Fuel injectors rely on a consistent supply of fuel at a specified pressure and volume. Early designs used a closed loop of sorts, called a return type system. In these, fuel rail pressure was regulated by a pressure relief valve located in the fuel rail that could be modified somewhat by referencing it to intake manifold vacuum. Excess fuel was bled off and returned to the tank via the return line, hence, it’s name.
It’s All Relative Pressure in the intake, and therefore at the injector tip, is affected by intake manifold vacuum. The difference between intake vacuum and atmospheric pressure is called manifold absolute pressure (MAP). For example, an engine at idle pulling 20 in/Hg will have a MAP of approximately 5 psi at sea level atmospheric pressure. To maintain a 40 psi differential across the injector tip, the fuel rail pressure would have to be 45 psi (40 psi differential + 5 psi MAP). However, the fuel rail sensor is already referenced to MAP, and the scan tool PID would read 40 psi. And if you connect a mechanical gauge that is referenced to atmospheric pressure, the gauge reading would be only 30 psi (45 psi minus 15 psi atmospheric pressure). This is why the two will not read the same on a running ERFS system. |
Later, returnless systems were designed that both reduced system complexity and heating of the fuel in the fuel tank. In these systems, the pressure relief valve was moved to the fuel pump assembly in the tank, allowing the excess to immediately return without traveling all the way to the engine and back. However, fuel rail pressure had to remain constant as it was not referenced to the intake as the older design was.
Maintaining a Constant
In 1998, Ford introduced ERFS, or the Electronic Returnless Fuel System. This system is unique in that there is no mechanical means of regulating fuel pressure. Instead, fuel pressure is maintained constantly electronically by varying the time the fuel pump is running.
This provides for a unique advantage in delivering fuel to the engine. To ensure that the correct amount of fuel passes through the injectors, the pressure differential between the rail side of the injector and the intake should remain as constant as possible. This system allows just that, using a dedicated fuel rail pressure/temperature sensor. The sensor is mounted on the fuel rail and is referenced to intake manifold vacuum. It is the key feedback sensor to the Engine Control Module (ECM), which in turn sends a varying duty cycle command to the Fuel Pump Delivery Module (FPDM) to maintain a constant pressure differential under varying loads.
The FPDM controls the fuel pump ground circuit, and by passing along the ECM command via a duty cycle of its own, the fuel pump can be rapidly turned on and off. This results in a varying pump speed and corresponding pressure variance.
The FPDM also is equipped with a fuel pump monitor circuit that sends one of three signals back to the ECM, and is used in diagnosing faults in the FPDM, fuel pump and related wiring.
How It Works
Fuel delivery is enabled during Key On, Engine Off for one second, and during crank or running mode once the ECM sees a signal from the Crankshaft Position Sensor (CKP). Based on feedback from the fuel rail pressure sensor, the ECM sends a commanded duty cycle signal varying from 5 percent to 51 percent to the FPDM. The FPDM doubles this duty cycle command and uses it to turn on the fuel pump. For example, a 40 percent ECM commanded duty cycle would translate to an 80 percent on time at the fuel pump. If all is well, the FPDM will send back a 50 percent duty cycle signal back to the ECM on the fuel pump monitor circuit, letting the ECM know that the FPDM is powered up and working correctly.
Testing Injector Flow with a Scope This can be time consuming, especially if you have to connect an activation tool to each injector. But wait a moment. The Ford ERFS system has a fuel rail pressure sensor, doesn’t it? Could I use that signal to monitor pressure drop across the injectors instead? It connects to the fuel rail at the Schrader valve, or it can be connected using most professional series fuel pressure testers that use a quick connect coupling. The ES300 shows pressure drops as a positive voltage shift on your scope, and the peaks should be relatively uniform if fuel flow through the injectors is equal. The location of the Schrader in the fuel rail on V6 and V8 engines will affect the amplitude of the pressure drops between banks, so take some time to try it on known good systems first. |
If the ECM wants the fuel pump off, it sends a 70 percent to 81 percent command signal to the FPDM. If any other duty cycle is sent or seen by the FPDM, the FPDM will return a 25 percent duty cycle signal on the monitor line, letting the ECM know that it either didn’t get the message or the message made no sense. A 75 percent duty cycle return on the monitor line lets the ECM know that the FPDM senses a problem in the fuel pump or the wiring between the pump and the module.
The fuel pump monitor signal can be viewed directly with a Digital Multimeter (DMM), graphing multimeter, scope, or with a scan tool using enhanced data. However, depending on the scan tool, the FP_M Parameter Identifier (PID) may display the actual duty cycle or one of the following:
* For actual 25 percent return: 15-60 percent PID
* For actual 50 percent return: 80-125 percent PID
* For actual 75 percent return: 250-400 percent PID
Testing Tips
Checking fuel pressure can be as easy as connecting your scan tool and viewing the FRP (fuel rail pressure) and FRP_DSD (desired fuel rail pressure) PIDs in the data stream. If there is a problem with the sensor circuit, look for codes P0192 (circuit low) or P0193 (circuit high). The sensor uses a 5-volt reference and ground supplied by the ECM that you can verify using voltage drop testing techniques. If they pass, make sure that the signal wire from the sensor to the ECM is intact before condemning the sensor.
If the ECM sees that the fuel rail pressure signal does not agree with what it commanded, it will record a code P0191 (FRP range/performance). This could mean a fault in the pressure sensor, its vacuum line, the pump or the delivery lines to the rail. You can verify the integrity of the fuel rail pressure sensor PID by measuring fuel pressure with a mechanical gauge. Keep in mind, though, that the sensor is referenced to intake manifold vacuum and your gauge is referenced to atmospheric pressure. That means they will only agree with the engine not running. Testing fuel pressure while running can result in a difference between the two of 7-10 psi. Once the sensor signal is verified, troubleshoot as you would any other fuel pressure complaint.
Last, a P0190 (FRP stuck in range) means the sensor signal to the ECM is not changing when it is expected to. Likely a sensor fault, verify by using your scan tool’s bidirectional capability to run the fuel pump at varying speeds and looking for the change in rail pressure on both your mechanical gauge and scan tool. If pressure varies on the mechanical gauge, but not on the scan tool, test the sensor to see whether or not the signal to the ECM is changing. Keep the tests brief, especially if running the pump at full speed, to avoid unnecessary wear and tear of the pump.
