Maximizing Performance: The Role of Fuel Tank Pressure Sensors

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Purpose

The Fuel Tank Pressure (FTP) sensor measures positive and negative (i.e., vacuum) air pressure in the fuel tank, which allows the engine or Engine Control Module or Powertrain Control Module (ECM/PCM) to test for leaks in the Evaporative Emissions (EVAP) control system.

The FTP sensor is a component of the “enhanced” EVAP system designed to prevent escape of fuel vapor into the atmosphere. This system was introduced in model year 1996 and became mandatory in 1999.

Location

The FTP sensor usually is located on top of the fuel tank, either mounted to the fuel pump module assembly or directly to the tank itself.

On some vehicles, the FTP is located away from the fuel tank and connected to it by a hose. On a few vehicles, it is located near the EVAP system charcoal canister that stores excess fuel vapor, and is connected to the line from the fuel tank to the canister.

Operation

The key component of the FTP is a flexible silicone wafer that changes its electrical resistance – the reduction in electrical current flow through a conductive material – as it flexes. The edges of the silicone wafer are held in a frame, so that one side of the wafer is exposed to tank pressure and the other to a known pressure in a sealed chamber.

When the ECM/PCM sends a 5 volt signal to one edge of the silicone wafer, the degree of flex caused by fuel tank pressure or vacuum determines the signal voltage sent from the opposite edge of the wafer to the ECM/PCM. The ECM/PCM calculates air pressure or vacuum in the fuel tank based on the voltage of this signal.

To test for air leaks, the ECM/PCM first seals the fuel tank and EVAP system by closing all valves and openings which would allow air to enter or exit. Depending on the vehicle manufacturer, the ECM/PCM then either pressurizes the tank by activating a pump (Chrysler) or allows the fuel pump to create vacuum as fuel is pumped from the tank (General Motors, Ford, and many other manufacturers).

A third system (developed by General Motors and adopted by some other manufacturers) allows the natural cooling of fuel after the engine is turned off to create tank vacuum. Once the system is sealed and the specified pressure or vacuum is created, the ECM/PCM checks for changes in pressure over a predetermined time.

Any change in pressure indicates an air leak. The rate of change indicates the size of the leak. If a leak is detected, the ECM/PCM stores a Diagnostic Trouble Code (DTC), illuminates the instrument panel Malfunction indicator Light (MIL), and stores Freeze Frame data on the second consecutive test failure.

Drivability Symptoms

Failure of the FTP sensor does not affect drivability, but will illuminate the MIL.

Inspection, Test, and Diagnosis

DTCs P0450, P0451, P0452, P0453, and P0454 pertain specifically to the FTP sensor and its electrical circuit. These codes indicate a short or open in the circuit or a defective sensor.

P0450 indicates the FTP reading fluctuated to a level above the normal signal range; P0451 indicates the signal fluctuated below the normal signal range (The normal signal range in most vehicles is between 0.5 and 4.5 volts).

P0452 is set when the signal remains above the normal range. P0453 indicates a signal constantly below the normal range. P0454 indicates an intermittent fault.

To exclude wiring as the cause of an FTP trouble code, disconnect the electrical connector and identify each of its three pins or slots: one each for the input or reference voltage circuit from the ECM/PCM, the signal circuit sending signal voltage back to the ECM/PCM, and the ground circuit.

Attach the red (positive) lead of a digital multimeter (DMM) to the connector’s input/reference circuit pin/slot and the black (negative) lead to the signal circuit pin/slot. With the ignition key on and the engine off (KOEO), a 5 volt reading verifies the circuit to the sensor.

Wiggle the connector to see whether the voltage reading changes. Any changes in voltage indicate a defective connector. Connect a 12 volt test lamp to battery voltage and the lamp tip or other lead to the ground pin/slot. If the lamp illuminates, the ground circuit is good.

Connect the red (positive) DMM lead to the signal circuit pin/slot and the black (negative) DMM lead to the ground pin/slot. With KOEO, on many vehicles a 5 volt reading verifies the signal circuit and establishes the FTP is defective. However, a 0 volt reading is inconclusive, as it may result from ECM/PCM design. If so, the ECM/PCM connector must also be removed, and the signal circuit tested with the DMM on the ohms setting for continuity between the FTP and the ECM/PCM.

Reprogramming

Replacement of the FTP sensor does not require reprogramming.

Repair/Replacement

Many vehicles provide an access panel in the floorpan under the rear seat or in the trunk floor that allows for removal of an FTP sensor located on top of the fuel tank. On others, the fuel tank must be removed to gain access to the FTP sensor. Access to a remotely mounted FTP sensor may require removing a rear wheel well inner fender.

Before removing an FTP sensor, the fuel system should be depressurized by releasing pressure at the fuel rail Schroeder valve – it resembles a bicycle tire valve - or by removing the fuel pump fuse and running the engine until it dies.

The negative battery terminal should be disconnected. A sensor mounted to the fuel pump assembly or fuel tank is removed by disconnecting the wiring harness connector, removing any retaining nut, bolt or clip that holds the connector in place and then gently prying the sensor from the sealing grommet through which the sensor tip projects into the tank.

Remotely mounted sensors are removed by disconnecting hoses and removing the sensor from any mounting bracket.

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