Auxiliary water pump on my W212, what is wrong?

[Threepointedstarlover]

Occasionally my temperature gauge doesn't move off 40 and the heater doesn't blow hot, but the main engine fans do run, indicating that the engine is hot.
Normally it runs at over 80 degrees.
If the auxiliary pump is faulty why doesn't it pack in altogether?

 

[Trickythemerc]

Hi. The fans running doesn't mean the coolant is hot ? If the aircon is running the fans will also run. The auxillary pump can fail comletely without affecting the heater, most likely culprit is the rhermostat.

 

[Threepointedstarlover]

Hi. The fans running doesn't mean the coolant is hot ? If the aircon is running the fans will also run. The auxillary pump can fail comletely without affecting the heater, most likely culprit is the rhermostat.

Oh it's hot, you can feel & smell the heat, sometimes I leave the ignition on for 10 minutes so the main fans can run til it cools down. If the thermostat was faulty surely it would be faulty all the time and not just occasionally? Also if the thermostat was closed, the heater would definitely get hot and if it was stuck open the engine would not overheat.

 

[Trickythemerc]

Either way if the gauge is only hitting 40 the thermostat is favourite, is your car fitted with duovalves for the heater ? These are often the cause of no heat.

 

[Supernoodle]

Oh it's hot, you can feel & smell the heat, sometimes I leave the ignition on for 10 minutes so the main fans can run til it cools down. If the thermostat was faulty surely it would be faulty all the time and not just occasionally? Also if the thermostat was closed, the heater would definitely get hot and if it was stuck open the engine would not overheat.

If ECU will keep the fans running after you've keyed off if it thinks it's too hot. No need to take control yourself.

 

[Trickythemerc]

I had a very odd fault at the start of the last cold spell. Everything working fine, heater blowing hot then went stone cold. New MB stat and auxilliary pump fitted in the summer. The pump had failed but the stat was done based on the age of the car. Xentry showed coolant at 88c stat opens at 92c so it was not opening fully. New stat ordered and then before I could fit it the fault vanished as quickly as it happened. Now blowing 50 odd degrees at the dash vents.