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Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide

Fuel Injection

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ADJUSTMENTS



Gasoline Engines
1974-75 MODELS

Adjustment should be made with the air conditioner off and transmission in park.

  1. Run the engine to normal operating temperature. The idle speed should not be adjusted immediately after hard driving for extended periods of time or when extremely hot. Be sure the cruise control cable is attached free of tension.
  2.  
  3. Remove the air cleaner.
  4.  
  5. Disconnect the connecting rod from the valve connection and check to be sure that the throttle valve closes completely without binding.
  6.  
  7. Reattach the connecting rod so that it does not bind.
  8.  
  9. Connect a tachometer and adjust the idle speed to specifications with the idle speed air screw.
  10.  
  11. Check the exhaust gas content with a Carbon monoxide (CO) meter. On 1975 cars, check the CO without air injection. Pull the plug from the oil temperature switch on the right-hand wheel well and ground it to cancel air injection. If necessary, adjust the CO content with the adjusting screw on the control unit. Turning the screw clockwise will give a richer mixture, while turning the screw counterclockwise will give a leaner mixture.
  12.  
  13. On 450SL and 450SLC, the control unit can be reached after removing the inner lining below the glove-box. On others, the control unit adjusting screw is behind a piece of trim on the right front kick panel.
  14.  
  15. Check and, if necessary, readjust the idle speed.
  16.  
  17. Install the air cleaner. Check the idle speed and exhaust emissions values and readjust if necessary.
  18.  
  19. Remove the tachometer.
  20.  
  21. Adjust the regulating linkage (on cars with a gate lever) so that the roller in the gate lever rests free in the gate. Move the transmission lever to Drive and switch on the A/C. Move the power steering to full lock. Adjust the speed so the engine runs smoothly.
  22.  

1976-79 MODELS

The mechanical fuel injection is the airflow controlled type known as Bosch K-Jetronicreg;. The idle speed should be adjusted with the air conditioner off and the transmission in Park.

  1. Connect a tachometer.
  2.  
  3. Be sure the cruise control cable is connected to the regulating lever with no binding or kinking.
  4.  
  5. Run the engine to normal operating temperature.
  6.  
  7. Be sure the throttle valve rests against the idle speed stop.
  8.  
  9. Set the idle speed to specifications with the idle air screw.
  10.  
  11. If possible, check the Carbon monoxide (CO) level.
  12.  

All 1976 enginesCheck CO without air injection. Disconnect the blue/purple vacuum line at the blue thermal valve and plug the opening at the thermal valve to stop air injection.

1977-79 modelsDisconnect the hose at the exhaust back pressure line and connect the CO tester to the line. On Federal V-8 engines, check CO with air injection connected. On all others, disconnect the blue/purple vacuum line from the blue thermal valve and plug the thermo valve to cancel air injection.

  1. Adjust the CO valve by unscrewing the plug and inserting the special adjusting tool (allen wrench). Turn the screw in to richen the mixture and out to lean the mixture.
  2.  
  3. Accelerate briefly and check the speed and CO again.
  4.  
  5. Reconnect the vacuum lines and check the CO value again. It should be below the specified value.
  6.  
  7. Adjust the regulating lever so that the roller rests in the gate lever without binding. Put the transmission in Drive and turn on the A/C. Turn the wheels to full lock and adjust the idle speed so the engine runs smoothly.
  8.  

1980 6-CYLINDER AND V8 MODELS
  1. Connect a tachometer and remote oil temperature gauge.
  2.  
  3. Run the engine to approximately 176-F oil temperature.
  4.  
  5. The automatic transmission should be in Park and the A/C off.
  6.  
  7. Be sure the throttle valve lever rests against the idle stop.
  8.  
  9. Adjust the Bowden cable with the adjusting screw so there is no tension against the throttle valve lever.
  10.  
  11. Check the idle speed. If necessary, adjust with the idle air adjustment screw.
  12.  

1981 6-CYLINDER MODELS
  1. Run the engine to normal operating temperature (167--185- oil temperature) and connect a tachometer.
  2.  
  3. The automatic transmission should be in Park and A/C off.
  4.  
  5. Be sure the throttle valve lever rests against the idle speed stop.
  6.  
  7. Be sure the cruise control actuating rod rests against the idle speed stop. Disconnect the connecting rod and push the lever of the actuating lever clockwise to the idle speed position.
  8.  
  9. Reconnect the connecting rod; make sure that the actuating element is approximately 0.04 inch from the idle speed stop. Adjust this clearance with the pull rod.
  10.  
  11. Check and adjust the idle speed. Idle speed is adjusted at the idle speed air screw.
  12.  

1981-84 MODELS

These engines have electronically controlled idle speed, using a solenoid connected to terminals 1 and 5 of the control unit.

Diesel Engines

1973-80 MODELS (EXCEPT 1980 300 SD)

See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Make sure the special spring is installed as illustrated

Since the diesel engine has no ignition distributor or ignition coil there is no way to connect an external tachometer to measure idle speed. While using the built-in tachometer on the dash is not the most accurate way, the only other possibility is to set the idle speed by ear.

