I drive a 2001 Buick Century. It has had some coolant problems/leaks on and off. My father in law was helping me with the car, and he told me that the radiator must be filled to the tippy top up to the cap (in addition to checking the overflow tank at the hot/cold levels appropriately). I've been unscrewing the radiator cap in the morning before work to check it, and it will consistently take about 4 oz of coolant to become filled to the top top of the radiator, to be filled all the way up the cap. This makes me wonder if I'm losing fluid, since I've been doing this everyday for a week or two now, but I also see no evidence of a leak like I used to...

So, my question is this:

I've been doing some research, and I've seen a few people say that the radiator cap only needs to be filled to the bottom of the neck, but the neck doesn't need to be filled. I've been filling the neck too, up to the top of the cap... Is it true that I only need to fill up to the bottom of the radiator's neck? Because the sources online saying 'fill up to the bottom of the radiator tank's neck' are making me suspicious of what my father in law has told me to do...Am I damaging the car if I'm filling the radiator to the 'tippy top' as I said? Or does that excess simply get drained into the overflow? Or just get spouted out somewhere?

I really don't know anything about cars, I used to live in the NYC area, please be easy on me.

  • Coolant leaks can be hard to find sometimes - especially slow leaks...if it isn't sending a cloud out the hood or tail pipe or spewing all over the ground it's pretty slow. As alphazero says a blown head gasket could cause this problem. Also check near the front and rear of the engine - especially near the water pump. Look for green staining and buildup in parts - some coolant will leave residue as it dries. Also, a pressure test would be a good idea here. I'm not sure where you live but generally you can rent the tool and it's an easy test you can learn to do quickly.
    – Techlord
    Jun 24, 2020 at 23:59

1 Answer 1


If you are putting 4 oz of fluid in the cooling system "every day," it is obviously going somewhere! If you have a blown cylinder head gasket, it will be disappearing down the tail pipe as steam and you won't see it.

The cooling system (including the radiator) should be completely full of fluid. When it heats up and expands the excess goes into the overflow tank and the level rises from the cold to the hot mark. When the engine cools down the coolant should be going back from the expansion tank into the radiator.

Check the level in the expansion tank visually when the engine is hot - but don't remove any filler caps till it has cooled down, because if you release the pressure the fluid may boil and spray you with very hot liquid!

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