Coolant Refilling

Topping up or replacing engine coolant may seem straightforward, but it actually tests the cooling system's sealing and air-bleeding efficiency. An improper refill method can trap air locks within the system, resulting in severe engine overheating.

Here is the standard procedure for coolant refilling and air bleeding:

Preparation 

Work on a cold engine: Always ensure the engine has completely cooled down to ambient temperature before starting.

Check coolant specifications: Always verify the coolant formulation (e.g. G12, G13, OAT, HOAT, etc.). Mixing coolants with different chemical formulations can form sludge and clog the coolant passages. It is best to purchase genuine OEM pre-mixed coolant; if using concentrated coolant, strictly dilute it with distilled water according to the recommended ratio.

Standard Refilling Steps

Drain the old coolant (for a full flush/replacement): Locate the drain plug at the bottom of the radiator or disconnect the lower radiator hose to drain the old fluid into a container. When replacing coolant, it is advisable to flush the system 2 to 3 times with clean water until the drained water runs completely clear.

Reseal the system: Tighten the drain plug securely or refit the radiator hose.

Pour slowly: Slowly pour the new coolant into the expansion tank (spare tank). Avoid pouring too quickly, as this can trap air inside the hoses and piping.

Fill to level: Fill up until the coolant reaches the MAX mark on the expansion tank. 

The Most Critical Step: Air Bleeding (Eliminating Air Locks)

Driving off immediately after filling is extremely risky. Trapped air in the coolant passages can cause the water pump to cavitate ("run dry") and trigger severe engine overheating. 

Start the engine: Let the engine idle and turn the cabin heater to maximum (highest temperature and blower speed). Operating the heater allows coolant to circulate through the heater core, helping to purge trapped air bubbles. 

Monitor the fluid level: Keep an eye on the coolant level in the expansion tank. As air bubbles escape, the level will drop rapidly; continue topping up coolant to keep it close to the MAX mark. 

Wait for the thermostat to open: Once the engine reaches normal operating temperature and the radiator fan kicks in, the main cooling circuit (full cycle) is fully open. Any remaining air will circulate and escape into the expansion tank. 

Final check: After the radiator fan stops and the engine cools down, recheck the expansion tank level and top it up to the MAX line.

Common Issues and Key Tips to Avoid Mistakes

Vacuum filling method: If you are carrying out a professional service, using a vacuum coolant refill kit is highly recommended. It evacuates air from the cooling passages before drawing in the coolant, completing the refill and eliminating air locks in one go—making it the most efficient and reliable method available.

Never use tap water: Never add tap water into the cooling system out of convenience. Minerals like calcium and magnesium in tap water will quickly form scale build-up under high temperatures, reducing cooling efficiency and potentially seizing the water pump impeller. 

Inspect the cap: The cap acts as both a seal and a pressure relief valve. If there are signs of coolant seepage around the cap or if it fails to hold pressure after refilling, it should be replaced immediately.