Tip & How-To about Mercedes-Benz E-Class

FPR - Fuel Pressure Regulator

2.1) FPR - Fuel Pressure Regulator (actuator/passive)

What is it? The fuel pressure regulator is a mechanical device that maintains a well regulated level of pressure in the fuel rail to the fuel injectors.

Where is it located? The FPR is a metal circular device most usually located on the end of the fuel rail on the engine. Attached to its top is a vacuum line from the intake plenum.

How does it work? The fuel pump(s) supply fuel at high pressure to the fuel rail. Excess fuel pressure, above that required for satisfactory injector operation, is released by the FPR by allowing fuel to return to the tank via the fuel return pipe. The FPR consists of a diaphragm valve held closed by the constant force exerted by a spring. Once the pressure of the fuel in the fuel rail exceeds the force of the spring on the valve then fuel bleeds past into the return pipe. The action of the spring is opposed by the effect of a vacuum line (from the intake manifold) acting on the diaphragm. When the car is at idle the high vacuum conveyed in the line from the inlet manifold to the FPR ensures that only a modest pressure of fuel is allowed in the fuel rail to the injectors. As the engine speed increases there is reduction in vacuum, the spring action is less impeded and the fuel pressure increases. By governing the period of each injection cycle at a given fuel pressure the ECU is able to maintain appropriate and economical use of fuel in response to engine demand.

Symptoms of faulty FPR

A faulty FPR has many similar effects to that of a faulty MAF namely:-

  • Rough running - if the diaphragm is inoperative or the vacuum line has a minor leak the fuel pressure in the fuel rail will be increased. The amount of fuel injected per cycle will be poorly regulated and this will make the engine 'lumpy' due to too rich mix. If the vacuum line to the FPR has a major leak or has become disconnected then the introduction of unmetered air into the inlet plenum will cause bad idling due to too lean mix.
  • Difficulty in starting - there are two effects here:
    • 1) If, as above, the effect of the vacuum line to the FPR has a small leak then the engine will run rich to the extent that excess fuel may flood the engine.
    • 2) Fuel may exit a faulty FPR diaphragm via the vacuum line and enter the intake manifold again either flooding the engine or making it too rich to ignite. In this circumstance the opening of the throttle fully by pressing hard on the accelerator pedal may result in the engine firing as more air is introduced creating a better mixture ratio.
  • Poor fuel economy - Rich mixture due to a faulty FPR allowing high pressures in the fuel line will cause excessively high fuel consumption

How to check? When the engine is at idle, disconnecting the vacuum line to a healthy FPR should result in a temporary surge in engine revs for a few seconds as the immediate loss of vacuum to the FPR causes an immediate build up in fuel pressure. At each injection cycle a little more is injected causing the engine to build up revs. In the next few seconds the gradual ingress of unmetered air into the plenum, also caused by the disconnected vacuum tube, begins to upset the stable idle of the engine. The engine will suffer from rough idle until the vacuum line is reconnected. Most of the injection inputs will be crudely governed by signals received from the MAP sensor and feedback from the oxygen sensor.

A technical check in FPR function is to connect a fuel pressure tester to the fuel rail. There is usually a threaded connection port on the fuel rail consisting of a Schrader type valve; more commonly seen on car tyres. Testing the fuel pressure not only shows whether or not the fuel pressure regulator is capable of maintaining a consistent pressure but it also checks the fuel pumps' capability of providing the necessary pressure. Pressure in the rail should be highest when the throttle is open and lowest at idle. Pressing the pin in the middle of the Schrader valve is a quick way of depressurizing the fuel rail, akin to letting the air out of a tyre.

How to Fix? Check for the presence of fuel from the open end of the disconnected vacuum line to help ascertain the health of the FPR diaphragm. If the FPR is faulty it is simple and cheap to replace it.

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hi . i am a mechanic,there is a mitsubishi l 200 pickup .it has a prableme ,not good runing from 3000rpm/minute,and came this code p0089 suction control valve stuck.what i can?please write me.

fuel pressure regulator
vacuum operated
on the Mitsubishi L200...vacuum operated
usually on the injector rail

The fuel pressure regulator is responsible for maintaining a steady fuel pressure.

Some vehicles have a fuel pressure built into the fuel rail assembly. Other vehicles with a returnless fuel supply,

the regulator is a part of the fuel pump module inside of the tank.


Oct 30, 2016 | Mitsubishi Cars & Trucks

1 Answer

where is fuel pump relay on 2009 gmc acadia

Your vehicle has no fuel pump relay . Has a fuel pump flow control module .
Fuel Pump Flow Control Module (FPCM)
The fuel pump flow control module (FPCM) is a serviceable GMLAN module. The FPCM receives the desired fuel pressure message from the engine control module (ECM) and controls the fuel pump located within the fuel tank to achieve the desired fuel pressure. The FPCM sends a PWM signal to the fuel pump, and pump speed is changed by varying the duty cycle of this signal.
Plus it has two fuel pumps !
The high pressure fuel pump (9) is a mechanical one cylinder design driven by an eccentric on the exhaust camshaft (1) of bank 2. High pressure fuel is regulated by the fuel rail pressure (FRP) regulator, which is a part of the high pressure fuel pump. The FRP regulator is a magnetic actuator which controls the inlet valve of the high pressure pump. The ECM provides battery voltage on the actuator high circuit and ground on the actuator low reference circuit. Both circuits are controlled by the ECM. When the ECM deactivates the FRP regulator, both circuits are disabled and the inlet valve is held open with spring pressure. When the ECM activates the FRP regulator, the low circuit driver connects the low reference circuit to ground and the high circuit driver pulse-width modulates (PWM) the high circuit. The ECM uses the camshaft and crankshaft position sensor inputs to synchronize the FRP regulator with the position of the eccentric on the camshaft. The ECM regulates fuel pressure by adjusting the portion of each pump stroke that provides fuel to the rail. The high pressure fuel pump also contains an integrated pressure relief valve.
Your best bet is have it towed to the dealer , you don't want to mess with the high pressure pump .

Feb 27, 2016 | 2009 GMC Acadia SLT-2

1 Answer

1992 325i bmw no fuel in fuel rail to start the car has a new fpr fuel filter fuel lines and fuel pump but no fuel is getting in the rail and rail is not cloged air flows through it freely

check for a vacuum in the fuel tank that is preventing the pump from moving fuel through the system. Air should not flow freely through the manifold as there is a regulator in the system that maintains the fuel pressure for the injectors. It should stay shut for a pressure of around 50 psi before it allows the air to flow out

Jan 21, 2014 | 1992 BMW 3 Series

1 Answer

1994 mazda check engine code 25 what does it mean?

Code 25 is for Fuel Pressure Regulator Control Solenoid (PRC).The FPR solenoid is connected to the fuel pressure regulator that's bolted to the fuel rails but a vac line. locate that vac line and follow it to the fpr solenoid. should look like any other solenoid. ----------- This should help.Thanks.Helpmech.

Mar 25, 2011 | 1994 Mazda MX-6

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