Hi all, im having serious cooling issues with my 2002 disco. After driving for bout 10 or 15 minutes the temp guage drops to nothing and engine is roasting. I have changed the thermostat and water pump but still the same. The rad isnt getting warm at all so im assuming therws a blockage or an air lock somewhere. Can anyone help on how to fix this. Its my first disco so im not that familiar with fixing it yet. Any help is much appreciated