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Signs of a Bad Fuel Pressure Regulator

By Andy Y, July 24, 2009

The fuel pressure regulator does exactly what its name implies – it regulates the pressure of fuel as it travels to the fuel injectors. Where the regulator is located, and the exact role it plays, depends on what type of fuel system the vehicle has.

fuel pressure regulator

Fuel pressure regulator

Continuous return fuel system

Older vehicles have what’s referred to as a continuous return fuel system. With this design, fuel is delivered from the tank to the fuel rail by the fuel pump. The fuel pump delivers more fuel than the engine needs. To maintain proper fuel pressure under all driving conditions, a fuel pressure regulator is used. The regulator maintains proper fuel pressure by routing excess fuel back to the tank, via a return line.

In a continuous fuel system, the fuel pressure regulator is typically mounted in the fuel rail. The fuel pressure regulator is controlled by engine vacuum.

Return-less fuel system

Most modern vehicles use a return-less fuel system. As the name indicates, this type of system has no return line to the fuel tank and no externally mounted regulator. Instead, fuel pressure is controlled in tank. The fuel pressure regulator is usually built into the fuel pump assembly. This design allows for more precise fuel control and reduced emissions.

Some return-less fuel systems regulate fuel pressure by varying the fuel pump’s duty cycle (on time). The fuel pump is usually either controlled by the powertrain control module (PCM) or a dedicated fuel pump control module (FPCM). A fuel pressure sensor is used to monitor system operation.

Signs of a bad fuel pressure regulator

A faulty fuel pressure regulator can lead to a wide range of engine performance problems. Some of the symptoms include:

  • An illuminated check engine light
  • Poor fuel economy
  • Black smoke from the exhaust
  • Poor engine performance
  • A vehicle that won’t start or stalls
  • Fuel leaks - Continuous return fuel systems may suffer from leaks if the fuel pressure regulator fails. The regulator is externally mounted and can leak if the diaphragm ruptures, or one of its seals fail.
  • Fuel pressure too high - A faulty fuel pressure regulator may result in fuel pressure that is too high and a rich running condition.
  • Fuel pressure too low - A bad fuel pressure regulator can also result in fuel pressure that is too low, causing a lean condition.
Get it diagnosed by a professional

How to troubleshoot a fuel pressure regulator problem

Before condemning the fuel pressure regulator, it should be tested. This is typically done by checking fuel pressure downstream of the regulator with a mechanical gauge. On many late model vehicles, fuel pressure can also be checked using a scan tool.

fuel pressure regulator location

Fuel pressure regulator location

On older vehicles with a continuous fuel system, the regulator can be checked by removing its vacuum hose. With the hose removed, fuel pressure should increase by 8 to 10 PSI. Its also a good idea to check for fuel inside the regulator vacuum hose, which would indicate a leaking diaphragm.

If the fuel pressure regulator is found to be bad, it should be replaced. On most return-less fuel systems, this typically means the fuel pump assembly must be replaced.

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