excessive diesel consumption, help!

Hi!
im Jesús and i have a 2004 jetta tdi.
everything was fine but a month ago i send my car to maintance bcs it's was having issue (not even one related with diesel consumption) then i got it back and it's giving me even less mpg than a regular fuel car. the people at the shop tell me that the engine was running at a lower temperature than the usual but i dont want to leave it with them because they're expensive and they took so much time to fix it, also for other reasons i don't trust them anymore, anyone knows why the engine is running at a lower temperature and also that's got something to do with the excessive consumption? thank you in advance
and sorry for my bad english.

The coolant temp is controlled by the thermostat. It may be stuck open and not allowing the system to come up to temp.

X2. Sounds like you need a new T-Stat. If the engine is running too cool because of a stuck open T-stat you will burn more diesel. Same with a gas vehicle.

Not only will you get bad fuel economy, but the internals can sludge up in the long run.

Failed thermostats are a pretty common issue these days (the cars are getting old). Replace it with a new one and you'll be back in business with good fuel economy.