Monday, April 30, 2012

W124 Fuel Pressure Regulator



The Fuel Pressure Regulator (FPR) in any modern car is quite a critical component of the fuel delivery system.

The purpose of the FPR is to ensure a constant fuel pressure that is higher than the intake manifold pressure. In most natural aspirated (NA) engines, the intake manifold maximum possible pressure is 1 bar, which is the atmospheric pressure of almost all places on earth. To ensure proper operations of the injectors, the fuel pressure must be higher than 1 bar, else air will enter the injectors instead of fuel exiting the injectors.
Bosch's stock Fuel Pressure Regulator for W124

For W124s running the KE Jetronic fuel delivery system (ie 200e, 230e, 300e etc), the minimum expected pressure from the fuel delivery system is 3.5 bar in order for the injection valves (injectors) to work properly. As the injectors are the "old school" type, which does not have any electrical/electronic control of the pulsing between opening and closing of the valves for fuel spray, it relies solely on the pressure of the fuel line. According to the KE Jetronic manual from Bosch (page 17), it is mentioned that the injector will start pulsing between the opening and closing of its valve once the fuel pressure of 3.5 bar is attained. It is thus critical to ensure that such a minimal fuel pressure is attained after ignition.

Referring to one of the official testing procedures for the CISE (aka KE Jetronic) system, the minimum expected pressure for new and old injectors are slightly different, taking into consideration of the age of the injectors.

eg, For new injectors to start pulsing, 3.5 - 4.1 bar of fuel pressure is required.
But for non-new injectors, only 3.0 bar is required.
(It is perhaps sound advice to replace the FPR along with injectors if either one of them requires replacement to ensure that the fuel is atomized properly when entering the combustion chamber).

So how does the FPR fails and what are the symptoms?

Cross Section of a typical fuel pressure regulator
For any fuel pressure regulator, the weakest link will be its diaphragm.

The function of the diaphragm is to separate the fuel from entering the vacuum of the intake manifold. This intake manifold vacuum line is critical to ensure that constant pressure is maintained even at high loads by pulling the spring thus opening the valve wider. If the diaphragm breaks, fuel will leak into the intake manifold and will be burnt off but this will cause a leak in the pressure of the fuel line and may cause the injectors not to pulse as expected.

Here are some of the symptoms experienced on a failing FPR:

  • Unstable RPMs during partial or full load of the throttle
  • Repeated stalling of engine after ignition [ due to insufficient pressure for injector valves to pulse ]
  • Fuel pump producing loud whirring noise [ as it is unable to provide pressure due to leakage in the system ]
  • Unstable idling and stalling [ due to excess fuel causing fouling of spark plugs ]
  • Poor fuel consumption

It is important to note that as the rubber hose, for connecting to the intake manifold vacuum line, is built into the body of the stock W124 Bosch's FPR, delicate care must be taken when handling it. Any breakage of the short rubber hose may cause vacuum to leak and the pressure may not be sufficient to pull the spring during high loads and it may just cause pinging due to insufficient fuel in the combustion chamber.

When that happens, it will be due for replacement of the entire FPR unit, which costs more than SGD$ 250 and that excludes the labour charges required.


This article was inspired by a fellow Singaporean W124 owner enthusiast,
 whom experienced the breakage of the short rubber hose on his FPR
and had to replace the entire unit.