Ford has used returnless fuel systems for quite some time now, but we still get many calls regarding the theory of operation and diagnosis of low or high fuel pressure problems.
There are two types of returnless fuel systems. One is a mechanical returnless system and the other is the electronic fuel returnless system. This article will deal with the electronic returnless system used on 2004 and newer F-150 trucks.
With the electronic fuel returnless system, the powertrain control module (PCM) monitors fuel pressure via a fuel rail pressure sensor (FRPS). This system utilizes a fuel pump driver module (FPDM) that the PCM sends a command to. The command varies the duty cycle to the pump to control fuel pump speed, thereby controlling fuel pump pressure.
Most often, the fuel pump has direct battery voltage applied to it at all times, and the ground side control is duty cycled to control average voltage applied to the fuel pump. Other returnless systems have the ground supplied to the fuel pump at all times and the power is duty cycled to control average voltage to the fuel pump. Either system works basically the same. The main point in diagnosing a defective fuel pump lies in monitoring the average power supplied to the fuel pump as the problem occurs.
When a fuel pump is in good working order, it only needs an average of 6 to 9 volts to maintain adequate fuel pressure. As the pump begins to wear out, or if the fuel filter becomes restricted, average voltage to the fuel pump is increased to maintain good fuel pressure.
When diagnosing fuel pressure issues, the first step is to retrieve trouble codes from the PCM. The PCM has the ability to recognize low fuel pressure from feedback from the FRPS. Typically, if the fuel pressure drops below 20 psi, the PCM will generate a P0191 trouble code. This code indicates that the PCM still has good communication with the FPDM, but sees that the fuel pressure is out of range via the FRPS.
If the PCM sets a code P1233, this indicates that the PCM has lost communication with the FPDM, either from a defective FPDM, loss of battery voltage from a tripped inertia switch, loss of ground, or wiring harness problems affecting the Monitor or Command circuits.
Retrieving the codes will dictate which diagnostic path to follow. Sometimes there are no codes set, although fuel pressure drops below specs.
A mechanical fuel pressure gauge should always be installed to compare actual fuel pressure to the FRPS. Although the FRPS seldom fails, it can send a false reading to the PCM indicating that the fuel pressure is actually higher than it really is. The FRPS is a typical three-wire sensor that utilizes a 5-volt reference voltage and ground wire and sends the sensor values to the PCM. In general, the higher the feedback voltage, the higher the fuel pressure is.
With Ford’s FRPS voltage and fuel pressure chart, specs are as follows:
4.5 volts = 70 psi
3.9 volts = 60 psi
3.4 volts = 50 psi
2.8 volts = 40 psi
2.2 volts = 30 psi
1.6 volts = 20 psi
1.1 volts = 10 psi
0.5 volts = 0 psi
This sensor can be found on most scan tool data streams and labeled as ‘FRP.’ It is important to note that the FRPS reading on the scan tool will read 8 to 10 psi higher than the mechanical fuel pressure gauge unless the vacuum hose is removed from the FRPS; then the readings will come close to matching each other. This is normal and does not indicate a defective FRPS. If the PCM sets a code P0191 and the fuel pressure actually is lower than printed specs, the power and ground to the fuel pump must be checked. The P0191 code usually sets if the fuel pressure drops below 20 psi or is above 60 psi.
To diagnose a 2004 F-150:
1 Verify the FRPS has 5 volts on the brown/white wire, a good ground on the gray/red wire, and that the signal wire orange/light green is not shorted to power or ground.
2 If the fuel pressure reads lower on the mechanical gauge than the FRPS reads and there are no wiring harness issues, replace the FRPS and retest fuel pressure.
3 If code P1233 is set, perform the same power and ground checks and service if necessary. The FPDM gets its power from the inertia switch on the white wire and should be battery voltage. If there is less than battery voltage on this wire, voltage checks will need to be done to determine where the voltage drop is coming from.
4 If OK, verify the FPDM is properly grounded on the black/yellow wire. A poor ground will not allow the FPDM to supply an adequate ground signal to the fuel pump.
5 If OK, check the power from the FPDM on the pink/black wire that feeds power to the fuel pump.
6 If OK, move the ground lead of the voltmeter to the brown/white wire so that average voltage to the pump can be measured.
7 If the voltage is near battery voltage and the fuel pressure is low, this would indicate the fuel pump is failing or the fuel filter is badly restricted.
8 Always recheck the voltage at the fuel pump in case the wiring between the FPDM and the fuel pump is bad.
9 If the power and ground are OK but the P1233 resets, check the fuel pump monitor (FPM) wire (light blue/orange) to pin 30 of C175b of the PCM for shorts or opens.
10 Do the same with the fuel pump command (FBC) wire (white/yellow) to pin 62 of C175b. If the wires test out OK, the FPDM is usually at fault.