I have a strange problem ...I have an e60 523i but with a z4 si m engine in it it also has full custom induction system and full custom exhaust system it has had a custom tune done on the ecu an it's fantastic........until yesterday when it started to over heat...I thought it a bit strange because it's never happened so I let it ool down topped up the Levels and drove it home, hen I got home I noticed that the heater was not getting hot so I did the following checks

Checked levels all here fine
Checked hose temps ..,top of radiator was lava hot and bottom was cold
Took top off expansion tank with engine running and I could see constant movement of coolant ....so I guess the pumps working


So I'm stuck ...what could it be!

 

Could be thermostat. Get the codes read.

 

Thermostat, or (less likely) clogged radiator. you're not getting sufficient coolant flow through the system. You should not have a big temperature differential between upper and lower hoses like that. More often than not, the symptoms you describe are due to a failed thermostat.

good luck with the car.

 

its must be airlock first do that before changing the thermostat

 

Could also be the water pump is dying.