Caution: Spark plugs should only be replaced when the engine is cold or slightly warm. Removing the spark plugs when the engine is hot can damage the threads in the cylinder head.
1. Turn off the ignition and remove the upper decorative engine cover (see Chapter 2).
2. On 1.6L engines, disconnect the spark plug connectors (1 in the illustration).
14.2. Spark plug connectors for 1.6L engine
3. On 1.8L engines, disconnect the ignition coil connectors (1 in illustration 14.3a), by pressing the lock and pulling the connector towards you. Remove the ignition coils using a puller (see illustration 14.3b).
14.3a. Ignition coil connectors for 1.8L engine
14.3b. Ignition coil puller
4. On 2.0L engines, remove the bolts (1 in the illustration) and disconnect the connectors (2) of the ignition coils by pressing the lock and pulling the connector towards you. Remove the ignition coils using a puller (see illustration 14.3b).
14.4. Connectors (2) and bolts (1) for fastening the wiring of the ignition coils of 2.0 l engines
5. On 3.0, 3.2 and 4.2 L engines, first remove the air cleaner cover (when working on the right side, see Section 16) or expansion tank (without disconnecting the hoses from it, - when working on the left side). Then unscrew the bolts (1 in the illustration) and disconnect the connectors (2) of the ignition coils by pressing the lock and pulling the connector towards you. Remove the ignition coils using a puller (see illustration 14.3b).
14.5. Connectors (2) and bolts (1) for fastening the wiring of the ignition coils (3) of 3.0, 3.2 and 4.2 l engines (shows the right side of the 3.2L engine)
6. Remove the spark plug using a long socket wrench, being careful not to tilt it.
7. Assess the condition of the spark plugs (see illustrated insert). If necessary, correct any possible faults that can be judged by the condition of the spark plugs. Carbon deposits on the electrodes can be removed with compressed air under pressure; do not use a wire brush for this purpose. Checking and adjusting the interelectrode gap between spark plug replacements is not required.
8. Install the spark plugs and tighten them to the required torque.
9. Install the ignition coils if they were removed and connect their wiring.
Coal deposits Symptoms: The presence of soot indicates an over-rich fuel-air mixture or weak spark intensity. Causes misfires, difficult starting and leads to unstable engine operation.; Recommendations: Check if the air cleaner is clogged. Try using spark plugs with a longer insulator, which increases resistance to contamination.
Oiling Symptoms: Oiling of the spark plug is caused by worn out oil deflector caps. Oil gets into the combustion chamber through worn valve guides or piston rings. Causes misfires, difficult starting and leads to unstable engine operation.; Recommendations: Perform mechanical repairs and replace spark plugs.
Overheat Symptoms: Porous, white insulator, erosion of electrodes and absence of any deposits. Leads to a reduction in spark plug life.; Recommendations: Check that the installed spark plugs are of the correct heat range, that the ignition timing is set correctly, that the fuel/air mixture is not too lean, that there are no vacuum leaks in the intake manifold and that the valves are not stuck. Also check the coolant level and that the radiator is not clogged.
Too early ignition Symptoms: The electrodes are melted. The insulator is white, but may also be contaminated due to misfiring or foreign particles entering the combustion chambers. May cause engine failure.; Recommendations: Check the glow number of the installed spark plugs, the ignition timing, and the quality of the mixture (isn't it too poor?), whether the cooling system is clogged and whether the lubrication system is functioning normally.
Conductive gloss Symptoms: The insulator has a yellowish color and a polished appearance. Indicates a sudden increase in temperature in the combustion chambers during sharp acceleration. Normal deposits melt, acquiring the appearance of a varnish coating. Leads to misfires at high speeds. Recommendations: Change the spark plugs (cooler, while maintaining the same driving style).
Short circuit of electrodes Symptoms: Combustion waste gets into the interelectrode space. Solid deposits accumulate, forming a bridge between the electrodes. Leads to ignition failure in the cylinder. Recommendations: Remove deposits from the interelectrode space.
Normal condition of the candle Symptoms: Gray-brown color and slight wear of the electrodes. The glow number of the spark plugs corresponds to the type of engine and its general condition. Recommendations: When replacing spark plugs, install spark plugs of the same type.
Ash formation Symptoms: Soft, brownish deposits on one or both spark plug electrodes. They are caused by oil and/or fuel additives. Excessive buildup can insulate the electrodes and cause misfires and rough running under acceleration. Recommendations: If deposits build up quickly, replace the valve stem seals to prevent oil from entering the combustion chambers. Try changing the fuel brand.
Wear Symptoms: Rounding of the electrodes with a small accumulation of deposits at the working end. Normal color. Causes difficulty starting the engine in cold, damp weather and increased fuel consumption. Recommendations: Replace the spark plugs with new ones of the same type.
Detonation Symptoms: Insulators may be chipped or cracked. Insulator damage may also result from improper spark plug gap adjustment. Piston damage may result.
For splashing Symptoms: After prolonged misfires, deposits may become loose while the combustion chamber remains at operating temperature. At high speeds, deposits flake off the piston and stick to the hot insulator, causing misfires. Recommendations: Replace spark plugs or clean and reinstall old ones.
Mechanical damage Symptoms: Can be caused by foreign material entering the combustion chamber or by the piston hitting a spark plug that is too long. Leads to cylinder failure and piston damage. Recommendations: Remove foreign material from the engine and/or replace the spark plugs.
[The article is a reprint of material from: audimanual]