Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide

Electric Fuel Pump

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Do not confuse the electric fuel pump with the injection pump.

All Mercedes-Benz fuel injected engines are equipped with electric fuel pumps. The electric fuel pump is located underneath the rear floor panel. The fuel return line was also eliminated and a check ball installed in its place.

Two types of fuel pumps have been used. One, the large pump, has been replaced with a new small design which has a bypass system to prevent vapor lock.

REMOVAL & INSTALLATION



  1. Jack the left rear of the car and support it on jackstands. This will provide sufficient working clearance.
  2.  
  3. Remove and plug the intake, outlet, and bypass lines from the pump.
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  5. Disconnect the electrical leads.
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  7. Unbolt and remove the fuel pump and vibration pads.
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  9. Install the fuel pump in the reverse order of removal. Be sure that the electrical leads are connected to the proper terminals. The negative wire (brown) is connected to the negative terminal (brown plastic plate) and the positive wire (black/red) is connected to the positive terminal (red plastic plate). If the terminals are reversed, the pump will operate in the reverse direction of normal rotation and will deliver no fuel.
  10.  

TESTING Fuel Pump Pressure



Temporarily reduce the pressure in the ring line by unplugging the connection at the starting valve. Connect the terminals of the starting valve to the battery for approximately 20 seconds. Reconnect the starting valve.

  1. Remove the air filter and connect a pressure gauge at the branch connection at the ring line.
  2.  
  3. Run the engine and measure the pressure. It should be 26.5-29.5 psi.
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  5. Stop the engine. The fuel pressure may drop to 21 psi, after approximately 5 minutes. If the fuel pressure drops uniformly to 0, check the following points for leaks.
  6.  

Starting valveSwitch on ignition and disconnect hose at starting valve. If there is no drop in pressure, the valve leaks.

Pressure regulatorSwitch on the ignition and disconnect the fuel return hose, as soon as the fuel pump stops. If there is no drop in pressure, the regulator leaks.

Ball valve in delivery connection of fuel pumpSwitch on the ignition and disconnect the fuel hose in front of the ring line the moment the fuel pump stops. If there is no drop in pressure, replace the fuel pump.

Injection Valvesremove the kick panel and bridge terminals 1 and 3 on the relay shown. This will energize the fuel pump with the engine stopped, and ignition ON. Check the valves for leaks.

  1. Before removing the gauge, reduce the pressure in the ring line.
  2.  

The fuel pressure can be adjusted on the regulator with the adjusting screw. Adjust to 28 psi. If a slight turn of the screw shows no change of pressure replace the regulator.

ADJUSTMENTS Fuel Pressure



See Figure 1

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Fig. Fig. 1: To check the fuel pump pressure, disconnect the fuel return hose from the fuel distributor (arrow)

The fuel pressure can be adjusted on the regulator with the adjusting screw. Adjust to 28 psi. If a slight turn of the screw shows no change of pressure replace the regulator.

Before replacing the regulator or removing the pressure gauge, reduce the pressure in the ring line by unplugging the starting valve and connecting it to battery voltage for 20 seconds.

Regulating Shaft

Step on the accelerator pedal up to the kickdown. The regulating lever should rest against the fuel throttle stop of the valve.

Loosen the hex bolt and push the linkage up to the full throttle stop if adjustment is required.

Regulating Linkage See Figure 2

Click image to see an enlarged view

Fig. Fig. 2: Location of the adjusting screw on the fuel pressure regulatorBosch electronic fuel injected models

  1. Check the linkage for ease of operation.
  2.  
  3. Check that the throttle valve closes completely. Disconnect the regulating rods.
  4.  
  5. Adjust the pushrod to a length of 4 in. from the center of the rubber mount to the center of the ball socket and attach.
  6.  

On vehicles with manual transmission:

  1. Adjust the connecting rod to a length of 3.5 in. (4.1 in. for cars with gate shift lever with bore next to ball head) and attach it.
  2.  
  3. Push the throttle lever against the idle speed stop. Adjust the connecting rod so that the regulating lever rests with the roller against the end stop of the gate shift lever.
  4.  

On cars with automatic transmission and no gate shift lever:

  1. Adjust the connecting rod (31) to 4.2 in. and the connecting rod (11) to 2.7 in. and attach.
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  3. Push the control rod to the rear against the stop and attach it tension free. During adjustment of the control pushrod, the ball socket must be held next to the ball head.
  4.  

On cars with automatic transmission and gate shift lever:

  1. Adjust the connecting rod to 4.2 in. and attach.
  2.  
  3. Push the throttle valve lever against the idle speed stop. Adjust the connecting rod so that the regulating lever rests with the roller against the end stop of the gate lever. Push the regulating lever to the rear against the stop pin.
  4.  
  5. Push the control pushrod against the stop and connect it tension free. The ball socket must be held next to the ball head.
  6.  

 
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