Changed the tag. Thanks Ariel for reminding me :)

Yes and no. It all depends on the car.

 In a Miata, if you remove the thermostat, cylinder #4 runs much hotter
 than the others. If you are also flogging the car on a hot day, it's
 more likely to overheat because #4 boils and then you lose coolant and
 you're in trouble.

The engine was originally designed for sideways mounting. When it was
re-engineered for the Miata, they reversed the direction of the water
flow, and a portion of the cooling for #4 became the heater core. The
water flow was calculated based on the restriction of an open
thermostat. With no thermostat, the flow is reduced through the heater
core and #4.

Lots of times when people turbocharge Miatas, they will re-plumb the
cooling system to restore the originally-designed direction, and they
get more even cooling that way.

Bob


On Sat, Nov 22, 2014, at 01:09 PM, Allen Mulvey wrote:
> A motor vehicle engine does NOT run hot if you remove the thermostat.
>=20
> > -----Original Message-----
> ...
> >      Or if you leave the thermostat out the engine will run hot because
> > the coolant is going too fast to pick up the heat. This violates one of
> > the laws of thermodynamics and I don't remember which one. The reason i=
t
> > runs hot is because most cooling systems have a bypass hole so that
> > coolant will circulate through the engine when the thermostat is closed=
..
> > If the thermostat isn't there hot coolant is being directed right back
> > into the engine without going through the radiator. On some engines the
> > hole was only about a quarter of an inch in diameter. In some engines
> > the hole could be close to two inches. That is a lot of coolant that
> > never makes it to the radiator.
>=20
> The bypass hole serves two purposes. It helps relieve the back pressure
> on
> the water pump when the thermostat is closed and it allows the HOT water
> to
> continue to circulate through the engine so it heats up faster. When you
> remove the thermostat most of the hot water goes into the radiator and
> cold
> water from the radiator enters the engine, keeping it cool. The only
> problem
> you have if you remove the thermostat is that sometimes, in cool weather,
> the engine never warms up. Try it. I have.
>=20
> Allen

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