My newly- bought BMW 525i has overheated.
Today's drive home from work was particularly congested thanks to Frankie Valli and the four seasons. I became conscious that the car smelled hot, but the gauge was reading at the 12 O'clock position, just as it should be. I wasn't far from home so I finished my journey and popped the bonnet with the engine running, just in time to watch the expansion tank overflow. All the while the gauge was reading normal and I have no warning lights.
There are no other symptoms, no mis-fires, no water in oil or visa versa. I haven't carried out any further investigation yet so I don't know whether I have a plastic water pump impeller problem, a viscous fan problem, a secondary fan problem or a thermostat issue.
The way I see it, the normal gauge reading is a major curve ball.
Thanks in advance for any help

MUSTARD!!! Why couldn't you have just sold me yours!

Those are pretty much my thoughts. I don't have time to waste as this car is supposed to be towing my Caterham to Thruxton at the end of the month. I think I'll just order a water pump, thermostat, viscous fan clutch, seals and coolant from BMW tomorrow and fit it all at the weekend. When I bought the car last Friday, I allowed it to idle for 10-15 minutes to check for overheating and it didn't, so I'm sincerely hoping it isn't a head / head gasket problem I have.

Just taking a tea break. The news is mixed!
It all stripped down fine but the more I stripped it, the more I thought "this has been replaced recently". The 'stat looked a bit dodgy, wasn't completely shut, but it did open in boiling water so it hadn't failed completely. The water pump looks newish and has a metal impeller. There was evidence of leaking around the 'stat housing and it had been fitted with sealant, so was probably re-used. I got a new one. It's going back together fine but as everything looked pretty recent, I'm worried I've got a cracked head or a gasket issue. Will update this thread once it's running and bled!
For today I'm re-fitting the old fan/coupling. If I don't have a terminal engine problem I will replace it!