs up with a greasy film.
The head gasket. When a head gasket fails, coolant can leak into the cylinders or the oil, and combustion gases can be forced into the cooling system. Signs include white exhaust smoke, a milky film under the oil cap, bubbles in the expansion tank and coolant disappearing with no puddle. A head gasket failure is often the result of an earlier overheating event rather than the original fault. Our guide to a blown head gasket goes into the signs in more detail, and if you are seeing smoke, read what each exhaust smoke colour means.
External leaks vs internal leaks
Working out where the coolant is going is the first big step.
External leaks leave evidence. You might see a puddle under the front of the car, drips from a hose or the radiator, dried coolant crust around joints, or steam from under the bonnet. Hoses, clamps, the radiator, the cap, the water pump and the heater connections all fall into this group, and they are usually the simpler fixes.
Internal leaks do not leave a puddle. The coolant level drops, but nothing shows on the ground. The coolant is being burnt in the engine or is mixing with the oil, which points towards the head gasket, a cracked head or, less commonly, a leaking heater core inside the cabin. These need testing to confirm rather than guessing.
Is it safe to drive with a coolant leak?
It depends on what the car is doing.
- Temperature gauge climbing, a warning light, or steam from the bonnet: pull over somewhere safe and switch off. Do not keep driving.
- Coolant level dropping slowly, gauge normal: you can usually drive gently to the workshop, checking the level when the engine is cold and topping up with the correct coolant. Book it in soon.
- Sweet smell, white smoke, or milky oil: stop driving it until it has been checked, as these point to an internal leak.
Never open the radiator or expansion tank cap while the engine is hot. The system is under pressure and hot coolant can spray out and cause serious burns. Wait until the engine has fully cooled.
How we diagnose overheating and coolant loss
Throwing parts at an overheating car gets expensive fast, so we test first. At our Frankton workshop that usually means:
- A visual inspection of the hoses, clamps, radiator, cap, water pump and heater connections for leaks and wear
- A cooling system pressure test, which pressurises the system with the engine cold to find leaks that only show under pressure
- A combustion leak test, which checks the coolant for exhaust gases to confirm or rule out a head gasket
- Checks on the thermostat and cooling fans, including whether the fans switch on when they should
- A fault-code scan for temperature sensor and overheating-related codes with manufacturer-level scan tools
Once we know the cause, we explain what we found and give you a free quote before any work goes ahead. You can read more about our cooling system and coolant leak repairs, or if a head gasket is confirmed, our head gasket repairs.
How to help prevent overheating
- Check the coolant level in the expansion tank regularly, with the engine cold
- Have the coolant condition checked at each service, since old coolant loses its protection
- Look under the car where you park for new drips
- Watch the temperature gauge, especially when towing or climbing hills in summer
- Get any sweet smell, steam or rising gauge checked straight away
A regular car service includes a coolant level check, which is a good chance to catch a slow leak early.
If your car is running hot or the coolant keeps disappearing, call us on (07) 847 3339 or use the contact form to book a cooling system check at 281 Kahikatea Drive, Frankton, Hamilton. We work on petrol and diesel cars, utes and light commercials.