Cherokee, Grand Cherokee, 1999-2005

Spark Plugs

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Inspection



Cold Fouling/Carbon Fouling: Cold fouling is sometimes referred to as carbon fouling. The deposits that cause cold fouling are basically carbon. A dry, black deposit on one or two plugs in a set may be caused by sticking valves or defective spark plug cables. Cold (carbon) fouling of the entire set of spark plugs may be caused by a clogged air cleaner element or repeated short operating times (short trips).

Click image to see an enlarged view

Fig. Cold fouling/carbon fouling spark plugs

Wet Fouling or Gas Fouling: A spark plug coated with excessive wet fuel or oil is wet fouled. In older engines, worn piston rings, leaking valve guide seals or excessive cylinder wear can cause wet fouling. In new or recently overhauled engines, wet fouling may occur before break-in (normal oil control) is achieved. This condition can usually be resolved by cleaning and reinstalling the fouled plugs.

Oil or Ash Encrusted: If one or more spark plugs are oil or oil ash encrusted, evaluate engine condition for the cause of oil entry into that particular combustion chamber.

Click image to see an enlarged view

Fig. Oil or ash encrusted spark plug

Electrode Gap Bridging: Electrode gap bridging may be traced to loose deposits in the combustion chamber. These deposits accumulate on the spark plugs during continuous stop-and-go driving. When the engine is suddenly subjected to a high torque load, deposits partially liquefy and bridge the gap between electrodes. This short circuits the electrodes. Spark plugs with electrode gap bridging can be cleaned using standard procedures.

Click image to see an enlarged view

Fig. Electrode gap bridging

Scavenger Deposits: Fuel scavenger deposits may be either white or yellow. They may appear to be harmful, but this is a normal condition caused by chemical additives in certain fuels. These additives are designed to change the chemical nature of deposits and decrease spark plug misfire tendencies. Notice that accumulation on the ground electrode and shell area may be heavy, but the deposits are easily removed. Spark plugs with scavenger deposits can be considered normal in condition and can be cleaned using standard procedures.

Click image to see an enlarged view

Fig. Scavenger deposits

Chipped Electrode Insulator: A chipped electrode insulator usually results from bending the center electrode while adjusting the spark plug electrode gap. Under certain conditions, severe detonation can also separate the insulator from the center electrode. Spark plugs with this condition must be replaced.

Click image to see an enlarged view

Fig. Chipped electrode insulator

Pre-Ignition Damage: Pre-ignition damage is usually caused by excessive combustion chamber temperature. The center electrode dissolves first and the ground electrode dissolves somewhat latter. Insulators appear relatively deposit free. Determine if the spark plug has the correct heat range rating for the engine. Determine if ignition timing is over advanced or if other operating conditions are causing engine overheating. (The heat range rating refers to the operating temperature of a particular type spark plug. Spark plugs are designed to operate within specific temperature ranges. This depends upon the thickness and length of the center electrodes porcelain insulator.)


CAUTION
If the engine is equipped with copper core ground electrode spark plugs, they must be replaced with the same type/number spark plug as the original. If another spark plug is substituted, pre-ignition will result.



Click image to see an enlarged view

Fig. Pre-ignition damage

Spark Plug Overheating: Overheating is indicated by a white or gray center electrode insulator that also appears blistered. The increase in electrode gap will be considerably in excess of 0.001 inch per 2000 miles of operation. This suggests that a plug with a cooler heat range rating should be used. Over advanced ignition timing, detonation and cooling system malfunctions can also cause spark plug overheating.


CAUTION
If the engine is equipped with copper core ground electrode spark plugs, they must be replaced with the same type/number spark plug as the original. If another spark plug is substituted, pre-ignition will result.



Click image to see an enlarged view

Fig. Spark plug overheating

Cold Fouling/Carbon Fouling: Cold fouling is sometimes referred to as carbon fouling. The deposits that cause cold fouling are basically carbon. A dry, black deposit on one or two plugs in a set may be caused by sticking valves or defective spark plug cables. Cold (carbon) fouling of the entire set of spark plugs may be caused by a clogged air cleaner element or repeated short operating times (short trips).

Click image to see an enlarged view

Fig. Cold fouling/carbon fouling spark plugs

Wet Fouling or Gas Fouling: A spark plug coated with excessive wet fuel or oil is wet fouled. In older engines, worn piston rings, leaking valve guide seals or excessive cylinder wear can cause wet fouling. In new or recently overhauled engines, wet fouling may occur before break-in (normal oil control) is achieved. This condition can usually be resolved by cleaning and reinstalling the fouled plugs.

Oil or Ash Encrusted: If one or more spark plugs are oil or oil ash encrusted, evaluate engine condition for the cause of oil entry into that particular combustion chamber.

