If there is a pressure drop during a cooling system pressure test without visible leaks, what could this indicate?

A pressure drop during a cooling system pressure test without visible leaks suggests that there is an internal leak in the system. This often points to a leak in the cylinder head gasket. When a cylinder head gasket fails, it can allow coolant to escape into the combustion chamber or the engine oil passages, creating a situation where coolant is lost without any external leaks being present.

This type of leak might not show evidence externally, which is why the pressure test reveals a drop in pressure even though coolant isn't visibly leaking out of the system. Other components, like the radiator cap or water pump, may cause pressure issues, but they would typically either be visible externally or involve other symptoms that can be identified during testing. Low coolant levels, while potentially a problem, would not necessarily result in a drop in pressure if tested correctly. Thus, an internal issue such as a faulty cylinder head gasket is the most logical explanation for the pressure drop observed.