I am pretty sure your problem is the the valve, if you also replaced that. When I was getting at the exhaust manifold bolts to remove my trap oxidizer, I broke the plug and pins on the top of the monovalve solenoid. I had a new valve I had not installed, so when I replaced the solenoid, I put the new valve in. I had no heat at all over engine idle. I put the original, corroded valve back in and the heat worked, despite a plugged screen and slightly ripped membrane (or whatever it is called. It is supposed to fold neatly up into the bottom of the valve body when the valve is open.) I could not tell a difference between the spring strength of the MTC valve and the old or new Mercedes one with a travel scale, but in the open, retracted position, the MTC membrane/bellows did not neatly fold up into the valve body. It was rubber, and just kind of scrunched up unevenly. The Mercedes valve seems to have a creased rubberized fabric, and it completely folds up into the valve. My thought was that the bunched rubber caught the flow of water to the heater core, especially at higher engine speeds, and pulled the valve shut.

Anyway, I ordered a Mercedes valve from my closest dealer at less than three times the price of an MTC, and I have no problems.

1985 300 TD, 368,000, daily driver