Coolant circulation question.

[VVC-Geeza]

Does the coolant in a cars header tank circulate around the rest of the cars cooling system while the car is running.Or does it purely stay in the tank as a means of replenishment for any lost coolant?

[macafee2]

I think it does circulate. I say this as I have removed the cap to the header tank on my diesels when the engine is running but cool and there is coolant coming in from a small tube at the top.

macafee2

[TourerSteve]

Originally Posted by VVC-Geeza

Does the coolant in a cars header tank circulate around the rest of the cars cooling system while the car is running.Or does it purely stay in the tank as a means of replenishment for any lost coolant?

The cooling system bleeds back into the expansion tank while the car is running and replenishes the system from the expansion tank so coolant is constantly circulating

[VVC-Geeza]

Originally Posted by TourerSteve

The cooling system bleeds back into the expansion tank while the car is running and replenishes the system from the expansion tank so coolant is constantly circulating

Thanks Steve,perfect

[SD1too]

Originally Posted by VVC-Geeza

Does the coolant in a car's header tank circulate around the rest of the car's cooling system while the car is running?

No it doesn't.

Originally Posted by VVC-Geeza

Or does it purely stay in the tank as a means of replenishment for any lost coolant?

That's correct, "lost coolant" includes the minor return flow to the expansion tank through the small bleed hose from the top of the radiator (Ian's diesel) and including the cylinder head (your's and Steve's 1.8).

The main flow, controlled by the thermostat, does not pass through the expansion tank.

I hope this clarifies and avoids any misunderstanding.

Simon

[VVC-Geeza]

Originally Posted by SD1too

No it doesn't.

That's correct, "lost coolant" includes the minor return flow to the expansion tank through the small bleed hose from the top of the radiator (Ian's diesel) and including the cylinder head (your's and Steve's 1.8).

The main flow, controlled by the thermostat, does not pass through the expansion tank.

I hope this clarifies and avoids any misunderstanding.

Simon

Thanks for the detailed explanation Simon.The crux of my question was did coolant cycle through the expansion tank.I wasn't aware that coolant is constantly bleeding into the tank and out again,so it's part of the circulatory system.

[bl52krz]

Originally Posted by SD1too

No it doesn't.

That's correct, "lost coolant" includes the minor return flow to the expansion tank through the small bleed hose from the top of the radiator (Ian's diesel) and including the cylinder head (your's and Steve's 1.8).

The main flow, controlled by the thermostat, does not pass through the expansion tank.

I hope this clarifies and avoids any misunderstanding.

Simon

Simon, you are wrong with regards to the coolant in the header tank not circulating around the engine. Where does the hot water go to when the thermostat reaches the opening temperature? It ends up in the header tank. This cooler water in the header tank is there to replace hot water entering the system from the thermostat opening. It does also act as a back up for any water that is lost from leaks, upto a point.So yes, the water does circulate through the water system, otherwise, how would it get very hot. Scalding if when you have just stopped after a drive, you open the tank screw top.

[SD1too]

Hi Dave, thank you for raising these important points.

Originally Posted by bl52krz

Where does the hot water go to when the thermostat reaches the opening temperature?

It flows through the radiator then directly enters the cylinder head. Remember that on our cars the thermostat is in the return from the radiator back to the engine.

Originally Posted by bl52krz

... how would it get very hot. Scalding if when you have just stopped after a drive, you open the tank screw top.

When the expansion tank cap is released, the cooling system pressure abruptly falls and so the benefit of the raised boiling point is lost. The entire cooling system therefore boils and that's why scalding coolant is ejected.

At the car's normal operating temperature try touching the expansion tank moulding. Is it scalding hot? I doubt it. Whatever heat is present will be due to conduction through the coolant from below.

RAVE has very good drawings of the circulatory system for all engine types if you'd like to have a look.

Simon

[TourerSteve]

Originally Posted by SD1too

That's correct, "lost coolant" includes the minor return flow to the expansion tank through the small bleed hose from the top of the radiator (Ian's diesel) and including the cylinder head (your's and Steve's 1.8).


Simon

That's incorrect Simon
It is not lost but excess coolant that returns to the expansion tank through the small bleed pipework from the top of the radiator, The 1.8 head bleed will only pass when required and this can be confirmed by the bleed hose temperature as it normally has a cold section between the head and the radiator tee piece. The hose than from the radiator to the expansion tank is hot to touch . Engine speed, coolant temperature and water pump flow differential dictate the flow to the expansion tank and will in turn be replenished back into the system ,
You are correct in saying the main cooling circuit does not pass through the expansion tank, but the coolant in the expansion tank is circulated by the flow from the bleed pipework

[Matt_75]