Phew, just a water pump ('99 E300)
 

Was thrashing the '99 E300 pretty hard down the highway yesterday when the coolant temp took off. Hit the defrost to draw off some of the heat, found no leaks but things were clearly hot, drove it gently back to the garage with temp at or above 100C (rather than the typical 78C) and defroster blasting.

Thought maybe a stuck thermostat or (yikes!) a head gasket. Pulled thermostat this morning and can feel the water pump shaft turning while the impeller is loose and stationary. I was trying to find the buzzing noise under the hood..no doubt it was the plastic impeller announcing it's impending demise. Can't fault it for only lasting 183,000 miles I suppose. Just glad it's an easy fix ! :)

Good for you! Glad it wasn't the head gasket! My H2O pump had already been done on my 98 when I bought it with 148K on it. So apparently the pumps are only good to about 150K -180K.

Funny that you say H2O pump, since water pump is already inaccurate.

Anyway, on the topic of coolant pumps, does anyone know if the 616 and 617 pumps are interchangeable?

gimme a break. Half the coolant is water. Call it what you want, it's fixed and the post might be useful to someone else whose ... coolant pump... plastic impeller fails.