I suspect there is a small crack on the engine head gasket. I saw a small air bubble in the coolant system.
Generally, a failed head gasket will leave evidence in one of four areas. The oil needs to be checked for a milky residue -- as this would indicate coolant leaking into the oil. The coolant needs to be checked to see if there is oil present.
Note: Never open the check or open the cooling system with the engine hot or the system pressurized as serious injury can occur.
The exhaust needs to be checked for the presence of white smoke. White smoke would indicate coolant is being burned in the combustion chamber. The last check would be the sealing surface where the cylinder head mates with the engine block. A white, flaky residue on in this area could be an indication of a head gasket that has failed as well.
If none of these checks present any evidence, then a cooling system pressure test may need to be performed. It would help determine not only if there is a cooling system leak, but also if the head gasket is the source. You may want tot enlist the help of a mechanic, like one from YourMechanic, who will have the training and tools to not only diagnose the cooling system but who can perform the repairs to get you vehicle back to top operating condition.
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