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!!

__________________
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!)

The new rattle could be a water pump bearing. Do you see a loss of coolant from the water pump weep hole?

Where is the water pump weep hole? Near the pump itself?

Thanks!!

__________________
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!)

FOrgot to mention! The Auxilliary Fan runs on high speed almost all of the time! eVEN THOUGH engine coolant temp reads a constant 100, until it's been driving for more than a few (est. 10-20 miles? combined) miles.
AND, THE AUX WAter pump literally JUST arrived! I'm going to install it now.

__________________
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!)

is the top rad hose semi hard ...??

The water pump weep hole is at the bottom of the water pump. It will start to weep coolant when the seal /bearing in the pump goes bad. If your car has in excess of 150k and has the original water pump it is certainly suspect.

No the top Radiator hose is not semi-hard. Although I will double check. I replaced it myself about 6 months ago.

Okay so I will double check that is the location of the leak. I'm always in such a darn hurry to kill the motor before coming to a stop to prevent overheating.

I should do water pump as preventative maintenance at this point anyway...
And I presume that the whole pump should just be replaced? And can't/won't replace just the bearing!

Thanks for all the help you guys!

__________________
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!)

The entire water pump should be replaced. Rebuilding the water pump yourself is normally beyond the capability of a do it yourselfer. Replacing the water pump is about 1/2 day.

I figured as much! I'll keep you guys posted on the results. Putting in aux pump now (just too easy not to!).

Much appreciation!

__________________
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!)

the hose question had to do with pumping ..
if the water pump is ok, the hose should feel hard/water flow when squeezed if the thermostat is open

a rattling bearing, or weeping seal is not going to make the car overheat
it sounds like you are far past boiling, if you have to relieve pressure
this has me confused, as the Cap should vent at the appropriate setpoint
why are you venting the tank ..???

in general that is a symptom of a bad head gasket. However I don't know the likelihood on that engine.

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Auxiliary water pump was as easy to install as it was to take out, EASY! And I definitely noticed a lower engine temp as the engine warmed.. ie it didn't jump to 100 immediately as it had been.

I don't see ANY leak too near to the water pump. However, I do see the leak (only occurs after a decent run), and it's rather plentiful, that is down at the bottom of the radiator. I think it might be the radiator drainage plug....

More to come...

__________________
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!)

No, the M104 motor was infamous for having the notorious head gasket failure (because the two different metals they used for the bottom vs the top engine apparently expanded under heat at different values/rates, thus ruining the gasket)...

Head gasket can be seen easily with hood open (atleast the passenger side one) and it looks a-okay. Also, doesn't leak a drop of oil, never has! Burns about a quart of synthetic every 5k miles- it's actually a nice "refresher."

I will atleast investigate further on your suggestion. Thank you.

Additionally, haven't noticed any oil in the cooland/water and by how well the engine performs, I doubt there's any water in the oil. ?

Thanks again!

__________________
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!)

David, you may want to replace the cap on the expansion tank. If faulty it will allow the cooling system to operate at an elevated pressure. If you determine you need a new water pump, recommend buying a remanufactured water pump from your local MB dealer. You will need to give them your old water pump, or pay a core charge.

__________________
Fred Hoelzle