C8 dreadful fuel consumption, engine sensor positions!

[PeteC8]

So, almost a year of owning the 04 2.0Hdi auto C8, (85000 on clock) and I'm actually getting to like it! However..
The fuel consumption round town has dropped from around 27mpg to 19mpg. On a moderate run (no long fast stretches near us) has dropped from 35mpg to 25mpg. Note this started in the warm times last October, not just to do with Winter!
I have fitted a new air filter, removed and cleaned the MAF unit on the end of the airbox, also tried car for about 50 yards with it unplugged to check it wasn't dead (very sluggish response then!), no warnings coming up on the dashboard, brakes not binding. Any thoughts?

I really struggle recognising all the pipes and sensors strung around the engine bay! Is there a visual schematic of what they do and importantly, where they are located?

Many thanks in advance, Pete

[GiveMeABreak]

Have you cleaned the fuel system lately using a diesel engine fuel system cleaner? I would consider adding one of these products regularly as specified and you should see an improvement:

https://products.carmagazine.co.uk/car- ... -cleaners/

I'd also suggest you have it diagnosed with a Lexia to see if there is anything flagging up. There may be something showing up that isn't a permanent fault.

Your particle filter may be using assisted regeneration more frequently as you are not doing long journeys.

If the exhaust temperature doesn't reach the 550° C naturally on a good drive, then assisted regeneration will attempt to regenerate the filter requiring only 450 °C. However, the process can be interrupted if the journey's are constantly short, stop and start traffic, or just small runs that barely allow the car to reach operating temperature. Despite various strategies engaged by the engine ECU to try and restart the process at the next opportunity, if it fails to start or complete the process, it will lead to a risk of the filter starting to clog. You should get a warning about this at some point, but this can affect the performance.

So a few pointers and suggestions, but do remember that the vehicle was built in July 2003, so is fast approaching 20 years old, so you'll have to factor wear and tear into the equation.

[PeteC8]

Thanks, Marc for your usual prompt response!
I fitted a new fuel filter in December. She starts and runs sweet, but lays some smoke visible at night in following cars' headlights. I will run some additive through and have an excursion to find a good long stretch of fast road! I understand the wear and tear aspect, but this has started quite abruptly rather than gradually worsening.
Also what, if any, effect would the EGR valve have on this? It's a thing that people seem to pluck out of the air as all-evil, and due to its inaccessibility I would rather let it be!

[GiveMeABreak]

EGR valves are a necessary evil as they reduce NOx emissions, but despite having a self checking / anti sticking routine (depending on version) they will ultimately start gunking up. But you should get a warning for a faulty or sticking EGR that prevents it from opening or closing. Air inlet manifolds can also get gunked up over time.