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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7 responses to “Fuel Level

  1. do both tank sensors have the same characteristics? One might wonder whether the transfer pump completely empties one side of the tank at low fuel levels.

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    • There should be two sensors in series. Haven’t measured the real thing yet.
      I wouldn’t be worried, Subaru has got many years of experience with saddle bag tanks. As in petrol models there’s probably a siphon pump that draws fuel from the other side, completely passive, powered by returned fuel flow.

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  2. it’s an electric pump on diesels, you can see it above the rear differential.

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    • That’s the electrical sub fuel pump for sure, powered via relay, controlled by ECU. Impreza has it, too.
      Can you tell if it has anything to do with levelling tank halves? Otherwise I’d guess it’s meant for pre-pressurizing fuel up to filter…

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  3. it connects the two tank halves, has the same function as the jet-pump in gassers. Does the ECU run it all the time? It doesn’t start at key-on, and sometimes I can hear it ticking, but other times not.
    Certainly, fuel flows from one side of the tank to the engine and is returned to the other, but how much more flow the sub-pump can muster than the return supplies is an interesting question.

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  4. actually, I was doubting myself and perused the service manual and parts infos again. Both the main and sub sensors have flow and return; the sub pump connects directly to the rear of the tank. In this case, does it mean the sub-pump has another purpose- circulating the fuel in the tank for cooling perhaps?

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    • Levelling + cooling purpose is possible, too. ECU will reduce power at very high fuel temps.
      Yes, sub fuel pump won’t run at ignition on, needs engine starting/running. Then, according to SSM2 logs, it’s operating all the time. Also, on Impreza its relay is pretty loud, located near glovebox, so switching would be noticable…
      Either engine noise is louder than pump or it isn’t that noisy with actual fuel flow going.
      You can test this pump and other things using test connector (passenger footwell, connect when IGN off, then IGN on).
      Update: Listening carefully near rear outside, I can hear pump ticking softly, fuel level roughly 3/4. Different sound compared to ENG OFF.

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