Does coolant in reservoir ever cycle through the system?

In my VW tanks, the coolant flows through. The tanks get too hot to touch after a drive and the only way they could do that is if coolant circulates.

I take advantage of this by sucking out the coolant in the tank when it is cold and replacing it with new coolant about six times a year.There is NO dirt in my tank and my coolant always looks fresh and clean. I never drain and fill the systems.
I like that idea! Usually I do a drain and fill on radiators every few years, but your method (very vehicle specific) sounds easier.
 
Some vehicles went to the circulation system before others. You can check the cap on the coolant tank. It will be a sealed cap capable of taking pressure. There will also be an inlet and an outlet. Coolant circulates through it. It’s more than just taking the expansion fluid. Here is one from Rock Auto for a 2005 Ford Taurus. The inlet and outlet are circled.
^^^ This.

My Nissan's have these, they did back to at least 2005. It has caused many owners a great deal of confusion - like the fact that the spring cap goes on the degas (overflow) bottle and the solid cap goes on the rad. Also, the rad cap is not the highest point in the system. If you pull the rad cap on a full system even cold, some coolant comes out.

It does make changing the coolant really easy. Fill the rad as much as possible. overfill the degas tank just a little, 1 hot cold cycle then top the tank to the full line. Done.
 
I believe the '13 HONDA Fit and '18 HYUNDAI Accent reservoirs cycle through the system . Did notice a difference in the green colors of the FIT and ACCENT over a period of time after adding new HONDA Genuine coolant to both .
 
"It depends."

If you are referring to an overflow tank, no. That is supposed to be a one way path, since the radiator cap only allows coolant to escape when the pressure exceeds the caps rating. If the cap has failed, then you will see some back and forth, but mostly overflowing.

If referring to an expansion tank, yes. It's part of the circulation.
A typical overflow expansion tank will push hot coolant out with dissolved air in it and when it cools down suck the now airless liquid back in. If it does not then there is a vacuum leak on cool down. This is easily seen by looking to see that the radiator is 100% full when cold. If it is not then there is some most of leak in the system. Weak OE spring clamps are an issue I have found with time.
 
If the reservoir/overflow tank has 2 hoses, the coolant is recirculated. If it has a single hose, then no recirculation.
 
If the cap is a pressure cap it is circulated if not then the old fashiond style. My Ford Taurus was the only pressurized coolant tank I had. Other than the tank not lasting and in this case the Dorman was the long lasting go to replacement I prefer that type of system.

They are nice as you never need to worry about air in the system. Usually have a small hose connected to the upper part of the motor and one to the top of the radiator. Air gets vented right there with the pressure cycles.
 
If the reservoir/overflow tank has 2 hoses, the coolant is recirculated. If it has a single hose, then no recirculation.
That's not a universal rule either! A Saturn S-series has two hoses. One goes around to the top of the engine and its top radiator hose and is marketed as a "degas" hose. It's higher than "mean high tide" eg the "hot full" line on the pressurized reservoir. I've run these cars with the caps off and anything coming out of that hose is spittle with air.

The other hose is a 5/8" output line that goes to a tee into the heater hose circuit. Sure, coolant put in here will mix eventually. But if someone wants to add "stop leak" and have it work, they need to drain a quart from the radiator. This will put air in the heater circuit and allow new coolant, introduced to the pressurized surge tank, to directly find its way into circulation.

If one were to put Bars Leaks (recommended for these cars) powder in a normally full reservoir it will sit there and turn to sludge with the air present, and do the car little good.
 
That's not a universal rule either! A Saturn S-series has two hoses. One goes around to the top of the engine and its top radiator hose and is marketed as a "degas" hose. It's higher than "mean high tide" eg the "hot full" line on the pressurized reservoir. I've run these cars with the caps off and anything coming out of that hose is spittle with air.

The other hose is a 5/8" output line that goes to a tee into the heater hose circuit. Sure, coolant put in here will mix eventually. But if someone wants to add "stop leak" and have it work, they need to drain a quart from the radiator. This will put air in the heater circuit and allow new coolant, introduced to the pressurized surge tank, to directly find its way into circulation.

If one were to put Bars Leaks (recommended for these cars) powder in a normally full reservoir it will sit there and turn to sludge with the air present, and do the car little good.
If there is air coming from the degas line, and you don't know where it came from, that's probably a head gasket leak. That is the purpose of it in fact.
 
^ Nope, it's just to automate the initial fill or service refills. A real easygoing procedure. Otherwise you'd need a pipe plug/ bleeder valve on the water outlet which can corrode in place.
 
Ford has a (very old) video explaining how the "DeGas" cooling system functions

I've had bad luck with non OEM caps



For an overflow style, yes, extremely slowly
In a degas or surge setup, also yes, frequently
 
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