Thermostat coolant level?

[accy]

Hi...when ever the car (a4 2.0tdi b8.5) reaches temp and then I turn on the heaters the temp drops to to 70 degrees or below.from the read ups here on the forum everything points to the thermostat..my question is my coolant level drops to just above minimum. Would this be a normal issue with thermostat playing up?
Thanks

[niall campbell]

I would replace the cap on the expansion tank for the radiator firstly


It sounds like the cap is opening to allow steam out before it should, allowing the pressure to drop in the cooling system .............................. it is a pressurised system after all

Resulting in coolant loss, without a leak !!

[niall campbell]

get a genuine one for around £10 to £15 from Audi

it sounds as if its stuck open & everyone forgets about the importance of them

[accy]

Thanks for the reply niall...wouldnt have thought the cap could open seems secure enough and no steam leak from it

[niall campbell]

thats because it operates under pressure

if the cap is stuck open .................................................. you wont be able to tell

its not like the old radiator cap & worked with a big spring, you could tell if they were stuck open or closed................................... a bit like a thermostat

As the engine heats the water, steam is formed, just like a kettle or boiling your tatties. If the steam escapes, then when the water cools down, then there is less water

Except, the expansion cap is designed not to let the steam escape in normal conditions, controlled by pressure; either in pounds or psi

So if the cap is letting steam escape , you will not see it unless you roast the car, then when the car is cold; the steam does not turn back to water as it has escaped and hence your water level is at the Minimum mark

At this stage, it is highly likely that

1) You have a leak

2) Expansion Cap is not working correctly & cheapest option at £15

3) Water Pump is not pumping water

[accy]

Thanks that makes alot of sense ..will be replacing the expansion cap