BMW Coupes and Sedans 1970-1988 Repair Guide

Spark Plugs

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See Figure 1

A typical spark plug consists of a metal shell surrounding a ceramic insulator. A metal electrode extends downward through the center of the insulator and protrudes a small distance. Located at the end of the plug and attached to the side of the outer metal shell is the side electrode. The side electrode bends in at a 90° angle so that its tip is just past and parallel to the tip of the center electrode. The distance between these two electrodes (measured in thousandths of an inch or hundredths of a millimeter) is called the spark plug gap. The spark plug does not produce a spark but instead provides a gap across which the current can arc. The coil produces anywhere from 20,000 to 40,000 volts which travels through the wires to the spark plugs. The current passes along the center electrode and jumps the gap to the side electrode, and in doing so, ignites the fuel/air mixture in the combustion chamber.



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Fig. Fig. 1: Cross-section of a spark plug

SPARK PLUG HEAT RANGE



Spark plug heat range is the ability of the plug to dissipate heat. The longer the insulator (or the farther it extends into the engine), the hotter the plug will operate; the shorter the insulator (the closer the electrode is to the block's cooling passages) 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 leads to plug fouling and consequently to misfiring. A plug that absorbs too much heat will have no deposits but, due to the excessive heat, the electrodes will burn away quickly and possibly lead to preignition. Preignition takes place when plug tips get so hot that they glow sufficiently to ignite the fuel/air mixture before the actual spark occurs. This early ignition will usually cause a pinging during low speeds and heavy loads.

The general rule of thumb for choosing the correct heat range when picking a spark plug is: if most of your driving is long distance, high speed travel, use a colder plug; if most of your driving is stop and go, use a hotter plug. Original equipment plugs are generally a good compromise between the 2 styles and most people never have the need to change their plugs from the factory-recommended heat range.

REMOVAL



See Figures 2, 3, 4 and 5

Depending on the engine type, some spark plug wires and plugs are covered by a decorative cover which must be removed prior to spark plug removal.

When you're removing spark plugs, you should work on one plug at a time. Whenever possible, do not remove the plug wires all at once, unless you number them. On some models, it will be more convenient to remove all the wires before working on the plugs. If this is necessary, take a minute before you begin and number the wires with tape. The time you spend here will pay off later on.



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Fig. Fig. 2: A variety of tools are needed for spark plug service



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Fig. Fig. 3: On early model 4-cylinder engines, the spark plugs are located on the passenger side of the engine



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Fig. Fig. 4: Using the proper sized socket, loosen the spark plug-6-cylinder engine shown



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Fig. Fig. 5: With the spark plug fully loosened, remove it from the engine

To remove the spark plug, proceed as follows:

  1. Twist the spark plug boot and remove the boot from the plug. You may also use a plug wire removal tool designed especially for this purpose. Do not pull on the wire itself. When the wire has been removed, take a wire brush and clean the area around the plug. Make sure that all the grime is removed so that none will enter the cylinder after the plug has been removed.
  2.  
  3. Remove the spark plug using the proper size socket. For most engines, a 13 / 16 in. spark plug socket is required.
  4.  

All BMW cylinder heads are made of aluminum. Extreme care should be used when removing the spark plugs so as not to strip the threads.

  1. If a spark plug is difficult to remove, drip some penetrating oil on the plug threads and allow it to soak into the threads, then remove the plug. Also, be sure that the socket is straight on the plug, especially on those hard to reach plugs.
  2.  

INSPECTION



See Figures 6, 7 8, 9, 10 and 11

Check the plugs for deposits and wear. If they 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 can be at most automotive parts stores. This type of tool does an acceptable job of cleaning with a stiff brush.

If the plugs are cleaned, the electrodes must be filed flat. Use an ignition point file, not an emery board or the like, which will leave deposits. The electrodes must be filed perfectly flat with sharp edges; rounded edges reduce the spark plug voltage by as much as 50%.

Check spark plug gap before installation. The ground electrode (the L-shaped one connected to the body of the plug) must be parallel to the center electrode and the specified size wire gauge (as listed in the Tune-Up Specifications chart) should pass through the gap with a slight drag. Always check the gap on new plugs, too; they are not always set correctly at the factory. Do not use a flat feeler gauge when measuring the gap, because the reading will be inaccurate. Wire gapping tools usually have a bending tool attached. Use that to adjust the side electrode until the proper distance is obtained. Absolutely never bend the center electrode. Also, be careful not to bend the side electrode too far or too often; it may weaken and break off within the engine, requiring removal of the cylinder head to retrieve it.



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Fig. Fig. 6: Under normal conditions, a worn spark plug can be identified by traces of tan or gray deposits on the firing tip



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Fig. Fig. 7: A carbon fouled spark plug is identified by the heavy black deposits around the tip. Likely causes of this condition are a poorly tuned vehicle, too cold a spark plug or a defective fuel pump



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Fig. Fig. 8: The damage to this spark plug indicates contact with some part within the combustion chamber. Carefully inspect the chamber for debris, then make sure the plug is not so long that it contacts the piston



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Fig. Fig. 9: An oil fouled spark plug indicates excessive oil in the combustion chamber, most likely from worn piston rings or cylinder head assembly



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Fig. Fig. 10: The widened gap on this spark plug, from extended use, will prevent proper arcing of the high voltage current. The result will be unburned fuel remaining in the combustion chamber and a noticeable loss of power



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Fig. Fig. 11: A bridged or almost bridged spark plug, identified by a deposit between the electrodes, is caused by excessive carbon or oil build-up on the plug

INSTALLATION



See Figures 12 and 13

  1. Refer to the owner's manual included with the vehicle for the correct spark plug gap width. On some models, this information can be found on the vehicle emission sticker in the engine compartment.
  2.  
  3. Using the correct spark plug gapping tools, gap each spark plug to the factory width.
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  5. With all plugs gapped, lubricate the threads of the spark plugs with a drop of oil or anti-seize paste. Install the plugs and tighten them hand-tight. Take care not to cross-thread them.
  6.  



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Fig. Fig. 12: Checking the spark plug gap with a feeler gauge



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Fig. Fig. 13: Adjusting the spark plug gap

  1. Tighten the spark plugs with the socket. Do not apply the same amount of force you would use for a bolt, just snug them in. If a torque wrench is available, tighten to 17-21 ft. lbs. (22-27 Nm).
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  3. Install the wires on their respective plugs. Make sure the wire are firmly connected. You should be able to feel them click into place.
  4.  

RETHREADING



Should you encounter uneven or unduly stiff resistance when removing or installing a spark plugs into the cylinder head, the threads may be stripped or cross-threaded. This will necessitate either rethreading the existing threads or the installation of a Heli-Coil®. Consult the Engine Rebuilding portion of Engine & Engine Overhaul for details on these procedures.

 
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