Apologies of this has been covered elsewhere but having done a search I cannot find anything that specifically covers this issue.
1.9CDTI 150, 2007. Generally does 42-44mpg on the sort of roads I use for a daily commute rising to 46-48 on motorways. With the instaneous fuel consumption readout selected at idle it shows 0.2-04gal/hr depending upon how warm the engine is and whether the climate control is on.
Occasionally - not normally more than once per tank of fuel and more often than not only once every 203 tanks of fuel, the instantaenous fuel consumption on the run will suddenly drop by at least 50%, the distance to run will count down 1 mile every 200 yards or so travelled and if I pull up to a halt the instantaneous readout at idle shows 1.1gal/hr. If I switch the engine off for 10 seconds and restart, the problem clears. The dealer has checked for fault codes and there are none.
Very occasionally (3 times in past 6 months - I've had the car for 3 years) it does it repeatedly. So for example a month or so back I was driving up the M3 to London and had to pull over every 2-3 miles to switch off and restart. After doing this about 10 times I gave up and accepted that I would have an average of about 20mpg, but about another 10 miles further on it cleared completely; the drive back was fine and I got it up to over 50mpg average.
When it's doing it the performance doesn't seem too bad although you can tell that it is over-fuelling from its slightly rough running. Since it is intermittent and there are no fault codes, the dealer (whom actually is very good normally) hasn't got any suggestions. So I wonder if the collective knowledge here can help?