Hello all
After 5 years of this thread, I would like to announce that I have the same exact issue bebe has.
Same thoughts came to me before even seeing this thread, I've done the same troubleshooting steps.
I've changed the G62, G83 and thermostats.
I've changed radiator and gearbox oil cooler as suspecting a clog in the system that might be expanding after getting hot.
Like you said bebe "It's obvious that something won't work after the engine heats up a few times"
I suspected the big fan and water pump as well. specially the water pump because the water flow in the return hose to the reservoir decreases after the engine reaches op. temp. or gets warm "not after a cold start".
BUT I am not going to change it because now I am at the point where my A/C died .. yes it is happening to me exactly.
Changing the water pump didn't solve the
problem for you so, I assume it wont solve it for me.
I've bought Auto polar FIS last October to precisely monitor coolant and oil temperature readings on my cluster screen while I am driving. The auto polar FIS has 3 parameters for coolant however there is only 2 known coolant sensors all over the 2.0tfsi engines.
1 for the G62 called motor out temp, 1 for the G83 called coolant temp and the 3rd is unknown to me called Radiator temp. I contacted them recently to know more about the 3rd one.
Anyway I've found out that the 3rd parameter "Radiator temp" starts fluctuating like crazy after the engine been running for a while and the fans starts working after the reading of that parameter exceeds 77 or 80 degrees but no luck, that reading goes from 80 to 50 back to 70's then drops 60's in seconds then to 90's then drops to 60's maybe 50's. This is where the fans doesn't work properly. I think I might caught a thin thread that might be causing the whole issue.
You are the only one on the WEB who has the same exact issue as mine. I've been studying and searching for a year. No one understands no one has it no one knows except you and I.