Note: Please also refer to the materials in the corresponding Section of the Chapter Engine electrical systems.
Caution! Replace spark plugs only when the engine is cold or slightly warm. If the spark plugs are removed when the engine is hot, the threads of the cylinder head, made of light metal, may be damaged.
1. Depending on the type, spark plugs are replaced every 96,000 km or 160,000 km. Before starting work, prepare all the necessary tools (some spark plug manufacturers supply special tools for measuring spark plug gaps):
- Spark plug wrench 16 mm, for example, HAZET 900AKF. Mercedes-Benz service stations use spark plug wrench 3122B for this.
- 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. Two spark plugs are used for each cylinder.
2. The best way to replace spark plugs is to first buy new spark plugs, adjust their gap, and replace each spark plug one by one. When buying spark plugs, it is very important to choose exactly those spark plugs that are intended for your car's engine. Information about them is provided in Specifications.
3. 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.
4. The spark plug gap is checked by inserting a gauge of the correct thickness between the electrodes at the end of the spark plug. The gap should be as specified in the Specifications. The wire should just touch each of the electrodes. If the gap is not correct, slightly bend the curved part of the electrode with a knurled adjuster until the correct gap is achieved.
Note: When adjusting the gap of a new spark plug, bend only the base of the ground electrode, not the end. If the ground electrode is offset from the center electrode, use a notched adjuster to align them. Check the porcelain insulator for cracks, indicating that the spark plug should be replaced.
5. Compare the removed candle with those shown in the photographs in Section Spark plugs, - this will give you useful information about the general condition of the engine.
6. Check the interelectrode distance with a feeler gauge. The required gap value is given in Specifications.
Note: New spark plugs usually have the correct gap setting.
[Article text obtained from the website: MercedesMan]
7. If the spark plug thread is damaged, it must be replaced. There is a suitable tool and repair kit for this, for example, BERU. Using a special drill, the old threaded hole can be drilled. The cylinder head does not need to be removed for this. After this, a new thread can be cut in the cylinder head and the spark plug can be screwed in using a special threaded insert. The threaded inserts sit securely and hermetically.
8. To set the distance between the contacts, bend the corresponding ground electrode. To do this, tap the side of the ground electrode. When bending, rest a small screwdriver on the edge of the thread. Do not lean on the central electrode under any circumstances, as it can be damaged.
9. 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 with your fingers until it stops, 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 Specifications.
Note: To prevent the spark plug from being screwed in incorrectly, place a small piece of rubber hose or plastic tube on the spark plug stem. The flexible tube will act as a universal joint, aligning the spark plug with the spark plug socket, and if the spark plug starts to become warped, the hose will slide on it, preventing the threads from being stripped.
10. Before connecting the high-voltage (HV) wire to the spark plug (models with separate coils) 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.
11. 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.
12. Repeat the above procedure for the remaining spark plugs in sequence to avoid incorrect connection of the spark plug wires.