MK1 Focus zetec 2001 1.6L: appalling fuel consumption

[CaptainBudget]

Hi all,

 

First of all please forgive my ignorance RE:engine mechanics. I want to learn but still don't understand how everything works or what it does; best assume I know nothing. Also apologies for the long post.

 

My focus has had an issue for a couple of years with very poor economy. I bought it five years ago and got ~500 miles out of 50 litres. Now it struggles to do 350-400. It also vibrates when it idles (to me sounds a bit rough; a couple of my mates have asked if it's a diesel before now). It struggles on hills and has no power below 2.5-3k revs. It does seem to hesitate at times when accelerating from stationary (this has caused a couple of brown trouser moments at junctions you could have got a bus out of...)

My local garage knows her well and knows their stuff, but they're stumped. They found part of the pipe after the air intake sensor had disintegrated and replaced it, this helped but didn't cure it. Annoyingly it runs very well on the motorway (if a bit shy on power).

Here's the thing, when I fill the tank, for the next 24hrs all this goes away. It idles butter smooth, goes like a stabbed rat and the throttle response is sharp. After this all the above comes back. My working theory: not enough oxygen is getting into the engine so it doesn't combust properly. When the tank is filled the splashing in the tank causes air to dissolve in the fuel, temporarily correcting the imbalance

A bit of Googling suggests the MAF sensor is dirty/dying (it certainly fits most of the symptoms, but she's never died on me whilst idling), but I can't see one just after the air filter. Am I looking in the wrong place? Am I even considering the right part, is there something else I should be looking at?

I'm stumped, my garage is stumped. She is in really good condition bar the above, the only advisories I've had with her are bald tyres and dead bulbs, and she's only just clocked 100k miles so I want to hang onto it.

[CaptainBudget]

No it's right. 

Thing is she was doing that up to a couple of years ago, then it just fell off a cliff. This co-incided with a noticeable drop in power and rougher idling.

Also an engine warning light has just come on. Grr...