UK Car Repair: declining fuel economy!, skoda octavia, declining economy

Question
in the last month my skoda octavia 1.9tdi has increased fuel consumption dramatically - previously averaging 550+ miles to a tank i'll be lucky to get 450 this tank, 500 the previous one and i'm driving in exactly the same way and circumstances - the car has done 79,650 miles and is due a belt change tomorrow, will this effect it? the engine does seem to sound a bit heavy, but that may be my imagination!

Answer
Check the air filter, gradually declining economy is usually due to something slowly blocking up.

then its off to diagnostics, skoda can diagnose accurately and I would go to them, it's expensive but should find the fault if there is one - maybe MAF sensor, MAP sensor.. or a worn turbo.. leaking manifold-

you have to have it diagnosed electronically - unless you are able to check for manifold leaks: if so do that first and check for anything obvious since its free.

(spraying WD40 around the inlet manifold with the engine running will show if its leaking, you'll see the WD40 get sucked up if there is a leak there)


I would expect it to be a blockage (air filter) or leak (inlet/exhaust manifold) with the gradually declining economy, but sensors also fail gradually; that said, when a sensor is failing you'll usually get running problems and starting problems.