Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide

Description

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The valve lash (clearance) can be checked with a feeler gauge on a hot or cold engine. Be sure to consult the illustrations of valve placement; the clearance on intake and exhaust valves is different.

Valve lash is one factor which determines how far the intake and exhaust valves will open into the cylinder.

If the valve clearance is too large, part of the lift of the camshaft will be used up in removing the excessive clearance, thus the valves will not be opened far enough. This condition has two effects, the valve train components will emit a tapping noise as they take up the excessive clearance, and the engine will perform poorly, since the less the intake valves open, the smaller the amount of air/fuel mixture admitted to the cylinders will be. The less the exhaust valves open, the greater the back-pressure in the cylinder which prevents the proper air/fuel mixture from entering the cylinder.

If the valve clearance is too small, the intake and exhaust valves will not fully seat on the cylinder head when they close. When a valve seats on the cylinder head it does two things; it seals the combustion chamber so none of the gases in the cylinder can escape and it cools itself by transferring some of the heat it absorbed from the combustion process through the cylinder head and into the engine cooling system. Therefore, if the valve clearance is too small, the engine will run poorly (due to gases escaping from the combustion chamber), and the valves will overheat and warp (since they cannot transfer heat unless they are touching the seat in the cylinder head).

While all valve adjustments must be as accurate as possible, it is better to have the valve adjustment slightly loose than slightly tight, as burnt valves may result from overly tight adjustments.

ADJUSTMENT 4 and 6-Cylinder Gasoline Engines



See Figures 1, 2, 3 and 4

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Fig. Fig. 1: Location of the intake and exhaust valves4-cyl. gasoline engines



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Fig. Fig. 2: Location of the intake and exhaust valves6-cyl. gasoline engines



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Fig. Fig. 3: Insert a feeler gauge between the heel of the camshaft lobe and the sliding surface of the rocker arm (at the arrow)



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Fig. Fig. 4: Measure the valve clearance with a feeler gauge at the arrow4-cyl. engines (except 190)

The 190E has hydraulic valve clearance compensation. No adjustment is either possible or necessary.

The valve clearance is measured between the sliding surface of the rocker arm and the heel of the camshaft lobe. The highest point of the camshaft lobe should be at a 90- angle to the sliding surface of the rocker arm.

Prior to rotating the 6 cyl. engine manually, disconnect the transmitter ignition distributor (green) plug from the switching unit.

  1. Remove the air vent hose and air cleaner from the valve cover. Remove the spark plugs. This makes it easier to crank the engine by hand.
  2.  
  3. Remove the valve cover and gasket.
  4.  
  5. Note the position of the intake and exhaust valves.
  6.  
  7. Rotate the crankshaft with a socket wrench on the crankshaft pulley bolt until the heel of the camshaft lobe is perpendicular to the sliding surface of the rocker arm.
  8.  

Do not rotate the engine using the camshaft sprocket bolt. The strain will distort the timing chain tensioner rail. Always rotate the engine in the direction of normal rotation only.

  1. Some models have holes in the vibration damper plate to assist in crankshaft rotation. In this case, a screwdriver can be used to carefully rotate the crankshaft.
  2.  
  3. To measure the valve clearance, insert a feeler blade of the specified thickness between the heel of the camshaft lobe and the sliding surface of the rocker arm. The clearance is correct if the blade can be inserted and withdrawn with a very slight drag.
  4.  
  5. If adjustment is necessary, it can be done by turning the ball pin head at the hex collar. If the clearance is too small, increase it by turning the ball pin head in. If the clearance is too large, decrease it by turning the ball pin head out.
  6.  
  7. If the adjuster turns too easily or the proper clearance cannot be obtained, check the torque of the adjuster.
  8.  

This adjustment is ideally made with a special adaptor ("crow's foot") and a torque wrench. The shape of the adaptor is dictated by the need for accurate torque readings and, by using it, the torque wrench can be directly aligned with the ball pin head.

If the torque is less than 14.8 ft. lbs., the adjuster will vibrate and the clearance will not remain as set. If the valve clearance is too small, and the ball pin head cannot be screwed in far enough to correct it, a thinner pressure piece should be installed in the spring retainer. To replace the pressure piece, the rocker arm must be removed. Refer to Engine And Engine Rebuilding for this operation.

  1. After all the valves have been checked and adjusted, install the valve cover. Be sure that the gaskets are seated properly. It is best to use a new gasket whenever the valve cover is removed.
  2.  

Two types of triangular rubber gaskets are used on 6-cylinder DOHC engines, but only the later type with 3 notches are supplied for service.

  1. Install the spark plugs.
  2.  
  3. Reconnect the air vent line to the valve cover and install the air cleaner, if removed.
  4.  
  5. Run the engine and check for leaks at the rocker arm cover.
  6.  

