Category Archives: Table

Fuel Level

Introduction

On Euro 4 and early EU 5 (MY till including 2011) diesel models fuel level is being measured by BIU and transmitted on CAN-bus as resistance value in ohms (Ω). The ECU parses this and does some processing. First it calculates artificial fuel level voltage using standard voltage divider formula. Voltage value is being damped because the level information fluctuates a lot with fluid moving. Then the damped result is being used for LUT (Lookup table).

Euro 5 gen3 and EU 6, = MY 2012+: Fuel level is being digitized by combination meter. ECU ROM does more sophisticated damping/averaging.

Math

Did some math, using exact ROM table data (ScoobyRom, export as CSV), applying formulas, creating plots.


For above voltage divider, where:

  • Vin: Battery Voltage (assumed constant in this case)
  • R1: constant series resistor
  • R2: variable fuel level resistance (two sensors in series, one per tank half)
  • Vout: fuel level voltage (artificial ECU value in this case)

This way I eliminated ECU’s artificial fuel level voltage, creating direct relationship between resistance [Ω] (CAN value) and volume [litre].

ROM Tables and Calculated Results

gen1 Euro 4 (MY 2008-2010)

Fuel tank capacity: 64 litres.

Fuel Tank Level, 2D table, Euro 4, ScoobyRom screenshot

gen2 Euro 5 (MY 2010-2011)

Fuel tank capacity: 65 litres.

Fuel Tank Level, 2D table, gen2 Euro 5, ScoobyRom screenshot

gen3 Euro 5 (MY 2012-2014) and gen4 Euro 6 (MY 2015+)

Fuel tank capacity: 60 litres.

There seems to be a calibration data error as the vertical step (at 19 ↔ 23 litres) does not make sense! ECU’s LUT code can handle this but technically it is a flaw. The averaging algorithm after table lookup will mitigate this issue. Same map data was found in newer Euro 5 and Euro 6 ROMs.

Fuel Tank Level, 2D table, Euro 6, ScoobyRom screenshot

Diagnostic Trouble Codes

DTCs are triggered when (artificial) fuel level voltage is outside range. As usual it’s not that simple, many conditions may affect this, too – battery voltage, CAN state etc.

  • P0462 Fuel Level Sensor Circuit (Low Input)
  • P0463 Fuel Level Sensor Circuit (High Input)

For some reason, above codes are not publicly exposed in Euro 6 spec models, also not available in service manual DTC listing. They are processed internally however.

Fuel Gauge

Fuel gauge and fuel warning light, Subaru Impreza diesel Euro 4. At depicted position, there are still 5 to 10 litres left in the tank.

I tested the fuel gauge by inserting a variable resistor after disconnecting a fuel level sensor connector from the wiring harness, below the rear seats. The Euro 4 fuel gauge at least reacts very slowly. It takes more than 20 seconds from empty to full and vice versa. The combination meter does its own damping obviously. Therefore, gauge hijacking, i.e. abusing the fuel gauge in order to display something fluctuating like manifold air pressure (MAP) is not feasible (without hacking combination meter ROM, of course).

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