Replacing spark plugs (Audi 100 C4)

Although spark plug replacement is recommended every 300 km, it is recommended to check at shorter intervals.

Removal spark plugs

Grasp the spark plug connector and pull it off the spark plug. Do not pull on the high-voltage wires.

Unscrew the spark plugs using a special key.

Arrange the spark plugs according to the firing order.

Tighten the spark plugs securely without using excessive force, otherwise the spark plug threads may damage the cylinder head.

Warm up the engine and now remove the spark plugs, being careful not to burn your hands.

When installing, do not screw cold spark plugs into the hot cylinder head too tightly, as they will then sit very tightly.



Spark plugs should be tightened to a torque of 2 Nm.

If you do not have a torque wrench, screw in the spark plug until the gasket is seated; it should not be screwed in further by hand or with a wrench without limiting the force.

Now tighten the new spark plug with a spark plug wrench by 90° (quarter turn).



A used spark plug, the gasket of which is already slightly flattened, should only be turned an additional 15°.

Checking spark plugs

Spark plugs are to some extent an indicator of the engine's condition. The appearance of the spark plug tip will help determine whether the engine is operating in optimal conditions. The car should be thoroughly warmed up on a country road or highway beforehand. Checking after a short drive can lead to errors. It is necessary to inspect the tip of the insulator together with the central and side electrodes:

• The tip of the insulator is painted from light gray to brown: good adjustment of the carburetor or injection system, the engine runs economically.
• Heavy deposits: May be caused by additives in engine oil or fuel, or increased oil consumption. It may be necessary to change the grade of oil or fuel.
• Black sooty deposits: spark plugs do not reach their self-cleaning temperature during frequent short trips, incorrect heat range, CO content is too high.
• The tip of the insulator is painted white: the ignition timing is too far in the advance direction, the ignition advance regulator is not working, the CO content is too low.
• Melted parts on the middle and side electrodes: glow ignition due to deposits in the combustion chamber, overheated valves, incorrect ignition timing, or overheating due to insufficient cooling.
• Fracture of the insulator end, which is determined in the initial stage as a very fine crack: detonation combustion due to low-octane fuel, incorrect ignition timing, insufficient engine cooling or too lean a mixture due to additional air.
• Yellowish shiny layer on the tip of the insulator: gasoline or motor oil additives have formed deposits that spread and become conductive under uneven engine load - as a result, ignition failures. After using the car for a long time only for short trips, the engine should not be immediately fully loaded.
• Oil layer on top of the electrodes and inside of the spark plug: damage to piston rings, valve guides or valve stem seals.
• If the spark plug electrodes and insulator do not show any peculiarities, but the engine starts with difficulty or runs jerkily, then the reason may also be in the spark plugs. Invisible cracks in the ceramic insulator when starting a cold engine can fill with condensed fuel, which is why the ignition spark does not pass. Also, under pressure, the spark plugs can fail, although the spark jumps when removed.

Interelectrode distances

The air-fuel mixture or burnt gases corrode the metal electrodes of the spark plugs. The high voltage when the spark jumps also chips away small particles of metal, causing the spark gap of the spark plug to increase over time.

For the engines described, spark plugs with different interelectrode distances are used.

If the distance is too great, higher voltage is required to create an ignition spark, and this can lead to misfires. The engine may not even start at all, so spark plugs with the same side electrode must be bent regularly. Precise bending is hardly possible for spark plugs with 3 side electrodes.

Selecting new spark plugs

Heat rating:the spark plug must match the temperature in the combustion chamber. This number indicates how much heat the spark plug can transfer, i.e. how much heat it can remove without overheating itself. If the spark plug removes too much heat, it does not reach its self-cleaning temperature and the electrodes of the spark plug become coated with carbon.

Electrodes: Spark plugs with either one side electrode or three ground electrodes are used.

Spark plug thread: on all spark plugs for the engines described, the thread length is 19 mm, the diameter is 14 mm.

Key size:the hexagon size for putting on the spark plug wrench can be 20.8 mm and 16 mm.

Sealing: all spark plugs use flat gaskets that do not require additional seals.

[The original publication in its entirety is posted on the website: AudiManual]