Spark Plugs
Both the 4.0L and the 4.7L engines use resistor type spark plugs. Standard 4.7L engines are equipped with "fired in suppressor seal" type spark plugs using a copper core ground electrode. High Output (H.O.) 4.7L engines are equipped with unique plugs using a platinum rivet located on the tip of the center electrode. Because of the use of an aluminum cylinder head on the 4.7L engine, spark plug torque is very critical. To prevent possible pre-ignition and/or mechanical engine damage, the correct type/heat range/number spark plug must be used. See SPARK PLUGS in SERVICE & ADJUSTMENT SPECIFICATIONS - GRAND CHEROKEE article. Do not substitute any other spark plug on the 4.7L H.O. engine. Serious engine damage may occur. Plugs on both engines have resistance values ranging from 6000 to 20,000 ohms when checked with at least a 1000 volt spark plug tester. Do not use an ohmmeter to check the resistance values of the spark plugs. Inaccurate readings will result.
When removing the spark plugs, examine them for burned electrodes and fouled, cracked or broken porcelain insulators. Keep plugs arranged in the order in which they were removed from the engine. A single plug displaying an abnormal condition indicates that a problem exists in the corresponding cylinder. Replace spark plugs at the intervals recommended. See SPARK PLUGS in SERVICE & ADJUSTMENT SPECIFICATIONS - GRAND CHEROKEE article. On all engines except the 4.7L H.O., spark plugs that have low mileage may be cleaned and reused if not otherwise defective, carbon or oil fouled. Never use a motorized wire wheel brush to clean the spark plugs. Metallic deposits will remain on the spark plug insulator and will cause plug misfire. On the 4.7L H.O. engine, never clean the spark plugs. Damage to the platinum rivet will result. Do not use a wire-type gapping tool, as damage to the platinum rivet on the center electrode may occur. Use a tapered-type gauge. See Fig 1 .