> In a message dated 11/11/2007 5:20:13 PM Central Standard Time,
> email@example.com writes:
>> Is there a guru with a definitive answer to whether it's better to have an
>> electric fan wired to run whenever the thermal switch says so?
>> ISTR that I've owned a Volvo and a VW, both of which had fans that would run
>> with engine off and ignition key out, so there must be an argument for
> Sure. You can save a $5.00 relay that way. (actually, the cost to the car
> manufacturer for that relay is under $2.00) Since fans pull a healthy
> you don't want it powered off the keyswitch so switching it off with the
> switch requires a relay.
> Is there a functional benefit? No. As Jim pointed out the engine is off and
> the waterpump is stopped so there is no movement of coolant through the
> radiator. Also, since the engine is off there is no more heat being produced
> there is no more need to run the fan.
Umm, this is sort of true, and sort of not. The question originally was
if the car makers are doing it, what's the reason? I think it's quite
simple. Ever since the thermostats on most production engines were
raised to 195-degree items (the higher temps were necessary to get
higher fuel effiicencies and better cold-weather heater performance),
engines are running hotter, and aluminum heads mean that heat is
transferred through the thermal mass more quickly. So, when the engine
is shut down, the water pump is no longer pressurizing the coolant and
accumulated heat in the engine causes nucleate boiling to begin. This
raises the gas volume in the cooling system and forces coolant out of
the overflow. If this is allowed to go on unimpeded, eventually the
overflow bottle fills up and coolant is released to the environment.
That's the reason for the fan running after shutdown--to minimize
groundwater contamination with ethylene glycol.
Running the fan after the engine stops gets heat out of the system in
exactly the same way as it does when the engine is running. Even though
the water pump is not turning, coolant thermosiphons through the system
as long as there's enough energy in the system to cause the coolant to
move, or the coolant temp drops to the point where the thermostat
closes. Of course, when the coolant volume goes down, vacuum pulls
coolant back into the radiator, except for the reserve in the overflow
The reason is mostly time-dependent. Running the fan after shutdown
ensures that the now-hotter-running engine cools more quickly than it
would statically, thus reducing the chance of coolant overflow.
Never let anyone drive you crazy when you know it's within walking distance....
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