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:
- 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?
- 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!