Maximizing Efficiency: Understanding Fuel Pressure Sensors

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Purpose

The fuel pressure sensor, also referred to as the fuel rail pressure sensor, measures fuel pressure in the fuel rail leading to the fuel injectors. Some vehicles also employ a sensor to measure pressure from the tank to a high pressure fuel pump at the engine. 

This information is used by the Engine Control Module or Powertrain Control Module (ECM/PCM) to adjust fuel rail pressure and fuel injection timing and duration. When a second sensor is employed, the data is used to adjust fuel pump speed.

Common rail fuel injection systems employ a long tubular rail to hold fuel at high pressure that is then fed to the individual fuel injectors. These systems are used on most modern diesel powered automobiles and trucks, and are also used on late model automobiles equipped with gasoline direct injection, such as the Ford Ecotec and General Motors 3.6 liter V-6 engines. 

These systems inject fuel directly into the combustion chamber, rather than into the intake manifold passages to the cylinders, and employ a second high pressure fuel pump mounted on the engine to substantially increase pressure above that created by the fuel pump in the fuel tank. The ECM/PCM must know fuel pressure in the common rail to calculate when to open fuel injectors and when to end injection.

Fuel pressure sensors are also used in many conventional gasoline multi-port fuel injections systems (which inject fuel into the intake manifold passages to the cylinders) employing pulse-modulated fuel injection. In these vehicles, timing and duration of fuel injection is affected by throttle position, so the fuel pressure sensor measures the difference in pressure between the fuel rail and intake manifold vacuum.

Location

The fuel pressure sensor is located on the fuel rail, mounted either to the top of the fuel rail –near the end of the rail or near the middle - or projecting from its end. Sensors used to measure fuel pressure from the tank are located in the fuel line, usually near the fuel tank.

Operation

A fuel pressure sensor is composed of a silicone wafer that changes resistance to electrical current flow through the wafer as the wafer bends in response to changes in pressure applied to the wafer. This functionality is called piezoresistance. 

The sensor usually is a three-wire sensor: ground, signal, and five volt supply. The sensor is supplied a 5 volt reference signal by the ECM/PCM, which measures sensor voltage at the signal side of the wafer to determine pressure. Sensor voltage and resistance increase as fuel pressure increases. On some vehicles using pulse-modulated systems, the sensor also incorporates fuel temperature measurement, adding a fourth wire for the temperature signal.

In common rail systems, the sensor compares fuel pressure to pressure in a reference chamber within the sensor. In pulse-modulated systems, a vacuum hose connects the sensor to the intake manifold, allowing fuel pressure to act on one side if the wafer and intake manifold vacuum to act on the other. The sensor then measures the differential pressure.

Drivability Symptoms

Failure of a fuel pressure sensor may prevent starting the engine. Malfunction of the sensor can lead to rough and surging idle, severe loss of power, and misfiring under load. The problem may disappear if the vehicle is restarted, but reoccur shortly after restart. If the engine starts, the instrument panel Malfunction Indicator Light (MIL) will illuminate.

Inspection, Test, and Diagnosis

CAUTION: Common rail fuel pressures can be 25,000 pounds per square inch (psi) on diesel engines, 2,500 psi. on direct injection gasoline engines. Fuel released at those pressures can easily penetrate skin, which can be fatal. Use extreme caution when inspecting the common rail, fuel lines, or fuel injectors with the engine running.

1996 and later gasoline powered automobiles and light trucks and 1997 and later diesel powered cars and trucks employ On-Board Diagnostics Generation 2 (OBD2). In these vehicles, generic Diagnostic Trouble Codes (DTCs) P0190 through P0194 indicate the fuel pressure sensor signal is either missing, out of the expected range, or is intermittent. 

These are Type “B” codes, which illuminate the MIL and store Freeze Frame data on the second consecutive ignition cycle in which the fault occurs. These codes indicate either a defective sensor, a short or open in sensor wiring, or poor contact at the sensor connector. DTCs P0087, fuel rail pressure too low, and P0088, fuel rail pressure too high, are based on fuel pressure sensor data received by the ECM/PCM, but do not usually indicate sensor malfunction.

A Digital Multimeter (DMM) may be used to verify integrity of sensor voltage and sensor signal circuits. Remove the sensor connector and identify the connector terminals for the voltage reference circuit, the return circuit (i.e., ground), and the sensor signal. (The voltage circuit is generally a red wire, the return is usually black.) Using the DMM, connect the red lead to the connector voltage terminal and black to the return. A reading between 4 and 6 volts verifies the voltage and ground circuits. 

Using an ordinary and inexpensive 5 Kohm resistor (available at any electronics or radio supply store), connect one end of the resistor to the voltage terminal in the connector and the other to the sensor signal terminal. (The resistor is important, as it protects the ECM/PCM if the signal circuit is shorted to ground.) With the black lead connected to a ground, such as the battery negative terminal, touch the red lead to the signal circuit terminal in the connector. The voltage should be just below 5.0 – 4.98, for example. This verifies the signal circuit. If all circuits are verified, the sensor should be replaced.

Reprogramming

Replacement of the fuel pressure sensor does not require reprogramming.

Repair/Replacement

Before removing the fuel pressure sensor, relieve any fuel pressure from the system: if the vehicle is direct injected, either wait two hours after engine operation or remove the fuel pump relay and start the engine, allowing it to run until it dies and then removing the key. If the vehicle has pulse-modulated injection, pressure may also be released at the Schrader valve on the fuel rail.

The fuel pressure sensor is removed by disconnecting the wiring pigtail and the vacuum hose, if any, then removing the sensor. When replacing the sensor, it is essential that it be tightened to the torque value specified by the vehicle manufacturer. After installation, verify that there are no fuel leaks.

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