Will a radiator suck fluid out of the overflow tank if I didn't fill the radiator completely?

Question

I recently changed the engine coolant in my 2006 Prius. The system is the type which has the radiator cap on the radiator and a separate overflow tank. The whole system yielded about 1.5 gallons of fluid drained.

When replacing the coolant and burping the air, I filled the overflow tank to the full line, and proceeded to put the rest of the coolant through the radiator cap. I ran the engine for 5 minutes at a time to burp the air, allowing for the thermostat to open, and filling the radiator neck more as the fluid level in it went down. It got to a point where the fluid was going down the filler neck extremely slowly, and there were hardly any bubbles. So, I assumed I had successfully burped and filled the system.

The problem is, I had only used 1.25 gallons of the new coolant, and I had initially drained 1.5 gallons. I closed the radiator cap back, and I took the car for a drive on the highway about 10 miles to fully pressurize and heat the system. When I got back, the level in the overflow tank was still at the full line.

My understanding is that as the system heats and cools, the pressure differences force coolant in and out of the overflow tank. I'm unsure of two things, however:

1. Will that process pull fluid from the radiator overflow tank if the radiator itself isn't sufficiently filled, allowing me to top up the overflow tank with the missing fluid?
2. Will any excess air I may have missed be forced out of the system as the coolant switches between overflow and radiator?

I don't think a quarter gallon of coolant is going to cause an issue, but I figured it's better to be safe than sorry!

Answer 1

For Q1 , if the overflow tank has the pressure cap on it the yes coolant will be drawn into the system. Otherwise the seal on the radiator cap will prevent flow in reverse.

Q2 not really - if you have low fluid in the radiator then you need to top it up. The fluid does not change in volume very much between cold and hot - the overflow tank is to deal with fluid escaping when the pressure and temperature cause fluid to get past the radiator cap seal. And when it does it changes to vapour (steam) as it has a lot of energy in it that has to be dissipated. That is why you are warned NOT to remove the rad cap when the system is hot...

So, you need to bleed and top up your system to the correct level.

Answer 2

There is no such thing as an overflow tank with a pressure cap, if it has a pressure cap, its called an expansion tank.

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.

in my opinion, about 90% of vehicles have a recovery tank, number 2.

To answer the original question, by design, so long as there is enough coolant in the pressurized part of the system for the pump to operate without cavitation, there should be enough for the system to burp out air through the radiator cap into the recovery tank to disappear as steam, instead of making its way round and round the cooling system. SO in short, yes.