e39 excessive fuel usage!

[Scamp's Walker]

'98 523i, outward journey 31-34 mpg, return journey (les the passsenger and luggage 18-19 mpg. Stopped turned ignition off resatarted and all seemed ok. Then range suddenly dropped to single figures and limped home after putting yet more petrol in! Any ideas to explain any, or all, symptoms. Checked air filter and it doesn't look particularly dirty.

[Defcon5]

Is the gauge correct?

[Scamp's Walker]

Sorry should have said guage seems ok. The reading were from the computer consumption readout.

[Defcon5]

Ah so it has the usual needle gauge and a digital readout? And they have ifferent values or the same?

I meant does the number on the gauge tie up with the amount of petrol you are actually putting in the car?

[Scamp's Walker]

It seemed to when I filled it up immediately before setting out. Was idly playing about with the OBC when I noticed the differences.

[PJ S]

Dirty MAF?
Coolant temp sensor?
Both would have the EMU enriching the mixture.

I would consider running a can of BG 44K through the system.

[hornetrider]

Fuel leak? Perished hose perhaps?

[steve_bmw]

is it running a bit rough? low on power at the bottom of the rev range ok in the middle and poor and stutery at the higher end of the rev range?
Is there a loudish clicking coming from your injectors?

Cam sensor would be my guess!