What causes MPG to decline over time?

imightbewrong

2010 Caravelle TDI - anecdotally noticed we seem to be getting fewer and fewer miles per tank - graphed and indeed - each point is the average of the last ten tanks:

                                                                               

Starts to trend badly downwards from early 2017.  Have confirmed no fuel leak - what gives?

Inked

A heavy foot

Bubblin

Plugs fouled with carbon, blocked fuel filter, clogged air filter, leaking exhaust, or many more reasons,

Has it been serviced? Have all the filters really been changed and are clean?

imightbewrong

Yes it is serviced every year by our local guy and/or VW.  Filters are on various invoices - have not checked them first-hand.

DOBLY

I'd guess at a problem with the fuel injectors.

494930

I haven't watched it yet, but this video may answer your question.

Over by there

Around July last year, change tyres? Mess with tyre pressures?

Edit. Add roof rack, tail rack whatever?

mikeysthoughts

Stick a bottle of Redex or similar in the next tank as a cheap option? It always helped my older bike engines.

Valve clearances are often 'overlooked' at major service time (read: too timely and fiddly for a less diligent garage to bother with). When they fall out of spec you'll see a steady decline in fuel economy.

mjn

Probably a dragging caliper. It’ll only be catching slightly harder than usual, maybe not enough to notice.

imightbewrong

It was two years ago actually.  Normally aim to keep the tyres at the right level.  Car came with roof bars - we did add a bike carrier around that time - the standard VW one.  Hmm.