Checking and replacing spark plugs, condition of high-voltage wires (Mercedes-Benz C-Class W203)

Please also refer to Section Checking and adjusting the ignition timing angle.

Warning! Warning! Replace spark plugs only on a cold or slightly warm engine. If the spark plugs are removed on a hot engine, the threads of the cylinder head, made of light metal, may be damaged.

Before you begin, prepare all the necessary tools. Some spark plug manufacturers supply special tools for measuring the spark plug gap.

Engines 111, 271

• Spark plug wrench 16 mm, for example, HAZET-2506 or -2776 (engines 111, 271) or HAZET-4766 (engine 112).
• Additionally, it is recommended to purchase special pliers, for example, HAZET 1849, which make it easier to unscrew the spark plugs. MERCEDES-BENZ uses the 3277A assembly key for this.
• Depending on the number of engine cylinders, the required number of spark plugs. The spark plugs used are listed in the Specifications.

Remove the spark plugs.





1. The best way to replace spark plugs is to buy new ones in advance, adjust their gap, and replace each spark plug one by one. When buying spark plugs, it is very important to choose the ones that are intended for your car's engine. Information about them is provided in the Specifications.

2. With new spark plugs on hand, allow the engine to cool completely before removing the old ones. During this time, you can inspect the new spark plugs for defects and check the spark plug gaps.

Warning! Do not touch high voltage parts. Persons with a pacemaker should not carry out any work on the electronic ignition system.

3. Remove the ignition coils, refer to Section Removal and installation ignition coils.


4. Remove the spark plugs using a spark plug wrench (1). Make sure that the wrench is straight and not crooked.

Examination

1. Compare the removed candle with those shown in the pictures in Chapter Engine electrical equipment, this will give you useful information about the general condition of the engine.

2. Check the interelectrode gap with a feeler gauge. The required gap value is given in the Specifications.

Note: New spark plugs usually have the correct gap setting.

3. To set the distance between the contacts, bend the corresponding ground electrode. To do this, tap it from the side. When bending, rest a small screwdriver on the edge of the thread. Never lean on the central electrode, as it can be damaged.

4. Apply a small amount of copper anti-seize compound to the threads of the new spark plug. Install the spark plug into the cylinder head and screw it in finger tight, then tighten it with the socket of a spark plug wrench. If possible, use a torque wrench to tighten the spark plug to ensure that it is installed correctly. The tightening torque for the spark plugs is included in the Specifications.

Recommendations: To avoid the possibility of screwing the spark plug in incorrectly, place a small piece of rubber hose or plastic tube on its tailpiece. The flexible tube will act as a universal joint, ensuring alignment of the spark plug with the spark plug socket, and if the spark plug starts to warp, the hose will slide on it, preventing the thread from being stripped.

5. Before connecting the high-voltage (HV) wire to the spark plug, carefully inspect it for damage, corrosion of the internal contact and measure the resistance. If it significantly exceeds 30 kOhm/meter, replace the entire set of wires.



6. Connect the wire to the installed new spark plug, again using a twisting motion and holding the connector until it is fully seated on the spark plug.

7. Repeat the above procedure for the remaining spark plugs in sequence to avoid incorrect connection of the spark plug wires.

Installation

1. Insert the spark plug and screw it into the cylinder head until it stops. Then tighten the spark plug to the torque specified in the Specifications. Keep the spark plug wrench (1) straight (A) and do not tilt it (B).

2. Reinstall the ignition coils, refer to Section Removal and installation ignition coils.

Repair of damaged spark plug threads

If it turns out that the spark plug thread in the cylinder head is damaged, it must be repaired. For this, there is a suitable tool and repair kit, for example, from BERU. The damaged thread is drilled out with a special drill. It is not necessary to remove the cylinder head for this. Then a new thread is cut in the head and the spark plug is screwed into a special transition threaded insert. Additionally installed spark plug threaded inserts sit firmly and ensure tightness.

Engine 112

Only differences from 4-cylinder engines 111, 271 are described.

Removal

Twin ignition coils for two spark plugs a and b of one cylinder



1. Remove the cover from the end of the engine.

2. Remove the upper part of the air filter housing (only for engines with air filter located on the side of the vehicle).

3. Remove the air mass meter with the intake pipe (only for engines with air filter located on the side of the vehicle).

4. Remove the engine cover or air filter, refer to Section Removal and installation the air filter.

5. Disconnect the ignition coils (1).

6. Disconnect the spark plug connector using a double-ended spark plug wrench above the cylinder head cover.

7. Remove the spark plugs.

Installation

1. Screw the spark plugs into the cylinder head by hand until they stop, then tighten to a torque of 28 Nm.

Attention! Each cylinder has two spark plugs. To avoid mixing up the plugs, they must be marked in the same way as the cover on the engine (2).

2. Connect the plugs to the spark plugs.

3. Install the ignition coils and tighten to a torque of 8 Nm.



4. Install the engine cover or air filter, refer to Section Removal and installation the air filter.

5. Install the air mass meter with the pipe (only for engines with air filter located on the side of the vehicle).

6. Install the upper part of the air filter housing (only for engines with air filter located on the side of the vehicle).

7. Secure the cover to the end of the engine.