Have your cap tested for relief pressure- 14# and insure the cap gasket has no tears and seats fully against the expansion tank filler tube stop.
When coolant cools, it contracts. The filler cap will open enough to allow outside air to enter to equalize the pressure within the system. This injested air will be burped (yes, I have a limited vocabulary) once the system is again at full operation temperature (coolant expansion). This continual cycling may introduce air pockets within the cooling system that are hard for the system to expell. The only true way to prevent this is to install a closed overflow bottle whereas it will accept the overflow and allow coolant makeup without exposing the system to the atmosphere (this is after ascertaining the cooling system has no leaks).
You may also be experiencing shut-down heat soak whereas the coolant cannot rid itself of heat immediately as it is no longer circulating. The relief pressure of the cap allows release of this increased pressure. The overflow bottle will also handle this.
There is an early seventies overflow bottle offered that will appear as somewhat correct in an earlier installation.
When coolant cools, it contracts. The filler cap will open enough to allow outside air to enter to equalize the pressure within the system. This injested air will be burped (yes, I have a limited vocabulary) once the system is again at full operation temperature (coolant expansion). This continual cycling may introduce air pockets within the cooling system that are hard for the system to expell. The only true way to prevent this is to install a closed overflow bottle whereas it will accept the overflow and allow coolant makeup without exposing the system to the atmosphere (this is after ascertaining the cooling system has no leaks).
You may also be experiencing shut-down heat soak whereas the coolant cannot rid itself of heat immediately as it is no longer circulating. The relief pressure of the cap allows release of this increased pressure. The overflow bottle will also handle this.
There is an early seventies overflow bottle offered that will appear as somewhat correct in an earlier installation.
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