Thanks a lot @73FJ40 . Appreciate your sound input. Am the 3rd owner of this FJ40. I guess the current cooling setup now is no longer original. I will check on its cap's type. Thanks again.@FES74
OEM coolant reservoirs started in January 1975. You can see from the illustration, below, no reservoir was in F engine FJ40's from 1969 through the end of 1974.
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You really need to post some pictures of the piping arrangement to your reservoir. If there is boiling water leaving the radiator and entering your reservoir, the coolant in the reservoir should act to quench the hot radiator water, collect it, and then let the coolant flow back into the radiator when the engine cools off, and the coolant contracts.
Note that is there is air in the engine cooling system, the heated air will force a lot more coolant out of the radiator than just expanding coolant.
The original radiator cap (non recovery system) is typically a single seal cap, whereas a recovery system cap is a double seal cap. Make certain you have the correct one.