Hot engine? Leave the cap alone
Never take the radiator or coolant cap off while the engine is warm. The system runs under pressure and can spray boiling coolant over you. If the gauge is climbing or steam is coming from under the bonnet, pull over somewhere safe, switch off and wait for it to cool. If anyone is hurt or in danger, call 111.
Water or coolant? Telling the drips apart
A car leaking clear water is usually fine. AA New Zealand explains that the air conditioning collects condensation in a tray and drains it through a small tube under the car, so a clear puddle after a drive on a humid day is normal. If the carpet inside is wet, though, that drain may be blocked, and it needs clearing before the water reaches the wiring.
Coolant is different. Makers dye it bright colours so leaks stand out, and it feels watery and smells sweet. If it drips onto the hot exhaust, you'll see steam.
Signs the leak is getting serious
- You're topping up the overflow bottle every few days.
- The temperature gauge sits higher than it used to, or climbs in traffic.
- Steam, or a sweet smell, after a drive.
- The heater blows cold even though the engine is warm.
- White smoke from the exhaust, or a milky film under the oil cap.
- A crusty white or coloured residue around hoses, the radiator or the water pump.
Where coolant leaks from
AA New Zealand lists the common culprits: a loose hose clamp, a leaking water pump or a failing radiator, plus overheating that pushes coolant out through the overflow. Add perished hoses, the heater core behind the dashboard and, at the far end, the head gasket.
A car radiator leak often shows as a damp, crusty patch on the core or where the plastic tanks join the metal. Ask the mechanic whether it's a repair or a replacement, and why.
What to do today
Check the level cold
Look at the see-through overflow bottle against its marks, with the engine cold and the car on level ground.
Top up with the right coolant
Use the type in the owner's manual. If you don't have it, plain water gets you by for a short trip; tell the mechanic you've added it.
Put cardboard under the front
Leave it overnight to see where it's dripping and how fast.
Watch the gauge on every drive
If it climbs, stop. Don't try to nurse a hot engine home.
Keep it away from kids and pets
Wipe up spills straight away. Wikipedia's antifreeze entry says ethylene glycol is poisonous and its sweet taste can tempt children and animals.
What will the mechanic check?
A pressure test is the standard first step: the mechanic pumps the cold system up to working pressure and watches for drips or a falling gauge. A dye that glows under UV light can find slow leaks. If coolant is vanishing with no drip at all, they'll test for combustion gas in the coolant to rule the head gasket in or out.
A good quote names the part that's leaking, the coolant it'll be refilled with, and whether the system needs the air bled out afterwards.
What you can do, and what's the mechanic's job
You can
- Check the overflow bottle level with the engine cold.
- Top up with the coolant the manual specifies.
- Clean up any spills straight away.
Leave to the mechanic
- Anything involving the pressure cap on a warm engine.
- Pressure and dye testing.
- Replacing hoses, the radiator, the water pump or the thermostat, and bleeding the system.
Our take
We'd treat a coolant leak as this week's job, not next month's. Top it up cold, put cardboard down, watch the gauge on every drive, and send it as a cooling system job with how often it needs topping up in the note.