Is the rail pressurized? If not we may need a fuel pump...

With the ever tightening emission standards the need to prevent hot fuel from returning to the tank became an obvious necessity. The fuel rail pressure sensor is a link in modern returnless systems, in which only one fuel line reaches the hot engine bay. The FRP sensor simply informs the ECM of fuel pressure value and it in turn regulates the furl pumps rotational speed.

Theory of Operation

In recent years the return-less fuel system has gained wide acceptance. With this type of system, the return fuel hose is eliminated in favor of a different type of fuel pump module, using a built-in fuel pressure regulator. In an in tank returnless fuel pump module system, the fuel is returned to the fuel tank right at the fuel pump itself without ever leaving the fuel tank. The reason for all this is to prevent an unnecessary amount of fuel from reaching the engine bay, where it will heat-up and cause excessive fuel vapors at the fuel tank. In other words, this system addresses the never-ending struggle to stop excessive EVAP emissions.

Another high-tech approach to the fuel vapor problem is the use of a fuel rail pressure sensor in conjunction with a variable-speed electric fuel pump. Ford, among others, has adopted this approach in a wide variety of their newer model vehicles. With the electronic return-less system, the ECM relies on the FRP (fuel rail pressure) sensor for fuel pressure input right at the fuel injectors. By monitoring the fuel pressure, the ECM can then adjust the fuel pump’s rotational speed and maintain a stable pressure. Once a stable fuel pressure is attained, the formation of fuel vapors in the fuel line itself is greatly reduced. The whole process happens very fast since it is electronically controlled.

The FRP sensor is a three-wire piezoelectric electronic pressure sensor. This means that the sensor’s resistance varies as pressure changes. The FRP sensor is also connected in line with an internal ECM voltage divider resistor network. So that as the sensor’s resistance changes with pressure the overall current flow varies as well. The higher the sensor’s resistance the less current flow and the higher the voltage. The higher voltage across the FRP sensor will cause a lower overall voltage across the ECM’s internal resistor and vise-versa. A typical FRP sensor voltage-to-pressure chart is shown next.

In some FRP sensor applications, the sensor is also connected to the intake manifold side. In this arrangement the sensor’s signal output is a differential signal of fuel pressure to intake manifold, which the ECM uses to control the fuel pump speed. Therefore, maintaining the fuel in the rail in a liquid state and preventing fuel vapors.

NOTE: The differential signal of the FRP sensor takes into consideration the amount of intake manifold vacuum of the engine. This way the ECM can properly control the actual amount of fuel leaving the injectors. In other words, the ECM actually tailors the fuel delivery according to the engine operating demands. It takes less fuel pressure to push a certain amount of fuel through an injector connected to a high vacuum manifold then otherwise. An engine having high vacuum creates a suction at the injector manifold ports, and the result is less pressure needed to inject the fuel.


The FRP sensor is connected directly to the injector fuel rail, which makes it susceptible to the same temperature variations as the injectors. A clogged fuel filter, a defective in-tank fuel pump module, dirty fuel lines, etc could also cause an erroneous signal reading. It is important to determine if the faulty signal reading is caused by the FRP sensor itself or some condition that is affecting it. Do not condemn the FRP sensor until all the necessary testing have been performed.


The FRP sensor is a three-wire type sensor. The ECM provides a reference voltage as well as a signal ground to the sensor. The sensor then sends a pressure signal back to the ECM thought the signal wire. It is also good to know that the FRP sensor may shares the reference and ground wires with other sensors and any electrical conditions that affect the other shared sensors will also affect it. Follow the steps bellow to determine the root of the problem.

• First determine if there is actual fuel pressure in the system. This will eliminate a faulty reading condition caused by a mechanical fuel system problem. Using a fuel pressure/volume gauge, ascertain that the system is working properly.

• Disconnect the FRP sensor and open the ignition switch.

• Using a voltmeter, probe between the FRP reference voltage wire and battery ground. Reference voltage (usually 5.00 volts) should be seen at the meter (tests for proper reference voltage).

• With a voltmeter, measure the voltage across the FRP sensor ground wire and battery positive. Battery voltage should be seen (tests the integrity of the FRP sensor ground circuit).

• With a voltmeter, measure the voltage across the reference and ground wires of the FRP sensor (double-checks the reference voltage and ground leads of the FRP sensor).

• Jump the FRP sensor signal wire to the ground wire at the connector. Using a scan tool, monitor the fuel rail pressure PID (with engine off). About 0.00 volts should be seen.

• Jump the FRP sensor signal wire to the reference voltage wire at the connector. Using a scan tool monitor the fuel rail pressure PID (with engine off). A reference voltage reading should be seen (usually 5.00 volts).

• Make certain that the vacuum hose going to the FRP sensor is not clogged or broken. The engine vacuum is taken into consideration by the ECM when adjusting the actual fuel pressure. The FRP sensor will actually output a differential signal, which takes into account the amount of vacuum at the tip of the injectors.

If these steps check out OK and there is no fuel system mechanical problems, the fault is probably at the FRP sensor itself. Take extreme caution when replacing the FRP sensor, since you will be dealing with flammable fuel. Always be aware of the fire extinguisher’s location and avoid any open flames while working on the fuel rail. Follow the manufacturer’s replacement procedures.


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