I am having trouble with the mono valve in my car resulting in no heat. I have replaced the cartridge in the valve twice with the same result. As the aux. pump starts the plunger is pulled down to the seat by the water flow and the water flow stops. I have cleaned the valve body completely and the ball check is in good shape. The plunger moves freely and seems to be open fully. Is there any adjustment for how far it is open when the coil is de-energized? Is there a way to put a stronger opening spring? The plungers are MTC brand. I have heard about problems with them. Did I get two bad ones? Has anyone measured what the proper spring force is to pull the plunger down? Any pointers with fixing the valve are welcome!

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1984 300CD (sold)
1973 220DPK 4 spd (died)
1978 350SL 4 spd gray market
2003 SLK32 AMG
1988 300CE (sold)
1997 E420 (Totaled)
1998 E320 Wagon