This is the expansion tank hose from the tank to the thermostat on the early style E30 325e and 325i (with the tank mounted on the passenger strut tower).
BMW Fitments:
3 Series
E30 (84-91) > 325e
E30 (84-91) > 325es
E30 (84-91) > 325i
E30 (84-91) > 325is
E30 (84-91) > 325ix