V-8 Engines See Figures 5, 6 and 7

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Fig. Fig. 5: Location of the intake and exhaust valves6.9 V8 engines. Black arrows indicate exhaust valves



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Fig. Fig. 6: Location of the intake and exhaust valvesV8 engines (except 6.9). Black arrows indicate exhaust valves



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Fig. Fig. 7: On 1974-75 V8 models, measure the valve clearance at the arrow. 1976-84 models use hydraulic lifters which require no adjustment

1976 and later V-8 engines use hydraulic valve lifters that require no periodic adjustment.

Valve clearance is measured between the sliding surface of the rocker arm and the heel of the camshaft lobe. The highest point of the camshaft lobe should be at a 90- angle to the sliding surface of the rocker arm.

  1. Loosen the venting line and disconnect the regulating linkage. Remove the valve cover.
  2.  
  3. Disconnect the cable from the ignition coil.
  4.  
  5. Identify all of the valves as intake or exhaust.
  6.  
  7. Beginning with No. 1 cylinder, crank the engine with the starter to position the heel of the camshaft approximately over the sliding surface of the rocker arm.
  8.  
  9. Rotate the crankshaft by using a socket wrench on the crankshaft pulley bolt until the heel of the camshaft lobe is perpendicular to the sliding surface of the rocker arm.
  10.  

Do not rotate the engine using the camshaft sprocket bolt. The strain will distort the timing chain tensioner rail. Always rotate the engine in the direction of normal rotation only.

  1. Some models have holes in the vibration damper plate to assist in crankshaft rotation. In this case, a screwdriver can be used to carefully rotate the crankshaft.
  2.  
  3. To measure the valve clearance, insert a feeler blade of the specified thickness between the heel of the camshaft lobe and the sliding surface of the rocker arm. The clearance is correct if the blade can be inserted and withdrawn with a very slight drag.
  4.  
  5. If adjustment is necessary, it can be done by turning the ball pin head at the hex collar. If the clearance is too small, increase it by turning the ball pin head in. If the clearance is too large, decrease it by turning the ball pin head out.
  6.  

If the adjuster turns very easily or if the proper clearance cannot be obtained check the torque on the adjuster with a special adaptor ("crow's foot").

The shape of the adapter is dictated by the need for accurate torque readings and, by using it, the torque wrench can be directly aligned with the ball pin head.

  1. When the ball pin head is turned, if the torque is less than 14.4 ft. lbs., either the adjusting screw, the threaded bolt, or both will have to be replaced. If the valve clearance is too small, and the ball pin head cannot be screwed in far enough to correct it, a thinner pressure piece should be installed in the spring retainer. To replace the pressure piece, the rocker arm must be removed. See Engine And Engine Rebuilding for this operation.
  2.  
  3. Install the regulating linkage, valve cover gasket, and valve cover. Be sure the gasket is seated properly.
  4.  
  5. Connect the cable to the coil, the venting line and the regulating linkage. Run the engine and check for leaks at the valve cover.
  6.  

Diesel Engines See Figures 8, 9, 10 and 11

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Fig. Fig. 8: Location of the intake and exhaust valves 4-cyl. diesel engines



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Fig. Fig. 9: Location of the intake and exhaust valves 5-cyl. diesel engines



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Fig. Fig. 10: Measure the valve clearance with a feeler gauge at the arrowdiesel engines



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Fig. Fig. 11: 1. Camshaft 2. Cam follower 3. Feeler gauge 4. Adjusting nut To check the valve clearance, use a feeler gauge and adjust the hex nut until a slight drag is felt on the gauge

The 190D utilizes hydraulic valve clearance compensators. No adjustment is either necessary or possible.

  1. Remove the valve cover and note the position of the intake and exhaust valves.
  2.  
  3. Turn the engine with a socket and breaker bar on the crankshaft pulley or by using a remote starter, hooked to the battery (+) terminal and the large, uppermost starter solenoid terminal. Due to the extremely high compression pressures in the diesel engine, it will be considerably easier to use a remote starter. If a remote starter is not available, the engine can be bumped into position with the normal starter.
  4.  

Do not turn the engine backwards or use the camshaft sprocket bolt to rotate the engine.

  1. Measure the valve clearance when the heel of the camshaft lobe is directly over the sliding surface of the rocker arm. The lobe of the camshaft should be vertical to the surface of the rocker arm. The clearance is correct when the specified feeler gauge can be pulled out with a very slight drag.
  2.  
  3. To adjust the clearance, loosen the cap nut while holding the hex nut. Adjust the valve clearance by turning the hex nut.
  4.  
  5. After adjustment hold the cap nut and lock it in place with the nex nut. Recheck the clearance.
  6.  
  7. Check the gasket and install the rocker arm cover.
  8.  

 
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