I have had a similar problem with three of my w123's. Heat would work fine around town, but as soon as I would drive on the highway, the heat would be lukewarm at first and then just plain cold. (Engine fully warmed to 80+ degrees.) I tried three MTC brand "mono valve repair kits" in a row from a usually reliable online parts supplier in Arizona, all with the same lukewarm to cold result. I might add that visually they appear fine. Finally I pulled a unit out of a salvage yard car and installed it in one of mine, and it works great.

I hope this helps,

__________________
Will



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