D'oh, You Kids!
1984 633CSi, 1985 635CSi, 1985 325e, 1987 325is, 1993 325is, 1995 318is, 1995 M3, 2003 F150
Correct. In fact, BMW called that a "heater return tube" at one time. Whichever way the coolant flows through it, if it's plugged, you get no flow through the heater.Ken can probably confirm this as he's far more familiar with the I4 engine than I am I believe but the coolant going to the heater control valve comes out of the end of the plastic manifold that fastens to the engine block. Item 1 in this diagram.