Ok, the other day last week I changed the air filter on my sister's car as it was pretty dirty, there was a few field mice and an owl nesting in there.

Since then she's been driving around in it for a week and is complaining of lower average mpg's, signifcantly.... by about 7-10. The car is a 2003 1.9Tdi hatch.

Has anybody else experienced anything like this?

I was thinking dirty egr or something but I've not had chance to look yet, and even then i'm not entirely sure how it would bring on this symptom.

Cheers.

Was it just the air filter? If so check that you haven't pulled out the MAF sensor plug. If all OK check that airways inside filter box are clear. Lastly, is the car now more energetic? Is she enjoying a heavy right stiletto?

Duncans Sister can not moderate her foot or tell us or even know if shs booting it more

Yes, very true unfortunately. Just the air filter. Don't think I did but I'll check the maf sensor plug. Does seem very strange...

If the old air filter had mice nesting in there they had probably gnawed their way through the filter creating straight through holes, thus allowing unfiltered air into the engine and good MPG.

And now that it's got a new filter, doing it's job, it is restricting the airflow and has obviously increased the MPG slightly to the normal rate.

DB.

haha, i wish that was the reason, i'd laugh at her for weeks

every single time i replace my air filter i get this effect, ive no idea why

Filter may have got damp while in storage. This has the effect of clogging up the element until it has been in use for a while.

There is now more air going since the filter change which allows more fuel to be metered in to match it - it should feel more powerful as well.

You could always put the mice etc back in, to double check all the listed theories. LOL

Is there a possibility that the airfilter cover wasn't seated back correctly? any disturbance in the airflow could alter the MPG

I'll ask her if she wants the field mice back in.

Thinking about it though, it seems like it could be a combination of your points, maybe more air flow now with new filter, so car has adjusted but her foot hasn't. And she probably hasn't noticed any increase in performance because she thinks it's an F1 at the best of times.