PeachParts Mercedes-Benz Forum - View Single Post - Crazy heater question, heats around town cold on the highway

Jeremy5848 · #2 12-13-2008, 05:37 PM

Sounds like your monovalve may not be opening fully, thus allowing a little hot water (but not enough) to enter the heater core. You can remove the electrical connector and four screws, then lift the monovalve out of its body (you'll lose a little bit of coolant but not enough to need to pre-drain the system) and look to see if it's all crudded up or has a corroded electrical solenoid that is blocking free travel of the plunger.

Another possibility is that the aux coolant pump is crudded up and is not allowing much hot coolant through the heater core. The fix is similar to the monovalve fix, just harder to get to.

A third possibility is that the heater core is itself partially blocked. That is a much more difficult and expensive repair so let's check the monovalve and aux pump first.

Jeremy