Fuel Gauge
Overview of the VR4 Fuel Gauge
The fuel gauge in the VR4 is an analogue gauge whose signal and reading is based on changing resistence of two rheostats (fuel senders) located in the fuel tank. The gauge itself is measuring current flowing through the gauge, with the amount of current changing based on the combined resistance of the two fuel senders.
The fuel gauge is notoriously inaccurate due to the design of the fuel tank shape and the lack of any computer controlled calibration of the gauge reading against the witnessed changes in resistance.
In addition to the fuel gauge, there is a low fuel warning light, operated by a separate rheostat and low fuel switch (on a different circuit).
Fuel gauge circuit
The diagram below (taken from the VR4 electrical wiring diagram workshop manual) shows the circuit path between the Fuel gauge and the fuel level senders. The sensors are wired in series with the upper (sub) sender altering its resistance first before bottoming out around the same time that the lower (primary) sender also begins to drop.
Fuel gauge inaccuracy
Without any modifications installed to resolve the issue, the fuel gauge is known to be highly inaccurate. The reasons for Mitsubishi not putting in the time and effort to make an accurate gauge are unknown, but the community consensus is that the gauge and sender setup are a "parts bin special" using components from the Mitsubishi Lancer Evolution.
Reasons for fuel gauge inaccuracy
By virtue of not using any form of gauge calibration, the inaccuracy of the fuel gauge is primarily caused by the design and shape of the fuel tank. The VR4, like the Lancer Evolution, uses a saddle-tank fuel tank design where the lower middle of the fuel tank needed to be shaped to fit over the driveshaft.
As a result of the saddle tank design, the fuel gauge is only accurate for about the first 20 litres of consumed fuel. At this point, fuel is located in both the driver and passenger sides of the fuel tank, with the "saddle" keeping them separate. The venturi (jet valve) located on the passenger side of the fuel tank will cause fuel to be lifted and pumped back over to the driver's side, but fuel can also "slosh" back over to the passenger side during right turns.
Given the saddle tank design and the act of fuel constantly sloshing back and forth over the saddle, an accurate fuel gauge required better calibration which Mitsubishi chose not to implement.
Solving fuel gauge inaccuracy
The accuracy issues with the VR4 fuel gauge have been resolved via development of the Fuel Gauge Recalibrator project. This is a module which sends calibrated readings to the fuel gauge based on an internal calibration table that compares the "seen" resistance of the fuel level senders and adjusts this in accordance with the calibration.
Low Fuel light circuit
The low fuel light circuit is separate from the fuel gauge circuit. It is combined of a switch located in the passenger side of the tank and a rheostat/sender located in the driver's side of the tank, wired in series.
The Low Fuel light is not permitted to operate unless the low fuel switch on the passenger side of the tank has been activated. Regardless of the position of the low fuel level sender in the driver's side of the tank, the Low Fuel light will not illuminate unless the level of fuel in the passenger side of the tank is low enough to activate the Low Fuel switch.
If the Low Fuel switch is active however, the Low Fuel light is allowed to illuminate based on the resistance reading of the Low Fuel sender. As this is a rheostat, this will cause the Low Fuel light to glow either dimly or brightly depending on how low the fuel level is.
Inconsistent Low Fuel light behaviour
The Low Fuel light may appear to behave erratically, coming on and off based on how you are driving. This is caused by the fuel in the tank sloshing around and between the driver and passenger side of the tank. The most common scenario is the low fuel light coming on dimly and then shortly turning off. Generally, this is caused by the fact that the venturi jet which collects fuel from the passenger side of the tank can take time to lift and pump fuel from the passenger side of the tank, but as it does so and the level of fuel in the driver's side increases, it causes the Low Fuel light to turn off.
If you then take a sharp right turn, much of this fuel that was just pumped to the driver's side of the tank can slosh back over to the passenger side of the tank, causing the Low Fuel light to again illuminate.
When to not ignore the Low Fuel light
If the Low Fuel light comes on very brightly, this means that you are very low on fuel - regardless of the saddle tank design - and should immediately drive more economically and find a petrol station.