Just went through the same carp with my 300 TDI. The "P" Gasket went behind the water pump, no signs of any C/H problems. Went on for three weeks kicking water out of the expansion tank. Head gasket went between the 1st and 2nd cylinder. Had head skimmed and pressure tested, put it back on and bingo, up and running fine once more! The funny thing is it never showed up on a gas test ?Replaced water pump, thermostat, bled system to no avail before finding the problem. Damned "P" gasket is a real pain!