The crankcase ventilation oil separator (also known as both the PCV and CCV) recirculates the small (but constant) amount of blow-by, created by gases leaking past the rings in the combustion chamber. Over time, the PCV can become clogged or even crack, causing oil leaks and oil to be pushed back and burnt in the combustion chamber. Replace the PCV on your BMW with this inexpensive part manufactured by Vaico. Photo for illustration purposes only.