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How is the fuel used calculated


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How is the Fuel Used calculated?

It used to be very accurate in that Fuel Used(U) + Fuel remaining® = Tank Capacity (68.1 L)

But over the last few months its not adding up anymore. It tracks fine for the first 10-12 L but after that the fuel remaining drops rather fast and there is a difference of more than 10-15L between U + R and the fuel tank capacity.

First I thought it was a defective sender unit in the tank but when I fill up on an almost empty tank(8L), the fuel pumped + fuel remaining adds up to a full tank, but the litres used is between 45-50 L only. It used to fine before.

A plausible cause is that the resistance value on the sender unit after 10-12L is way off and it reports a value less than what is in the tank at that volume.

Any comments?

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Our good friend bbobynski posted this on the old board, Mike


Kevin is correct, the "fuel used" is calculated using the pulse width calculated for the injectors. The computer just adds all the pulse widths from the time you reset and converts that to gallons using the flow rate of the injectors. Generally speaking, I would trust the fuel used over the numbers on the pump....however....I have personally compared the fuel used to the pump readings many many times over the years and have very rarely seen much of a descrepancy. You will probably see more descrepancy on a one tank basis due to the variability in the shutoff of the nozzles and differences in tank fills due to topping off differently, etc. Try running the fuel used continuously for many tank fulls and I think you will see any variability average out and the total fuel pumped will equal the fuel used closer and closer over time. On the cars I've driven I usually see only a few tenths of a gallon descrepancy on a tank by tank basis.

The only way the system can not be accurate is if the fuel presure is off or the fuel pressure regulator is leaking allowing "un measured" fuel to enter the intake....if the wrong injectors are in the engine the flow rate will be off since the computer is calculating based on the flow rate of the production injectors but this is unlikely.

Seems like some time ago someone posted about their fuel used and displayed economy being inaccurate and it turned out that their fuel pressure regulator was acting up or something was found wrong....


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