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!

  • Turn on heater so the fluid not going there yet.
    – Adib
    Commented Jul 1, 2019 at 15:03

2 Answers 2


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.

  • Can you further explain why a normal radiator cap with overflow won't draw fluid back into the radiator when the system cools? Isn't that the purpose of the overflow to hold any fluids for just this purpose? Commented Oct 15, 2017 at 15:00
  • An overflow tank traps fluid expelled so it does not land on the road , an expansion tank is designed to receive and return fluid into the cooling system. If the radiator has a cap with a spring loaded valve or pressure refief valve then it is an overflow tank. If the cap on the radiator is just a simple cap with a seal (similar to that on a jam jar for example) and the pressure cap is on the tank then that is an expansion tank - usually with a larger diameter hose compared to an overflow tank.
    – Solar Mike
    Commented Oct 15, 2017 at 21:25
  • 2
    I take it you've never heard of a bi-flow radiator cap? If you look at just about any overflow reservoir (not an expansion tank), the hose from the radiator run off goes to the bottom of the tank (or dips down to the bottom of the tank from the lid). This puts the outlet in coolant when at proper level. This allows for excess pressure to vent to the reservoir as well as any coolant. Then, once the radiator is cool and loses pressure, it can draw on the reserve of fluid, pulling it in instead of air. This keeps the radiator full as long as it's all working correctly. Commented Oct 15, 2017 at 22:56

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.

  • I'd almost completely agree with you, but with one correction ... about 90% of vehicles will have either #1 or #2. Most GM and VAG vehicles built today have expansion tanks. That's a lot of vehicles. I'm not sure about other manufacturers, but I'm sure they aren't the only ones. Commented Jan 16, 2019 at 13:35

You must log in to answer this question.

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