BMW Coupes and Sedans 1970-1988 Repair Guide

Mixture Control Unit and Sensor Plate

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REMOVAL & INSTALLATION



  1. Disconnect the large intake pipe (made of rubber) from the unit by loosening the clamp(s) and pulling it off.
  2.  
  3. Loosen the 3 screws in the top of the fuel distributor. Open the clips holding the fuel line to the control unit, and remove the clamp linking the 4 fuel lines together.
  4.  
  5. Lift the distributor off the control unit, using tape to hold the piston up inside the unit, move it aside.
  6.  
  7. Disconnect the electrical plug at the air horn, and the small vacuum hose and large air line connected to the vacuum regulator, mounted near the rear of the mixture control unit.
  8.  
  9. Loosen the 2 mounting nuts located on the wheel well side of the unit and lift it out.
  10.  
  11. Remove the bolts in the flange holding the upper and lower sections of the unit together. Then remove the 6 bolts (3 located inside the lower housing) which retain the air cleaner housing to the mixture control unit.
  12.  

To install:
  1. Replace the mixture control unit lower and upper housings together with an entire new unit. The new unit will have to be split (top and bottom sections separate at the flange). This will permit the air cleaner bolts to be attached from inside. Use a new gasket, and reassemble the upper and lower sections of the new mixture control unit after the air cleaner housing is attached; then, reverse the remaining procedures to install it, using a new seal under the fuel distributor.
  2.  

ADJUSTMENT



See Figure 1

49 state and California control units are not interchangeable.

  1. Remove the air intake cowl at the mixture control unit and throttle housing.
  2.  
  3. Turn the ignition ON for approximately 5 seconds, and during this time, slowly raise the sensor plate with a magnet. Turn the ignition switch OFF .
  4.  

The amount of resistance should be constant when raising the sensor and no resistance should be felt when pushing the sensor plate down quickly.

  1. The sensor plate should be flush to 0.02 inches (0.5mm) below the beginning oft he venturi taper. If adjustment is necessary, remove the mixture control from the intermediate housing and bend the spring accordingly. Center the sensor plate in the bore by loosening the center plate screw. Tighten when aligned.
  2.  



Click image to see an enlarged view

Fig. Fig. 1: Cross-sectional view of valve and opening. Dimension "A" represents the opening

With the sensor plate too high, the engine will run on and with the sensor plate too low, poor cold and warm engine start-up will result.

  1. If the sensor plate movement is erratic, the control piston can be sticking. Remove the fuel distributor and inspect the control piston for damage and replace as necessary.
  2.  


WARNING
Do not drop the control valve.

SYSTEM PRESSURE TESTING



Install a shut-off valve and an oil pressure gauge between the control pressure line and the fuel distributor, with the pressure gauge next to the fuel distributor.

Cold Engine Pressure Test
  1. Disconnect the terminal plug at the mixture control unit to avoid excessive heat.
  2.  
  3. Open the valve for oil flow and turn ON the ignition switch to operate the fuel pump, but do not start the engine.
  4.  
  5. Control pressures is dependent upon the engine coolant temperature. At a temperature of 50°F (10°C), oil pressure should be 10-11 psi (68-75 kPa) and at 77°F (25°C), oil pressure should be 22.0 psi (150 kPa). At coolant temperature of 104°F (40°C), the pressure should be over 29.4 psi (200 kPa).
  6.  

If the oil pressure is too low, the warm-up regulator may be defective. If the oil pressure is too high, the fuel return flow may be insufficient or the warm-up regulator may be defective.

  1. When complete, turn the ignition OFF .
  2.  

Warm Engine Control Pressure
  1. Open the shut-off valve for oil flow. Disconnect the mixture control terminal plug and turn the ignition ON to start the fuel pump. Do not start the engine.
  2.  
  3. The control pressure should be 48-54 psi (326-367 kPa) after 3 minutes, with the engine coolant at normal operating temperature. If the control pressure does not rise, check the wire plug terminal for current at the warm-up regulator. If current is present, the heating coil may be defective and would necessitate the replacement of the warm-up regulator.
  4.  

Testing With Engine Running
  1. Connect the wire plugs to the auxiliary air regulator and to the mixture control unit. Install the air intake cowl.
  2.  
  3. With the engine running at idle speed and at normal operating temperature, the control pressure should be 48-54 psi (326-367 kPa).
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Cold or Warm Engine System Pressure
  1. Close the pressure shut-off valve with the engine stopped and disconnect the mixture control unit terminal plug.
  2.  
  3. Turn the ignition ON , but do not start the engine.
  4.  
  5. The speed control pressure should be 64-74 psi (435-503 kPa).
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  7. Turn the ignition OFF and if the pressure is not within specifications, one of the following defects may be the cause:
    1. Leakage at the fuel lines or connections.
    2.  
    3. Fuel filter clogged.
    4.  
    5. Engine overruns.
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    7. Defective fuel pump.
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    9. Pressure adjustment incorrect.
    10.  

  8.  
  9. If the pressure is too high:
    1. Fuel return flow is restricted.
    2.  
    3. Incorrect pressure regulator setting.
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    5. Control piston stuck.
    6.  

  10.  

Shims may be used to change the pressure. Shim thickness changes will vary the pressure as follows:



0.01mm-0.85 psi (6 kPa)
 
0.50mm-4.3 psi (29.2 kPa)
 

  1. The transfer valve of pressure regulator must open at 50-57 psi (340-388 kPa).
  2.  

Cut-off Pressure Testing

This is a good test for checking if there is any system leaking.

  1. Open the pressure shut-off valve and turn the ignition ON .
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  3. Disconnect the wire plug at the mixture control unit and then reconnect the plug. Turn the ignition OFF .
  4.  
  5. Cut-off pressure must not drop below 24 psi (163 kPa) after several minutes.
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  7. If the pressure drops too early, one of the following may be leaking.
    1. Pressure regulator O-ring.
    2.  
    3. Warm-up regulator or supply line.
    4.  
    5. Fuel pump check valve.
    6.  
    7. Pressure reservoir.
    8.  

  8.  
  9. Remove the pressure gauge and shut-off valve and reconnect the pressure line.
  10.  

 
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