Poor fuel economy

[PetersOnA6]

Hello everyone,

I have this A6 C6 3.0 tdi from 2005. Engine BMK. Recently I am experiencing a big drop in fuel consumption and seems like I am in need of a litle bit of experienced help with it. There are things I know as following:

- Engine starts, run and warms up to 82°C aprox within 5-10 minutes and stays there regardles on outside temperature

- on board 37MPG or less after steady drive or max up to 75mph (that is still steady drive for this engine)

- immediately after cold start, as soon as car gets to move, MPG seems to go nicely up even touching 30s but then after aprox 1 minute drasticaly drops to low 20s and is a heroic achievement to get back to 30s even after 20 minutes of driving with already warm engine (nursing it - painfull becasue that is not what this engine likes the most)

- somewhere between 50 - 70°C is where idle gets lumpy and jumping up and down but if you give it a throtle it stabilize after few seconds, newer does that hot or cold just with mild warm engine

- broken intercooler pipe replaced, no more system leaks, same behaviour remains as above but with a lot better response from the engine

- actual boost pressure is a bit bellow spec values, but in general follows the curve, i'd say turbocharger may be on its way out - if you'd like screeshots I'll try to get some done.

- right hand (drivers side) cylinders bank injectors were resealed and two wants to be replaced tho they are within allowed deviation limits yet one is -2mg/str and the other 1.5mg/str the rest of injectors is between -0.7-0.1mg/str

- injectors tend to clog up very shortly after I've cleaned them up. They seemed to have black plastic like layer on them when removed again to fit bigger coppers. 

- car sometimes smokes sometimes not and just has that odour of failing injector diesel engine like, ussually it doesn't smoke after run but starts smoking on idle - slightly

- EGR operating valve and bypass cooler shuter seems to operate according to VCDS

- passengers side swirl flaps sometimes throw an itermitent fault with open circuit and recently also couldn't reach the end - it is on TO DO list now

- now this is something i do not get why - fuel temperature rises from environmental (same as coolant at the point of cold engine) to only low 20+°C never gets over 30°C and i do not know if that was like that always or if that happened recently with following MPG drop. I've tried 3 fuel temperature sensors and they all have returned same values.

I'd like to mention that this car used to do average 59-60MPG with cruise set to 70mph throught and across the europe. I only had to refuel once on 1100 miles long trip so don't try to tell me that 40 mpg is normal for this engine and I am just slightly bellow it 😁. Also things such as tyrepressures - irelevant as i check quite often.

I think of fuel being cooled down that much that it has no chance to reach the working temperature and so ECU may things engine is still cold and choosng wrong injection map? only i can think about is some issue with fuel cooling system - any ideas to check are welcome - or maybe it can be just faulty fuel termostat on fuel filter bypassing any returning fuel right back in fuel tank. 

Would really appreciate somebodys help with it as it used to be really great on fuel and very smooth long distance cuter