Why is my Volkswagen Bora overheating if the coolant is full?

I own a Volkswagen Bora 1.9 TDI 115PS and when I drive for about 3-4 miles the car overheats and sometimes there is no coolant at all. I asked a few friends and they reckon it's the head gasket but the last couple of times I've done that journey the car fans come on but the coolant is still full.
Usually it's an airlock in the cooling system. Might be caused by the thermostat sticking. Might simply be from using the heater after not using it all summer. But the first thing to do is, from stone cold, take the cap off the expansion tank, switch the heater to maximum heat, start the engine, and run it up to temperature (until the thermostat opens, the fans start and coolant starts to blow out of the tank) then replace the expansion tank pressure cap and, with a bit of luck, you'll have purged the airlock.
Answered by Honest John on

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