its the small round silver cap over the back left of the engine.
for cleaning :-
http://forums.tdiclub.com/showthread.php?p=677178
http://forums.tdiclub.com/showthread.php?p=682383
for cleaning :-
http://forums.tdiclub.com/showthread.php?p=677178
http://forums.tdiclub.com/showthread.php?p=682383