thermostat Info

[Bernard Littau]

Hi Kate,

A thermostat is a device used to regulate temperature in some way, generally
to keep a temperature constant or static, thus thermo (temperature) stat
(static or constant)

One a car, the thermostat is in the engine and is used to try and keep the
engine at a constant temperature.

How it works - the thermostat regulates how much engine coolant flows
through the radiator.  The radiator cools the engine coolant, so the more
coolant that flows through the radiator, the more the engine is cooled.  The
car has a water pump, which makes the engine coolant flow.  When the engine
is cold, the coolant flows from the pump to the engine and back to the pump.
When the engine gets hot, the coolant, under the control of the thermostat,
flows from the pump through the engine then through the radiator and back to
the pump.

How it works II - the thermostat blocks the path to the radiator.  When the
thermostat gets hot, it slowly opens up the blocked passage by moving a
small disk out of the way.

One should replace the thermostat when it no longer regulates how much
coolant flows through the radiator.  I would also consider just replacing
the thermostat every two years -- they are inexpensive, and not too
difficult to replace.  If the car is new to you, just replace the
thermostat.

What to look for when replacing a thermostat - the thermostat is usually
sandwiched between the engine block and a housing or cover of some sort.
There is typically a gasket or O-ring between the cover and the block.  You
will need both a new thermostat and a new gasket.  The surfaces that touch,
or mate, need to be clean - free of old gasket material, free of corrosion
or old dried coolant, and they need to be flat.  The gasket is used to make
sure the mating surfaces seal and do not leak.  A gasket can make up for
some imperfections in the mating surfaces, but in cannot fix a big scratch
or gouge, or deal with significant corrosion.  You will need to clean the
mating surfaces, usually with a metal scraper.  Take care to not scratch the
surfaces.  Don't scrap from inside to outside or outside to inside,
scratches going from inside to out are hard for a gasket to seal.  Scrap
radically, along the surface as if you were driving it like a road.

When you take the old thermostat out, note the orientation.  It is possible
to put the thermostat in backwards on some cars.  There is usually an arrow
on the thermostat showing flow direction that you can use to help orient,
although usually thermostats are asymmetrical enough that you can easily
tell on end from the other.  Make sure you know which end is supposed to go
into the block by using the old one as a guide.

When purchasing a thermostat, they usually have more than one "temperature"
choice.  Make sure you know which is recommended for your car.  They are
usually 180 degree or 195 degree, which is the temperature at which they
open.

What happens when they don't work - depends on how they fail.  There are
three basic failure modes 1) it does not open.  2) it is stuck partially
open, and 3) it opens too soon.

If the thermostat does not open, or does not open enough, the car will
typically overheat every time you use it. The temperature gauge will show
the car as hot hot hot.  The coolant will boil, and the pressure will spew
coolant out causing dripping; steam will rise from the engine compartment.
It is dramatic.

More typically, a thermostat will be stuck partially open.  They usually get
stuck in a position that works correctly for your typical driving, so the
car does not normally overheat.  If you drive atypically, like climbing a
long grade, the car will run hot.  If you go down a long grade, the car will
run cool.  Your two clues that a thermostat is stuck are: 1) the car takes a
long time to warm up, especially on cold days, and 2) when you go down a
long grade, the engine temperature cools way down.  If you don't have a
temperature gauge, #2 is sometime hard to tell.

The opens too soon case  makes the car take a very long time to warm up.
Sometimes, the temperature needle hardly moves from the cold position.
Sometimes, you'll notice the heater doesn't seem to warm the car up well
anymore in this case.

Just for fun, after you take your old thermostat out of the car, put it and
the new thermostat into a pot of water, put the pot on the stove and heat it
up on high.  Watch the behavior of the two thermostats.  The new one will
open rather suddenly, and quite fully.  If you have a thermometer, put it in
the pot and watch the temperature, too.  Kind of fun!  Rinse the pot with
soap and hot water when you are done.

Best,

Bernard Littau
Woodinville, WA
'88 5ktq

> Hello,
>
> Can anyone give me some info on thermostats.
>
> Such as, What are they, what are they used for, how do
> they work.  When to replace them, what to look for so
> that you know when replacing them.   What happens when
> they don't work?
>
> By the way, I got an 87 4000S audi.
>
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