The Fuel rail pressure sensor (FRPS) is 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 return-less 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.

FRP sensor voltage chart.

0.50 volts - 0 PSI 1.2 volts - 10 PSI 1.65 volts - 20 PSI 2.2 volts - 30
PSI 2.75 volts - 40 PSI 3.45 volts - 50 PSI 3.9 volts - 60 PSI 4.6 volts
- 70 PSI

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.

CONDITIONS THAT AFFECT OPERATION

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.

COMPONENT TESTING

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.