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Fuel Pressure And Manifold Vac/Press Interface


One of the most commonly confused functions of the fuel supply system is the interaction of manifold Vac/Press and the fuel pressure regulator.  Simply stated, the function of the fuel pressure regulator is to maintain a constant pressure at the fuel injector’s metering orifice.

It is important to the calibration of the EFI system that the ECU know how much fuel will be injected into the cylinders per a given pulse width.  Therefore, the ECU’s calculations are based on the injector’s flow curve which is established at a constant fuel pressure.  As was show earlier, fuel pressure changes will change the injector’s flow rate. If the pressure was allowed to change continuously, the ECU’s calibration tables would be incorrect since the flow rate versus pulse width values would be constantly changing due to the pressure fluctuations.  Therefore, fuel pressure at the injector’s metering orifice must be kept constant.

Most EFI fuel systems achieve constant pressure regulation through a back-pressure regulator.  The regulator receives a reference signal from the intake manifold and will raise or lower the fuel pressure to maintain a constant pressure (called Delta P) at the injector metering orifice.  The regulator function can be shown by Figure 1 and the following formula:


Press @ METERING ORIFICE (Delta P) = Fuel System Pressure – Manifold Pressure

                                                          Figure 1


The formula shows the relationship of manifold vacuum or pressure with fuel line pressure (the pressure you would see on your gauge).  As can be seen from the examples in Figure 1, when no vacuum or pressure is applied to the regulator, fuel is metered at the set pressure.  When vacuum is applied, the orifice is operating in a negative pressure environment.  Less pressure in the fuel line is required to maintain the constant pressure of 40 psi at the metering orifice.  Likewise, when there is positive pressure (boost from turbos and superchargers) more pressure is needed in the fuel line to overcome the additional pressure surrounding the metering orifice.

When checking or adjusting the pressure regulator, it is always best to disconnect the manifold line to the regulator before taking a reading.  This reading is the true fuel system pressure set point.


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