I had many happy months pursuing this problem on my Euro 95 E300D.

I would suggest to start by temporarily grounding both valves under the hood so you know they have good ground. This should keep them closed when engine is warm.

If this works then you know its the ground signal from PBU that is the problem. If they still leak hot then you know its the valve itself.

In my case one side leaked hot and I eventually stumped up for a new valve -not cheap. But that sorted it. Dont buy one unless you are convinced that valve is leaking. Its an easy job to change them about 1 hour not rushing (be prepared for grazed knuckles) . I stripped my old one down to see what the problem was and it seems that coolant was leaking into the solenoids.

Hope this is useful

Keith