Original water pump on 2003 E320 w/ 208k miles.
Aux water pump currently bypassed (broke with high pressure after new expansion tank and cap) until new one arrives... now Monday, 3/3 due to weather!
Have replaced radiator hoses, thermostat, expansion tank and cap, radiator was tested and has no stop/clogs.... now the damn thing wants to overheat after any semi-extended period of driving and is leaking out of the new expansion tank and cap....
I have to often pull over, relieve pressure, and add more water (a lot more).

My question is... as it "feels" /seems as though the water is not circulating itself through the cooling system... could the water pump have failed? It is not leaking from the unit itself, and Poly-V-belt is fine... however, I have noticed an new slight rattle of sorts....

Does this sound symptomatic of a failed water pump? I'm having the whole thing checked out VERY soon. Please,any advice?

Thanks a bunch!!

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)