Easy test. With the engine cold remove the cap, then start the engine and let it run. Once warm give her a little throttle, not much just a little. If coolant starts running over through the neck on the tank, chances are good your need a head gasket (or head). If nothing happens hmm...nothings wrong. I don't think the water pump will circulate hard enough to flow out the expansion tank on this system.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.