hello got a mk4 xr3i temp gauge does not ssem to go up and fan does not ssem to cut in.I got a new thermostat today going to fit tomorrow will i need to drain whole system as housin seems to be right hand side near the top.Do i refill system via water bottle as it seem rad is a sealed unit no filler cap on top.Do i just run car with cap off untill no air comes out.Would a knackered thermostat stop fan coming on as it is not opening any pic would be a help cheers all