How is the correct way to top off one's radiator when it has a recovery tank?

In a radiator that has a recovery tank beside it (as described below), is it necessary to get most of the air out of the top of the radiator before closing the cap, OR do I have to fully fill the radiator neck up to the brim before capping it?

After a coolant change I have been using a no-spill funnel to get air bubbles out, and as recommended to avoid spillage when you remove the funnel, you can first squeeze the top radiator hose to displace a little fluid back into your funnel, and then plug the funnel. This helps avoid the final drip when you remove the partially full funnel.

But I'm wondering if this might also create a problem by not having the radiator neck full when I'm done, that is unless I go back and fully fill the neck before capping the radiator.

I have an engine overheating problem that's stumping me, and am looking to see if this might possibly be related.

By 'recovery tank' I mean a plastic tank plumbed to the side of the radiator cap, with a high and low level line on it's side, and as further distinguished as #2 i this list by thommo:

There are 3 different types of tanks.

1) An expansion tank is plumbed in and always receives the same pressure as the radiator.

2) A recovery tank works on the hot/cold pushing fluid back and forth principle through a radiator cap with 2 seals.

3) An overflow tank is the one which just catches it and does nothing to put it back.

1 Answer 1


It's best to minimize the amount of air below the cap, so topping it off up to the ring that the cap seals against is best BUT if you've just replaced the coolant, there's likely to be air elsewhere in the system anyway. Some vehicles have a defined bleeding process you have to follow to get the air trapped in high points out (usually the cylinder head and heater matrix), ones that do not will have the cooling system plumbing arranged so that the air naturally accumulates at the radiator cap, and it will bubble out through the recovery tank as the system comes up to pressure, and then draw fluid back in as it cools. This can take a few heat cool cycles to complete.

Your overheating issue may be that the system isn't full, and the air isn't purging properly.Some engines need to rev fairly high to get the coolant flow up to the point that the air gets entrained and ends up where it can be vented, and unless the thermostat is opened, there'd minimal flow through the engine's cooling passages.

You should check in the manual or search online to see if there is a specific bleeding sequence you need to follow.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .