Engine Coolant Temperature Gauge question

Wendell Roy erwendell at mac.com
Tue Jan 22 16:41:11 PST 2008

> Question 1:
> Should the thermostat keep the coolant temperature relatively
> constant? Even at low temps and highway driving?

Yes. But the thermostat is only one part of the equation. At low temps it is
restricting coolant flow to the radiator in order to keep the engine warm.
At high temps it is all the way open and then the radiator fan and its
control system is in charge. At highway speeds the fans will never come on
but in traffic there is quite a temperature delta between the normal coolant
temp and the one that causes the fan to kick on. This is to prevent rapid
cycling of the fan.
> Question 2:
> What is the usual position of the needle when driving? And what is the
> position of the gauge when the radiator fan kicks in?

On mine, cruising, it's at 1/3rd scale. Temperature climbs to 1/2 scale in
traffic before the fan comes on, and 2/3rds scale is fan high overheat mode.
Just a hair above that is boil over. If the thermostat is stuck open then
the gauge won't register at all.

Keep in mind that these gauges are not the greatest. They consist of a
heating element wrapped around a bimetallic strip that bends to produce
needle movement. Not only is there the opportunity for mechanical errors,
the temperature of the car interior and therefore the instrument cluster has
some effect as well.

I think that troubleshooting this boils (ha ha) down to one simple test: If
you are getting good heat during highway cruising then the thermostat is


More information about the quattro mailing list