Engine overheating Audi A3

Hi,
Engine is overheating coolant level is normal, if I turn on the heat in the car all I get is cold air ???. Noticed that if I open the coolant/expansion tank and leave loose and run the engine. indicator
for engine tempature stays normal and I ahave heat if I turn back on the heating.
Anyone anyideas.
Thanks in advance
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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.
Thermostat faulty Radiator blocked Fan faulty Airlock in system Expansion cap faulty...........could well be this.
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In article
snipped-for-privacy@esatclear.ie wrote:

Maybe the radiator cap has gone bad. A failed inner seal causes the coolant to boil during hard acceleration. A failed outer seal causes air to be sucked in when the car cools.
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Kevin McMurtrie wrote:

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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