BMW Cars 1999-06

Spark Plugs



  1. Inspect the spark plug for the following:

    Deposit build-up that is closing the gap between the electrodes. Deposits are caused by oil or carbon fouling. Clean the spark plug..
    Check for wet, black deposits on the insulator shell bore electrodes, caused by excessive oil entering the combustion chamber through worn rings and pistons, excessive valve-to-guide clearance or worn or loose bearings. Correct the oil leak concern. Install a new spark plug.
    Look for black, dry, fluffy carbon deposits on the insulator tips, exposed shell surfaces and electrodes, caused by a spark plug with an incorrect heat range, dirty air cleaner, too rich a fuel mixture or excessive idling. Install new spark plugs.
    Normal burning.
    Light tan or gray deposits on the firing tip.
    Pre-ignition. This identified by melted electrodes and a possibly damaged insulator. Metallic deposits on the insulator indicate engine damage. This may be caused by incorrect ignition timing, wrong type of fuel or the unauthorized installation of a heli-coil insert in place of the spark plug threads. Install a new spark plug.
    Overheating. This is identified by a white or light gray spots and with bluish-burnt appearance of electrodes. This is caused by engine overheating, wrong type of fuel, loose spark plugs, spark plugs with an incorrect heat range, low fuel pump pressure or incorrect ignition timing. Install a new spark plug.
    Fused deposits. These are identified by melted or spotty deposits resembling bubbles or blisters. These are caused by sudden acceleration. Install a new spark plug.


Inspection & Gapping

Check the plugs for deposits and wear. Look carefully at the center electrode protrudes through the center of the porcelain. If the center electrode is eroded or rounded, replace the spark plugs. If the plugs are not going to be replaced, clean the plugs thoroughly. Remember that any kind of deposit will decrease the efficiency of the plug. Plugs can be cleaned on a spark plug-cleaning machine, which are sometimes found in service stations. These machines do a good job of cleaning the spark plug, although they tend to remove the protective anti-corrosive coating on the spark plug threads. They also cause the surface of porcelain around the center electrode to become slightly porous, allowing deposits to bond more easily to the porcelain. If a plug cleaner is used to clean the plugs, be sure the plug is thoroughly cleaned. The abrasive material used in the spark plug cleaners is very hard, and if allowed to enter the engine's combustion chamber, could cause internal damage. An acceptable job of cleaning the spark plug can be accomplished by using a stiff wire brush. Once the plugs are cleaned, the spark plug gap must be checked and reset to specification.

If the original equipment spark plugs are platinum tip spark plugs, or have more than one electrode, the plug gap must not be adjusted.

Always check the spark plug gap before installation. Using a suitable spark plug gap gauge, check the spark plug gap. Make sure the L-shaped electrode connected to the body of the spark plug is parallel to the center electrode. If necessary, adjust the L-shaped electrode to attain the correct gap and proper alignment. Make sure to use the specified size wire gauge, which must pass between the electrodes with a slight drag; the next larger size should not be able to pass, while the next smaller size should pass freely. When adjusting a spark plug gap, always set the gap to the minimum specification to allow for electrode wear.

Always check the gap on new plugs as they are not always set correctly at the factory. Do not use a flat feeler gauge when measuring the gap because the reading may be inaccurate. A round-wire type gapping tool is the best tool for checking the plug gap. The correct gauge should pass through the electrode gap with a slight drag. If in doubt, try one size smaller and one larger. The smaller gauge should go through easily, while the larger one shouldn't go through at all. Wire gapping tools usually have a bending tool attached. Use this to adjust the side electrode until the proper clearance is obtained. Never attempt to bend the center electrode. Be careful not to bend the side electrode too far or too often as it may weaken and break off inside the engine, requiring removal of the cylinder head to retrieve it.

The recommended plug gap for original equipment spark plugs for spark plugs where gapping is permitted is as follows:

M3 models: 0.024-0.028 inches (0.6-0.7mm)
E30 and E36 models (except M3 and multiple electrode spark plugs): 0.028-0.032 inches (0.7-0.8mm)
Multiple electrode spark plugs: 0.032-0.035 inches (0.9-1.0mm)

Multiple electrode spark plugs with three or four electrodes can be checked, but should not be adjusted.

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Fig. Inspect the spark plug to determine engine running conditions

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Fig. A variety of tools and gauges are needed for spark plug service

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Fig. Checking the spark plug gap with a feeler gauge

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Fig. Adjusting the spark plug gap

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Fig. If the standard plug is in good condition, the electrode may be filed flat-WARNING: do not file platinum plugs

Removal & Installation

  1. Before servicing the vehicle, refer to the Precautions section.
  3. Switch off ignition.
  5. Disconnect the negative battery cable.
  7. If equipped, removal engine cover.
  9. Remove ignition coils.
  11. Unscrew spark plugs with special tool 12 1 171.
  13. To install, reverse removal procedure. Tighten spark plugs to 18 ft. lbs. (24 Nm).

Spark Plug Heat Range

The spark plug heat range is the ability of the plug to dissipate heat. The deeper the insulator recedes into the body of the spark plug, the more heat the spark plug will retain and the hotter the plug will operate. If the amount the insulator recedes into the body of the spark plug is decreased, the less the plug will retain heat, and the cooler it will operate. A plug that absorbs little heat and remains too cool will quickly accumulate deposits of oil and carbon since it is not hot enough to burn them off. This causes carbon to build up on the porcelain insulator and the center electrode, creating an alternate path for the high energy electrical spark which eventually leads to spark plug fouling and consequently to misfiring.

A spark plug that absorbs too much heat will burn off deposits, however, due to the increased combustion temperature, the electrodes may also burn away more quickly and the excessive heat may cause pre-ignition or internal engine damage. Pre-ignition, also referred to as detonation, takes place when the combustion chamber gets hot enough to ignite the air/fuel mixture before the actual spark occurs. This early ignition may cause a "pinging", knocking or rattling noise during low speed acceleration or when operated under a heavy load condition, such as climbing a steep hill. Note that detonation can occur without being heard.

On engines equipped with Knock Sensors (KS), the Engine Control Module (ECM) will retard the ignition timing to prevent internal engine damage when detonation is detected. This will allow the engine to operate safely, however performance will be compromised.

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Fig. Spark plug heat range

The general rule of thumb for choosing the correct heat range when selecting a spark plug is: consult the vehicle's owner's manual or a spark plug manufacturer's supply catalog for recommendations. If only one heat range of spark plug is listed, use the recommended plug, and note the recommended spark plug gap. If more than one heat range of spark plug is listed, depending on operating conditions, evaluate the type of driving the vehicle is most often subjected to. If the vehicle is used for extended high speed, long distance travel for long periods of time, in warm weather, use the colder plug of the recommend spark plugs. If, however, most of the driving is stop and go, or the vehicle is operated in extremely cold climates, use the hotter of the recommended spark plugs. Usually, original equipment plugs are a good compromise between the 2 styles of driving, and most vehicles rarely need to have their plugs differ from the factory-recommended heat range.