Tip & How-To about Mercedes-Benz E-Class
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:-
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.
NEXT 2.2) ECT - Engine Coolant temperature sensor
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