Z4 cooling issue
 

I’m at a loss. 2003 z4, 3 liter auto trans, 125k miles. Car starts and runs fine when cool once the thermostat opens coolant fills expansion tank to over flow and pressure relives out the radiator cap. Temperature gage centered position then low level warning illuminates, level in tank then required about 1to 2 quarts.
Thermostat was replaced and opens to maintain temp. Correctly, no leaks in expansion tank, new radiator cap, new coolant,
No combustion gas entering cooling system(tested), car runs fine, no white smoke from exhaust, no misfire, removed radiator attached garden hose to radiator inlet water freely flows to outlet port. I’ve started the car with rad cap off as soon as thermostat opens rad overflows then low level light illuminates.
What am I missing? Any help Thanks

Water pump maybe??