Exactly, without the waterpump turning, no coolant is circulated.
@BEGIN(sarcasm)
And this is why all of the US nuclear submarines suffer meltdowns when running their reactors at low power with the coolant pumps turned off.
Oh, wait -- that's standard practice for low-speed silent running. I guess that pressure differential between the hot and cold side of the reactor and condenser are enough to keep the circulation going.
@END(sarcasm)
Seriously, though, a liquid cooling system
will get some cooling effect from both the pressure-generated circulation and from heat transmission through the working fluid. However, for most automobile and aircraft engines, the fundamental limits of design (ships don't normally have weight or size as a major consideration), the amount of cooling from passive heat transfer, so you will get a heat spike when the cooling system shuts down.
In point of fact, though, the heat spike is bogus. Think about it for a moment; at a constant temperature, the engine is producing
X amount of waste heat, and the cooling system is dissipating the same amount of waste heat. You shut the engine off, and it's no longer producing waste heat; it's
not going to get any hotter, it's just going to cool down. The
coolant, however, which during operation has to be at a lower temperature than the engine in order for it to be able to remove the waste heat from the engine,
will become hotter, as it absorbs heat from the engine but isn't being pumped to the radiator to dissipate that heat.
The reason that the 'engine' temperature rises when you turn off the engine is that it's
not the engine temperature you are seeing; it's the
coolant temperature.
A liquid-cooled engine relies on the coolant remaining a liquid; this is why a coolant leak is such an urgent problem. As the temperature of the coolant goes up, more of it boils, creating increased pressure in the coolant system, which raises the boiling point of the coolant, keeping it from boiling. When the pressure exceeds the system tolerance (the spring holding the pressure vent valve on the radiator in a car gets forced open) or the system is punctured, the loss of pressure causes part of the coolant to flash-boil. Steam, because it is much less dense, has a much more limited ability to cool the engine, and the temperature rises much more quickly, until it reaches a point at which the engine is damaged and fails.
