If you don’t have a pressurized coolant recovery system, you have to add liquid directly to the radiator. If you must add liquid to the radiator if the engine is still warm, always do so slowly with the engine running. This way, the cold liquid joins the stream of hot water that’s circulating through the system rather than falling all at once into the system when you start the engine again.
To add liquid to your radiator, follow these steps:
Open the radiator cap.
Place a cloth over the cap and turn it counterclockwise just to its first stop. Turning to the first stop allows some of the pressure to escape, but if you see liquid or a great deal of steam escaping, retighten the cap and wait for things to cool down. If nothing escapes, continue turning the cap counterclockwise to remove it.
Look into the radiator fill hole to see how high the liquid level is inside.
If you’re unsure about what the liquid level should be, just make sure that it covers the radiator tubes that are visible when you look down the hole, or that it reaches to within a couple of inches below the cap.
Add water and coolant, or pre-diluted coolant, as necessary.
Under normal conditions, a 50/50 mix of water and coolant is preferred for most vehicles. If the day is extremely hot or cold, a higher proportion of coolant/antifreeze may be necessary.
Replace the cap by screwing it on clockwise.
If you have a safety pressure cap, push the lever down.