Hi

I've got a Mk5 1.9TDI 2004 (engine code BKC) and am having ongoing issues with coolant. In the last 12months the coolant expansion tank has required topping up in fits and starts. Added to this, opening the coolant tank cap would result in a a big whoosh of pressure and the coolant level rising rapidly (even though the car had been sat for several hours), although this pressure build up has dissapeared in the last 4 months. On starting the car and then checking the tank level (once the car was up to running temp) the coolant level was then low again.

First thing I thought of was the head gasket, however, engine temp climbs to the standard 90degC and stays there. Plus there's no oil film in the coolant, or white gunk in the oil (on the dipstick or filler cap, plus did an oil change yesterday and the oil appeared uncotaminated). I also couldn't see any signs of dried coolant on the plastic engine tray, either, so ruling out rad leaks etc? I've also tried bleeding the coolant systems by running the engine at high revs with the heaters on full and coolant tank cap removed.

One new thing I did notice yesterday was a build up of brown foam scum around the filler neck of the coolant tank - this is new. Coolants have never been mixed, so this isn't the cause of the gunk.