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300e Fuel Pressure


300e Fuel Pressure

problem I know I posted here several times before...I have replaced many parts to this car and still cant figure out why the car keeps flooding out on me. I am 100% sure it is flooding out after a few minutes of running. The car is fine on a cold start but cuts out after a few minutes. When the OVP relay is DCd the car runs o.k. when warmed up. I installed a new fuel pump relay, fuel pump check valves, fuel accumulator, fuel pressure regulator, the OVP relay is pretty new, cap , totor, plugs, wires, o2 sensor, catalyitc converter... The old cat had 2 precats on it, this one only has the one cat in the middle, doe sthis make any difference? I appreciate all the help, I am on a fixed income in school and need this car to run....-chuck, nj

The fuel pressure (= mixture) is primarily controlled by the fuel distributor. It works in a way that the air mass sensor (the moving plate under air filter) controls the position of a plunger inside the fuel distributor, which again determines the fuel pressure to the injectors. The mixture is adjusted by the screw inside the tube, which leads to the top of the air filter.

The pressure is then fine adjusted by the fuel pressure regulator (EHA) through the incection computer.

If overvoltage occurs, the fuse in the OVP relay will blow and thereby disconnect the power supply to the injection and ABS computers.

When the injection computer has no power, the injection system works without fine adjustment of the pressure. Taking out OVP relay will simply remove power supply likewise.

Since the engine runs fine without the OVP relay, you can conclude that the basic system works and has almost correct mixture.

Since you see a difference with the OVP relay in place, you can also conclude that something is changing the pressure. That has to be the injection computer, which again means that it does get power supply through the OVP relay (which is then not the fault).

But apparently it gradually increases the pressure until the engine finally drowns. This indicates to me that the computer does not get correct feedback from the oxygen sensor (it should indicate too rich mixture, whereby the computer should reduce the fuel pressure). So check again the oxygen sensor and its connections.

Other inputs to the computer are:

  • coolant temperature sensor
  • EGR temperature sensor (if fitted)
  • idle switch
  • start signal (crank)
  • speed
  • rpm
  • airflow (potentiometer mesuring the sensor plate position)
  • altitude correction (if fitted)
  • reference resistor (under sealed cap)

All these inputs are used to preset the fuel pressure fine adjustment. But at the end of the day the oxygen sensor will measure whether the combustion is correct and make the computer fine adjust accordingly.

Therefor the major suspect is the oxygen sensor measuring incorrectly or not being connected correctly. Alternatively it could be the fuel pressure regulator not acting correctly according to its control signals. Or the computer is failing.

Jens, Denmark