Hi, spoke to my BMW man today and he did a test on the gas to check its not exhaust gas, and he’s happy the head gasket is ok, which was one of my concerns.
New cap is genuine but not through a dealer. has same part number and manufacturer as the one I took off.
The only thought now is that as it did not do this before the rad was changed, that because the expansion tank is markedly smaller on the aftermarket one this is all it is. He’d never come across this exact issue but he’d said when he worked for BMW he’d seen some 5’s that constantly seemed to have a lower than normal level but were fine when the cap was removed.
The considered opinion is not to worry about it & just keep a check on the level.
Thanks for the help though.
