Never open the coolant cap on a hot engine. The system is pressurised. Opening it when hot can cause boiling coolant to erupt and cause serious burns. Wait at least 30 minutes after the engine has been running before touching the coolant cap.
What Is Engine Coolant?
Engine coolant (also called antifreeze) is a mixture of ethylene glycol and water. It circulates through the engine to absorb heat and keep the temperature in the correct range. It also protects against freezing in winter and corrosion year-round. Coolant needs replacing periodically as it degrades and loses its protective properties.
Coolant Types
- Green/blue OAT coolant, Older formulation, needs replacing every 2–3 years
- Orange HOAT coolant, Most modern European and Japanese cars. Replace every 5 years
- Pink or purple OAT coolant, Some newer cars, especially Asian brands. Can last 10 years/150,000 miles
Always use the coolant type specified in your owner's handbook. Do not mix coolant types, this can cause gelling and block the cooling system. If unsure, drain and flush with fresh water before adding new coolant.
Step-by-Step: Topping Up Coolant
- Wait until the engine is cold, at least 30 minutes after driving, ideally longer.
- Open the bonnet and locate the coolant reservoir. It's a translucent plastic bottle with red, blue, or yellow markings. There should be MIN and MAX marks visible through the side. The cap usually has a symbol showing waves or a thermometer.
- Check the level. If the coolant is between MIN and MAX, no action is needed. If it's at or below MIN, you need to top up.
- Prepare a 50/50 mixture of the correct coolant type and distilled water (not tap water, tap water contains minerals that cause scale buildup). Many coolants are sold pre-mixed, check the label.
- Slowly remove the cap (engine cold only) by pressing down and turning. Some caps require pressing a release tab.
- Pour in the coolant mixture slowly until the level reaches MAX. Don't overfill.
- Replace the cap and twist firmly until it clicks.
- Run the engine briefly and check the level again when cold, it may drop slightly as the coolant circulates.
When to See a Mechanic
- The coolant level drops frequently between checks
- You see coolant puddles under the car (sweet-smelling, often green or orange)
- A sweet smell from the heater vents inside the car
- White or sweet-smelling smoke from the exhaust
- The temperature gauge rises above normal
- The engine warning temperature light comes on
These symptoms may indicate a coolant leak or, in more serious cases, a failing head gasket. Both need professional attention promptly.