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.
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.
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
• 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.