How does the fuel consumption gauge work?

[bostogrun]

Is the fuel consumption gauge really just a vacume gauge.

Low vacume = high consumption

High vacume = low consumption

Is that all their is to it?

[Jared at Pelican Parts]

It is not a vacuum gauge. It's electric.

It works off data coming from the injector pulsing and also the speed of the vehicle. Using this data, the OBC can calculate how much fuel is being used.

[bostogrun]

Thanks Jared,

Someone else on this board said thats how it worked. It kind of made sense but I didnt really think it was that simple.

[A930Rocket]

My speedo works, but my MPG gauge doesn't.

I've read the problem might be in the circuit board/instrument cluster.

[Jedan71]

So if it is electric not vacuum, i wonder why my mpg gauge pulses when driving along, around 2k - 2.5 k around 30mph and even more so when going up hill easy?
i have 97 BMW 318i auto with M43 engine

[coolbear1]

It gets its signal from several sources, none of which are vacuum. The electrical schematics show connections to the ECM, speedo, and service indicator board (SI on E-30 only).
There is very little use of vacuum to control the engine functions other than idle, crancase vent (re-introducing crankcase gasses and unburnt fuel back to the intake to be burned), and fuel pressure regulator.
Does it jump back and forth when the tank is full? Could be gas sloshing around in the tank, also the ecm may be sending more fuel and air to the engine due to changing load conditions which would directly effect consumption.