Hey. I have a general understanding of how the cooling system works in cars but I have a question about the reservoir tank. I know that when the engine warms up to operating temperature the coolant being circulated will expand and so the excess coolant gets sent to the reservoir via the hose connect from the radiator cap to the reservoir tank. However, once the engine cools down and the coolant shrinks back down does the radiator take the coolant in the reservoir tank to fill the radiator back up? I suspect it does which is why we need to keep a certain amount of coolant in the tank but how does that work? Is there vacuum created in the connecting hose to suck the coolant from the tank to the radiator? If that's the case and there is constant going back and forth with the coolant then is it reasonable to assume the coolant in the tank could be "contaminated" with coolant that had been circulating in the engine along with fresh coolant you put in there? TIA.

 

The tank is higher than most of the system and should also be the only place that there is air space. So when the fluid heats, it expands up into the tank. When it cools, gravity drains it back into the system.

Yes, the fluid is expected to mix. This does not mean it's contaminated. The coolant passages in the engine block are separated from oil passages, intake/exhaust runners, etc. Coolant CAN become contaminated, but usually that happens because of head gasket failure or other significant problems that require repair.

 

There are two types of systems, overflow and non-overflow/degas bottle types.

The latter doesn't have a separate cap or rad cap, everything is added via the degas bottle and it is at or near the top of the system and in most cases can purge air itself without assistance. There are normally 1-5 very small lines that lead from the "top" of areas on the engine to allow all air to be returned to the top of the degas bottle so no air lock can occur anywhere, when the engine is done purging these small lines flow a minor amount of coolant out from these areas you can see at the top of the bottle. Also makes a very handy/lazy way to change coolant, I just remove one of these purge lines and have it pump out coolant as I add it back to the bottle lol!

Overflow systems have a rad cap that acts both to regulate overall system pressure and to act as a gate between the rad and overflow tank, these systems generally speaking are more likely to need help to bleed and will have bleeder screws in various locations.

 

So the bottom line here is there's coolant going back and forth between the reservoir tank and the radiator through the hose connecting the two?

 

Right. I understand the coolant coming from the radiator to the reservoir tank isn't really contaminated which is why I put that word in quotes. I just mean it is the fluid being circulated through the engine that's being used getting mixed with new fluid in the tank. Thanks.