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2000 Mercury Sable GS Wagon. OHV engine, 120,000 miles. I just replaced the coolant reservoir because the old one was leaking. I filled the new one up to the cold fill line with engine cold. The engine comes up to temp normally, and the gauge reads mid level like it always did. Now, when I stop the vehicle after coming up to temp, the water level in the reservoir increases until it blows out from the pressure relief cap. Even when level in reservoir is low, when the engine is shut off, the level rises and then lowers within minutes. Has anyone ever seen this before? No other mechanical issues.

Fairly normal, especially in older engines. When you turn off the engine, coolant circulation stops and engine heat can cause localized hot spots and boiling which will force coolant into the overflow reservoir… As the engine cools, the coolant is drawn back into the radiator…It takes several hot/cold cycles to purge all the air from the cooling system and this event may diminish or stop completely…

Do you have the right concentration of antifreeze in there (typically a 50:50 mix with water)?

Thank you for the reply. This is really not an overflow reservoir, but part of a sealed coolant system. Coolant from the engine is returned to this tank and taken from the tank as part of its normal operation.

Did you compare markings and size between the old & the new reservoir? The fill lines could be different or the amount stored in the new res. could have changed. Also you could have air trapped in the system that is working it’s way out. (last week’s show talked about these)

Yes, the tank was an exact replacement. I guess I will give it a little time and hope that it is just air working its way out…