Hope everyone can help. I have searched through the forum looking for answers. Cooling system in my 07 xc70 gets pressurized after engine is shut down. Overflow happens at the reservoir with no other signs of leak elsewhere. Area around the water pump is dry except when coolant is pushed from the reservoir. The pressure will empty the entire reservoir and I get the warning on the instrument panel.

I have replaced the reservoir cap. I have done a combustion leak test and results are negative. Test was done with car at various temps. No gases were detected.

The last time I turned the car off and the reservoir began to overflow, I turned the car back on and ran the heater at high for a minute. The pressure released and the coolant level stayed level. Any suggestions? Does this lead to the idea of a failing heater core or a thermostat issue? Thank you for your time.

 

Have you connected a radiator pressure tester and measured this "pressure rise" when it's happening?
Is it possible the reservoir itself is cracked?
Has nothing to do with a heater core or thermostat.

 

Thanks for your response. I have not pressure tested the system. I have read where people discuss a comprised coolant reservoir tank and will follow up on the test.

My question is that if there is a crack in the tank, would the coolant not flow through the crack rather than pouring out of the cap. I can watch the fluid rise and hear a hissing sound around the cap until it just starts to flow from the seal. I hope a replacement tank will be the answer but if there is enough pressure in the system to basically empty the tank from the cap why would the crack not be better exposed.

I appreciate any recommendations as I am trying to understand the potential cause. I can squeeze the upper radiator hose and see the fluid in the tank react. Could it be as simple as a pocket of air in the system?

 

No - air will not be "caught" in the system - Unless - (and you have tested for this) combustion gasses (compression) is getting into the cooling system due to a compromised head gasket or a cracked cylinder (the r cars do that, and some others)