Spark Plug Inspection and Replacement

PROCEDURE
1. Before starting work, prepare all required tools, which include a 16 mm spark plug socket and a set of feeler gauge blades. Some spark plug manufacturers supply special tools for measuring spark plug gap. Spark plugs used on BMW vehicles are listed in the Specifications.
2. The best way to replace spark plugs is to buy new plugs in advance, adjust their gap, and replace each plug one at a time. When buying plugs, it is very important to select the ones intended for your vehicle's engine. Information about them can be found in the Specifications of the Engine Electrical Equipment section.
3. With new plugs at hand, allow the engine to cool completely before starting to remove the old ones. During this time, inspect the new plugs for defects and check the spark plug gaps.
4. Check the spark plug gap by inserting a gauge of the required thickness between the electrodes at the tip of the plug. The gap must correspond to the value specified in the Specifications. The wire should only touch each of the electrodes. If the gap is not within specification, carefully bend the ground electrode with a notched adjusting tool until the required gap is obtained.

* When adjusting the gap of a new plug, bend only the base of the ground electrode, without touching its tip. If the ground electrode is offset relative to the center electrode, use the notched adjusting tool to align them. Check the porcelain insulator for cracks, which indicate that the plug must be replaced.

5. With the engine cold, remove the ignition coils. Refer to Ignition Coil Removal and Installation.
6. If possible, use compressed air to blow dirt and foreign particles from the area around the spark plugs. A regular bicycle pump can be used for this. This is done to prevent debris from falling into the cylinder after the plugs are removed.
7. Fit the spark plug socket onto the plug and remove the plug from the engine by turning it counterclockwise.
8. Compare the removed plug with the illustrations in the Engine Electrical Equipment section. This will give you useful information about the general condition of the engine.
a) Grey electrodes and insulator of medium density - correct plug operation and optimal air-fuel mixture.
b) Black electrodes and insulator - mixture too rich.
c) Light grey electrodes and insulator - mixture too lean.
d) Oil-fouled electrodes and insulator - plug misfiring or poor piston ring sealing (check compression).
9. Check the plug insulator for signs of current leakage. Current leakage appears as thin, irregular tracks on the insulator surface. A loose connection of the plug connector may be the cause of current leakage. In this case, the connector assembly must be replaced.
10. Apply a small amount of copper-based anti-seize compound to the thread of the new plug. Insert the plug into the cylinder head and screw it in by hand until it seats, then tighten it with the spark plug socket. If possible, use a torque wrench to tighten the plug to ensure it is installed correctly. The tightening torque for the plugs is listed in the Specifications.

* To prevent cross-threading the plug, slide a short piece of rubber hose or plastic tubing onto its terminal end. The flexible tube will work as a universal joint, allowing the plug to align with the plug bore. If the plug starts to tilt, the hose will slip on the plug, preventing the threads from being damaged.

11. Install the ignition coils and engine cover. Make sure the connectors are correctly connected and the ground connection is secure. Refer to Ignition Coil Removal and Installation.
12. Repeat the above procedure for the remaining spark plugs in sequence to avoid incorrect connection.