Why measure fuel coming back?
On a return-style fuel system the pump always delivers more fuel than the engine uses, and the surplus flows back to the tank through the regulator. A single flow sensor in the feed line therefore reads pump output, not consumption. Put a second sensor on the return line and subtract one from the other: what's left is the fuel that went through the injectors.
That's the main reason to fit a return flow sensor. It's paired with a fuel flow sensor on the supply side to give real consumption figures. Endurance racers use those numbers for fuel strategy, and they're a useful cross-check on your fuel model when tuning.
Setting it up in NSP
The sensor has four setup nodes. We'd work through them in this order:
1. Wiring
Assign the input pin and tell the ECU what kind of signal the sensor produces:
- Analogue - Voltage for a sensor with a 0-5V output. The pull-up is normally left disabled for this type.
- Digital - Frequency for a sensor that outputs pulses, where the pulse rate rises with flow.
Check the sensor's datasheet if you're not sure which you have. Picking the wrong type gives readings that are nonsense rather than slightly off.
2. Configuration
This is where you calibrate the sensor, entering its scaling so the ECU converts voltage or frequency into a flow rate. The unit the ECU displays (for example gallons or liters per hour) is chosen separately under Preferences > Unit Options, so you can calibrate from a datasheet in one unit and view the result in another.
3. Display
Two warning limits control when number displays, gauges and bars for the return flow channel change to their warning colors, one at the low end and one at the high end. They don't change how the engine runs. They just make an abnormal reading hard to miss.
4. Diagnostics
Here you can enable fault codes (DTCs) for a signal that is too high or too low, in voltage or frequency depending on the sensor type. Each fault can be given a severity, and that severity decides which engine protection level is triggered. A broken wire or failed sensor then shows up as a clear code instead of a strange consumption figure.
Getting good numbers
- Both sensors need accurate calibrations. An error in either one ends up in the calculated consumption.
- Mount the return sensor where the line stays full of fuel. Air pockets make flow readings jump around.
- Log supply flow, return flow and injector duty together. When the difference between the two flows tracks injector duty, your setup is trustworthy.
FAQ
Do I need a return flow sensor on a returnless system?
Generally no. With no return line, the supply sensor alone reflects what the engine uses.
Will it change my fueling?
No. It's a monitoring and protection input. Fuel delivery is still calculated from your fuel model.
Related Nexus sensor pages: fuel pressure and wideband O2.
