the water pump continually forces water around the system diverting it
(short cut) when the thermostat opens and closes. Cooling being carried out
by fans and temperature sensors.
Airlock in system
Expansion cap faulty...........could well be this.
Check also the water pump impellor, depending on the age of the A3 some
of the earlier ones (1997-) had plastic ones. During a particularly
cold winter, the plastic fins snapped off on mine and this caused the
car to overheat.
Not sure whether the fact that opening the expansion tank fixes the
problem negates this theory however.
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