This is in fact done deliberately by most manufacturers to ensure that the engine receives its full level of coolant whilst in operation, even if the system is a little low. It would only be when the pump was stopped that the low coolant level would be equal in both the expansion tank, thermostat housing and radiator. If the coolant level was low it will show up in both the radiator and the expansion tank, before a low level was experienced in the thermostat housing or top radiator hose (whilst the water pump is operating).
In our Land Rover engines, the water pump draws coolant in through the bottom radiator hose and pushes the coolant from this hose up through the engine and out through the thermostat housing into the top radiator hose. When the engine is running, these are the two worst places to be looking for low coolant level because of the operation of the water pump. I’ve always had an issue with trying to detect a low coolant state either in the thermostat housing or the top radiator hose.
Some time ago I fitted a low coolant alarm to my Defender, and since my version is slightly different to some, I thought I’d share the ideas and reasoning.
