JFGC220

Good day MB lover.
I have one problem that I wish some of you could help me solve this. I have Diesel mixing in the Cooling System of the car, it shows on the expansion tank.
I am no Mechanic or MB expert but, I have search a lot on the internet to try and solve this problem, even call MB mechanic shop.

My W203 has one auxiliar circuit to warm up the diesel before it goes to the engine. One valve and one radiator, in this close circuit, it is a diesel/coolant heat transfer system. This is what I have search... and what the MB mechanic told me to change, but, diesel keep showing in my water tank.
At this moment I have eliminated the Radiator circuit, so diesel only pass by the valve, it is brand new, so it showed have no problem....there for, I am now out of options, were does the diesel comes in to the water circuit.....
In case any of you, expert on this matter, please let me know.

The red circule parts I have changed, still same problem, diesel keeps showing in the water tank.