i have just purchased a golf mk3 1.9tdi, occasionally i have smelled coolant when driving, i have also noticed a small amount of coolant around the bottom of where the header tank sits, after doing a little more investigating, i have noticed a lot of very tiny air bubbles in the pipe that goes into the header tank, is this normal, are header tank/pressure caps prone to fail? are headgaskets known to fail? the engine is ahu (i think) the car does not overheat.