Click image to see an enlarged view

Fig. Oil or ash encrusted spark plug

Electrode Gap Bridging: Electrode gap bridging may be traced to loose deposits in the combustion chamber. These deposits accumulate on the spark plugs during continuous stop-and-go driving. When the engine is suddenly subjected to a high torque load, deposits partially liquefy and bridge the gap between electrodes. This short circuits the electrodes. Spark plugs with electrode gap bridging can be cleaned using standard procedures.

Click image to see an enlarged view

Fig. Electrode gap bridging

Scavenger Deposits: Fuel scavenger deposits may be either white or yellow. They may appear to be harmful, but this is a normal condition caused by chemical additives in certain fuels. These additives are designed to change the chemical nature of deposits and decrease spark plug misfire tendencies. Notice that accumulation on the ground electrode and shell area may be heavy, but the deposits are easily removed. Spark plugs with scavenger deposits can be considered normal in condition and can be cleaned using standard procedures.

Click image to see an enlarged view

Fig. Scavenger deposits

Chipped Electrode Insulator: A chipped electrode insulator usually results from bending the center electrode while adjusting the spark plug electrode gap. Under certain conditions, severe detonation can also separate the insulator from the center electrode. Spark plugs with this condition must be replaced.

Click image to see an enlarged view

Fig. Chipped electrode insulator

Pre-Ignition Damage: Pre-ignition damage is usually caused by excessive combustion chamber temperature. The center electrode dissolves first and the ground electrode dissolves somewhat latter. Insulators appear relatively deposit free. Determine if the spark plug has the correct heat range rating for the engine. Determine if ignition timing is over advanced or if other operating conditions are causing engine overheating. (The heat range rating refers to the operating temperature of a particular type spark plug. Spark plugs are designed to operate within specific temperature ranges. This depends upon the thickness and length of the center electrodes porcelain insulator.)


CAUTION
If the engine is equipped with copper core ground electrode spark plugs, they must be replaced with the same type/number spark plug as the original. If another spark plug is substituted, pre-ignition will result.



Click image to see an enlarged view

Fig. Pre-ignition damage

Spark Plug Overheating: Overheating is indicated by a white or gray center electrode insulator that also appears blistered. The increase in electrode gap will be considerably in excess of 0.001 inch per 2000 miles of operation. This suggests that a plug with a cooler heat range rating should be used. Over advanced ignition timing, detonation and cooling system malfunctions can also cause spark plug overheating.


CAUTION
If the engine is equipped with copper core ground electrode spark plugs, they must be replaced with the same type/number spark plug as the original. If another spark plug is substituted, pre-ignition will result.



Click image to see an enlarged view

Fig. Spark plug overheating

Removal & Installation




NOTE
On the 4.0L engine, the spark plugs are located below the coil rail assembly. On the 4.7L engine, each individual spark plug is located under each ignition coil.

4.0L Engine: Prior to removing spark plug, spray compressed air around spark plug hole and area around spark plug. This will help prevent foreign material from entering combustion chamber.

4.7L Engine: Prior to removing spark plug, spray compressed air around base of ignition coil at cylinder head. This will help prevent foreign material from entering combustion chamber.


NOTE
On the 4.0L engine, the coil rail assembly must be removed to gain access to any/all spark plugs. On the 4.7L engine, each individual ignition coil must be removed to gain access to each spark plug.

  1. Remove spark plug from cylinder head using a quality socket with a rubber or foam insert. If equipped with a 4.7L V-8 engine, also check condition of coil O-ring and replace as necessary.
  2.  
  3. Inspect spark plug condition. Refer to Inspection.
  4.  

To install:


CAUTION
The 4.7L V-8 engine is equipped with copper core ground electrode spark plugs. They must be replaced with the same type/number spark plug as the original. If another spark plug is substituted, pre-ignition will result.

  1. Special care should be taken when installing spark plugs into cylinder head spark plug wells. Be sure plugs do not drop into plug wells as ground straps may be bent resulting in a change in plug gap, or electrodes can be damaged.
  2.  
  3. Always tighten spark plugs to specified torque. Over tightening can cause distortion resulting in a change in spark plug gap or a cracked porcelain insulator.
  4.  
  5. Start spark plug into cylinder head by hand to avoid cross threading.
  6.  
  7. 4.0L 6-Cylinder Engine: Tighten spark plugs to 26-30 ft. lbs. (35-41 Nm).
  8.  
  9. 4.7L V-8 Engine: Tighten spark plugs to 20 ft. lbs. (27 Nm).
  10.  
  11. 4.7L V-8 Engine: Before installing coil(s), check condition of coil O-ring and replace as necessary. To aid in coil installation, apply silicone to coil O-ring.
  12.  
  13. Install ignition coil(s).
  14.  

 
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