Every full tank I have 2 occasions of rough idle and the radiator stays on

[Teekay]

Every full tank I have 2 occasions of rough idle and the radiator stays on

Hi, I have had my audi a6 c6 Le Mans 2.0 tdi le mans 2010 for over 2 years now. Car looks and runs beautiful but every full tank I have 2 occasions of rough idle (revs fluctuating up and down) and with that the radiator stays on for 20-30 mins after I stop the car. Also, car states on every full tank that I should get about 460 miles but I get between 320-350 miles per full 70 litres.

I had changed the pressure sensor last year as I went to get air conditioning refilled and it was not picking it up and the chap told me to change it. After changing the sensor, my air con got refilled which works now and my car was giving me 400 miles per tank after the pressure sensor change, so I don't know if this has anything to do with my car issues. Also, on a recent service, I noticed some green lubricant on the condensor when I was underneath the car. 

Can you please recommend what I should do? 

[PAPA]

Check underneath your ECU. There could be some water and your relays could stop working due to short cut 

[Teekay]

Where is the ecu located? I also noticed that sometimes my lights on dashboard e.g. Fuel indicator flashes. What does this mean?