  1. On models before 1977, turn the knob on the instrument panel completely clockwise. Turn it again counterclockwise. The travel before the idle speed is raised should not exceed about 1 /2 turn. If required, adjust the travel with the nut.
  2.  
  3. On 1977-80 models, turn the knob on the instrument panel completely clockwise and check the distance between the adjusting ring and the specially shaped spring. It should be approximately 0.04 inch.
  4.  
  5. With the engine stopped, depress the accelerator pedal while turning the idle knob counterclockwise.
  6.  
  7. Start the engine. The idle should be 1,000-1,100 rpm. Adjust this with the adjusting screw, but do not exceed 1,100 rpm.
  8.  
  9. On 1977-80 models, be sure the special spring is installed correctly.
  10.  
  11. Run the engine to operating temperature.
  12.  
  13. Turn the idle adjusting knob on the dash fully to the right.
  14.  
  15. Disconnect the regulating rod and adjust the idle speed with the idle speed adjusting screw. 1977-80 models have a locknut on the idle speed adjusting screw.
  16.  
  17. Reconnect the regulating rod.
  18.  

1980 300SD AND 1981-84 DIESELS EXCEPT 1982-83 240D AND 1984 190D MODELS
  1. Run the engine to normal operating temperature.
  2.  
  3. On normally aspirated engines, turn the idle speed adjuster on the dash completely to the right.
  4.  
  5. Disconnect the pushrod at the angle lever.
  6.  
  7. Check the idle speed. Adjust by loosening the locknut and adjusting the idle speed screw. Tighten the locknut.
  8.  
  9. On all except Turbodiesels, adjust the pushrod so that a clearance of approximately 0.2 inch exists between the cam on the lever and the actuator on the switchover valve. The lever on the fuel injection pump must rest against the idle stop.
  10.  
  11. On all models except the 1981 turbodiesel, depress the stop lever as far as possible. The cruise control Bowden cable should be free of tension against the angle lever. Use the adjusting screw to alter the tension. Let go of the stop lever. The Bowden cable should have a slight amount of play.
  12.  
  13. On turbodiesels, adjust the pushrod so that the roller in the guide lever rests free of tension against the stop.
  14.  
  15. Put the automatic transmission in Drive and turn the steering wheel to full lock. The engine should run smoothly. If not, adjust the idle speed. Disconnect the cruise control connecting rod, and push the lever clockwise to the idle stop. Attach the connecting rod, making sure the lever is about 0.04 inch from the idle speed stop.
  16.  


WARNING
If the engine speed is adjusted higher, it will be above the controlled idle speed range of the governor and the engine can increase in speed to maximum rpm.

1982-83 240D MODELS
  1. Run the engine to normal operating temperature.
  2.  
  3. Turn the idle speed adjuster knob on the dashboard completely to the right.
  4.  
  5. Disconnect the pushrod at the operating lever.
  6.  
  7. Move the guide lever to the idle speed position. Set the edge of the guide lever at the mark (arrow) on the cap.
  8.  
  9. Check, and if necessary, adjust the idle speed. Use the idle speed adjusting screw.
  10.  
  11. Attach the pushrod to the injection pump lever so that the rod is free of tension when the lever is against the idle speed stop.
  12.  
  13. Check to be sure the cruise control rods are free of tension.
  14.  
  15. Move the automatic transmission into Drive. Turn on the A/C and turn the wheels to full lock. The engine should run smoothly. Adjust the idle speed if necessary.
  16.  

1984 190D MODELS

See Figure 2

Click image to see an enlarged view

Fig. Fig. 2: Digital tester (001) and TDC impulse transmitter (021) hook-up on the 190D

Testing the idle speed on the 190D will require two special tools. A digital tester (Sun-1019, 2110 or All-Test 3610-MB) and a TDC impulse transmitter; not commonly available tools. Without these two special tools, idle speed adjustment is impossible and should not be attempted.

  1. Run the engine until it reaches normal operating temperature.
  2.  
  3. Connect the digital tester and the TDC impulse transmitter as indicated in the illustration.
  4.  
  5. Check all linkages for ease of operation.
  6.  
  7. Disconnect the pushrod from the adjusting lever.
  8.  
  9. Start the engine and check the idle speed. If required, adjust by loosening the locknut on the vacuum control unit and turning the unit itself in or out.
  10.  
  11. After the idle speed is correct, tighten the vacuum control unit locknut and reconnect the pushrod so that it is tension free when the lever is against the idle speed stop.
  12.  
  13. Switch on all auxiliary power accessories and check that the engine continues to run smoothly. Readjust the idle speed if necessary.
  14.  
  15. Disconnect the two special tools and turn off the engine.
  16.  

See Figures 3, 4 and 5

Click image to see an enlarged view

Fig. Fig. 3: Troubleshooting Engine Performance



Click image to see an enlarged view

Fig. Fig. 4: Troubleshooting Engine Performance



Click image to see an enlarged view

Fig. Fig. 5: Troubleshooting Engine Performance

 
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