Only recently, in the past week or so, I've been getting really
crap MPG.

I had about 3/4 of a tank last week and up to 1/2 a tank, I had
over 120 miles out of it and the MPG gauge showed I was getting
around 40mpg at idle and during accelartion.

However, now, the MPG gauge is showing only 15mpg when idle and
during acceleration and it's used 1/2 a tank plus an extra £15
worth in less than 60 miles.

This only happens when idle or accelerating. During normal
driving, the gauge returns to above 50mpg, but then when slowing
down, It goes back to 15mpg .

Any ideas whats causing this? Faulty MAF perhaps? Worn cams
aswell?

how do you get any miles per gallon at idle when your not moving?? Should read 0mpg at idle, and your MPG is always crap whne you accelerate from stand still. Read the mpg when going a steady 40mph etc not when pulling off. to achieve a good mpg you must be traveling atleast 60mph constant, what your saying sounds about right to me. also you can not judge the amount of fuel used by the gauge as the shape of tank will vary this eg the tank could be smaller at the bottom and so when the float falls it falls quicker the lower it gets. In other word you may put £60 in for a full tank and when it is half way on the gauge it doesnt mean their is anotheer £30 in it possible more like £15-£20

Yeah, I know that it should read 0 at idle, and when you
accelerate, but according to the gauge it's a lot worse than it
normally is, unless the gauge is faulty.

Even if it is faulty, for the same amount of mileage it's been
using a lot more fuel the in past week then it ever has done

reasons for excess fuel comsuption are either faulty sensors, making the car think its not providing enough fuel when it is,therefore it piles far too much in, or ur plugs/leads/packs are weak and it takes more fuel just to get keep the power there. Does the car feel any different performance wise,and is there a smell of petrol from the exhaust?

ur car doesnt drive at idle mate,unless ur reffering to slotting it into neutral and coasting? in which case ur MPG guage shud read LOADS of miles per gallon,do u mean "closed throttle"? (which is when ur foot is off the peddle and the car is slowing down/revs are dropping?

The MPG gauge on the BMWs work of the vacume on the inlet system - check all the breather hoses are clear.

Sorry, yeah I meant closed throttle when I get good mpg.

If I coast, mpg is still good, but as soon as I
slow down as to stop, the gauge goes back to only 15. I know it
seems more logical, infact it should read 0 instead of 15, but
it's never done this before and fuel consumption is being
affected

Thanks Stuy, I'll check the breather hoses tomorrow