I'd noticed over the past few months that mpg on my 1.9 pd tdi b5.5 passat had dropped off to low 30's urban and struggling to get 40 on a long run. I checked MAF requested / actual, boost requested / actual, fuel temp etc with VCDS, all looking ok but noticed that although the dash temp gauge came up to 90'C within 5 miles and 10 minutes or so, that VCDS measuring blocks was only showing mid 70's coolant temperature. I thought it might be the CTS but checked around the engine with an infra red thermometer gun and it did seem to be running cool.
I changed the stat the other day while doing oil and filters, it was easier stat access to swing the alternator out the way on the bottom bolt.
After topping up coolant and setting off with VCDS measuring and logging, the temp quickly came upto 90 on the dash and VCDS showing high 80's then settling between 90 and 93 while steady driving.
Dash indicatedMPG was showing mid 40's for urban driving with several stops with engine idling to check coolant level andtraffic lights. This was much better than previously indicated urban and long distance MPG. the car also felt a bit more responsive, but a new fuel filter might also have helped that.
I'm no expert but think the engine has a different fuelling map at lower temepratures which was giving me the poor mpg, the dash temp indicator is so heavily damped to be little use in showing actual engine temperature.
The old stat looked ok and even opened and closed at expected temperatures in a pan on the cooker, I can only think it had got sluggish with age and was responding poorly to changes in engine temeprature tending to overcool the engine?
Anyway, hope this might point someine in the right direction of poor mpg, it might have been more difficult to accurately measure engine temp without VCDS or some other OBD tool showing actual temperature though.