Hi
My audi a4 1.9tdi for the last 6 months has been very poor on performance and fuel.
It used to do 43-47 mpg local trip and now im lucky if it does more than 30mpg. The performance is also down.
Had the car fully serviced 2 months ago and made no difference but did notice less than a month after the air filter was filthy so replaced it. Checked the air filter again today and its filthy again.
What would be causing this? I assume its the reason for poor performance/economy.

Forgot to mention on cold startup it idles rough at 1000rpm for a few seconds before it settles down