1. Cool the engine to a temperature that your hand can withstand.
2. Remove the air filter cross tube.
3. 4-cylinder engine: unscrew the high voltage cable guard on the cylinder head cover (5/1).
4. 6-cylinder engine: unscrew the protective cover of the high voltage wires (5/1) on the cylinder head cover. Remove the ignition coils with the connected control circuit wires and set them aside.
5. Loosen all spark plug connectors, then remove them. At the same time, pull only on the tips, and not on the wires.
6. If possible, carefully blow out the spark plug recesses with compressed air.
7. Remove the spark plugs (R4) using a suitable spark plug wrench or HAZET 2776 wrench and check the appearance of the plug socket. With some experience, you can draw conclusions about the operating condition of the engine from this. The following rules are valid.
Electrodes and insulators:
- medium gray color - normal combustion and normal operation of the spark plug.
- black color - too rich combustible mixture.
- light gray color - too lean combustible mixture.
- purple color - malfunction of the corresponding spark plug or bad sealing piston rings.
8. Check the gap between the electrodes with a feeler gauge. Set value: 0.8 mm.
9. To establish the gap between the contacts, bend the side electrode. To do this, tap the side electrode on the side. When bending, support the edge of the thread of the spark plug with a small screwdriver, but in no case the middle of the electrode, otherwise the electrode may be damaged.
10. Clean the threads on the spark plugs and in the cylinder head.
11. Screw in the spark plugs by hand until they are firmly seated on the cylinder head.
Attention! When inserting candles, do not twist them. You should pay attention to this, because in 4-valve engines, the plugs are deeply seated in their sockets. In view of this, it is recommended to use the corresponding HAZET 2776 spark plug wrench especially for Mercedes engines.
12. Tighten the spark plugs to 25 Nm.
Attention! Spark plugs that are too tight can break or damage the threads in the cylinder head when they are removed. In this case, the thread of the screw-in part of the candle can be restored using UTC-inserts (or Heli-Coil) .
13. Fit the spark plug connectors. Alternately moving back and forth, check the hard cap of the contact tips of the spark plugs and high voltage wires.
14. 6-cylinder engine: mount ignition coils with wire sockets.
15. Screw protective casings on a cover of heads of the block of cylinders. Install the air cleaner cross pipe and secure it with hose clamps.
Fuel quality matching
C-class engines generally require unleaded gasoline for high performance engines with an octane rating of 96-98 (according to ROZ 95). If the supply of this fuel is not guaranteed, e.g. abroad, it is also possible to temporarily fill up with unleaded occasional petrol (according to ROZ 91). The octane number, determined by ROZ, indicates the knock resistance of the fuel.
With 2.2 and 2.8 liter engines, thanks to the knock control system, an automatic adjustment to the corresponding fuel quality is carried out.
For engines with a volume of 1.8 and 2.0 liters, the adjustment is carried out by removing the equalizing plug. Due to this, the ignition switches to a different ignition characteristic. This new ignition characteristic allows the vehicle to be operated using unleaded gasoline for vehicles with moderately boosted engines, up to ROZ 91 octane rating.
1. Loosen the plastic screw securing the washer reservoir and lift the reservoir.
2. Remove the equalization plug (R16/7), which is located next to the engine control device (PMS).
3. If petrol for vehicles with high-powered engines, octane number 96-98 is used again, fit the equalizing plug.
Attention! When using gasoline for cars with moderately boosted engines, you can reduce power and increase fuel consumption. Try to avoid full throttle.
Gasoline with a higher octane rating, such as "super plus" (ROZ 98), can always be used without changing the equalization plug.