My long outstanding overheating issue was finally solved. It was the upper radiator hose that leaking somewhere around the area where it is clamped. The leak was so tiny I could not spot it. It leaves no coolant trace either.

After having to replaced the radiator due to leak at the bottom of the expansion bottle and fan clutch about a year-and-a-half ago the car running with this frustrating overheating issue ever since. I had to replace the thermostat housing twice since the first one was non-OEM part it didn't seal correctly especially when pressure inside the system rise while driving the S or M mode.

The same situation happened with the old hose. The leak was not that obvious it still can hold pressure while driving with D mode. Ed, Craig, Jimlev and many others were right: I should have done pressure test months ago!

Now I am sure the system is perfectly sealed. Bleeding process was not hard at all. I have been using the S and M mode for the last couple of days without worries. The side effect was good too: the auto transmission shifts much smoother! I remember this advice posted in E34 forum when I joined the group few years ago: bleed...bleed and bleed some more!

Thank y'all!! :)

Enrico Tobing

528i E39 1997
Automatic ZF 5HP18