Isuzu KB P190. Manual - part 893

Engine Management – V6 – Service Operations 

Page 6C1-3–48 

 

2.22 Spark 

Plugs 

Service Precautions 

1 

Allow the engine to cool (to at least 50

°C) before attempting to remove spark plugs. Attempting to remove spark 

plugs from a hot engine may cause the plug / cylinder head threads to bind, resulting in tearing of the alloy cylinder 
head threads. 

2 

Clean the spark plug recess area before removing any spark plug. Failure to do so could result in engine damage 
because of dirt or other foreign material entering the cylinder head or by the contamination of the cylinder head 
threads. The contaminated threads may then prevent the correct seating of the new or replaced plug. If required, 
use a thread chaser to clean the threads of any contamination where this is suspected. 

3 

Under no circumstances should the spark plug/s gap 
be adjusted. If the gap is not within specifications, 
replace the spark plug. 

Figure 6C1-3 – 66 

Remove 

1 

Turn the ignition switch off. 

2 

Remove the ignition coil/s, refer to 2.15 Ignition Coil. 

3 

Using a suitable spark plug socket, loosen the spark plug slightly and then re-tighten to break away any carbon 
deposits on the threads. 

 

Wear eye protection to avoid injury. 

4 

Loosen the spark plug once again one or two turns, then use compressed air to remove any foreign material that 
may otherwise enter the combustion chamber. 

5 

Remove the spark plug (1). 

6 

Repeat as required for the remaining spark plugs. 

N O T E  

Place each spark plug in the same order as that 
of removal. This will enable any abnormal spark 
plug condition to be identified with the cylinder. 

N O T E  

If the spark plugs are removed for an indefinite 
period before installation, plug the spark plug 
openings to prevent foreign particle ingress. 

7 

Repeat steps 2 to 5 for the remaining spark plugs as 
required. 

Figure 6C1-3 – 67 

Inspect 

The spark plugs must not be re-gapped. If the gap of a spark plug is outside the specified range, replace the spark plug. 
In addition, replace spark plugs that shows excessive dirt deposit or broken insulators. 

Engine Management – V6 – Service Operations 

Page 6C1-3–49 

 

1 

Replace any plug that has cracked and / or broken insulation or loose electrodes. 

2 

Inspect the spark plugs for defects. Refer to Analysis of Spark Plug Condition in this Section for identification of the 
condition of spark plugs. 

3 

Ensure the threads are clean and in good order. 

4 

Use a round wire feeler gauge to check the spark plug 
gap. If the gap is not within specifications, replace the 
spark plug. 

Spark plug gap........................................... 1.1 – 1.2 mm 

Figure 6C1-3 – 68 

Spark Plug Inspection 

Poor Spark Plug Performance 

A spark plug can perform poorly due to wear, dirt, carbon fouling, excessive electrode wear, a broken insulator or 
excessive gap. 

Worn or Dirty Plugs 

Worn or dirty plugs can give satisfactory operation while the vehicle is idling, but break down under load. 

This can cause: 

• 

poor fuel economy, 

• 

power loss, 

• 

acceleration loss, 

• 

difficult starting, or 

• 

poor engine performance. 

Carbon Fouling 

Carbon fouling is indicated by black carbon deposits. The black deposits are usually the result of slow-speed driving and 
short runs. In these circumstances, the optimum engine operating temperature is seldom reached. 

Fouling can also be caused by: 

• 

worn piston rings, 

• 

faulty ignition, 

• 

rich fuel mixture, or 

• 

spark plugs that are rated too cold. 

Excessive Electrode Wear 

This often indicates: 

• 

the engine is operating at high speeds, 

• 

the engine is operating at levels that are consistently greater than normal, 

• 

a plug that is rated too hot, 

• 

excessively lean fuel mixture, or 

Engine Management – V6 – Service Operations 

Page 6C1-3–50 

 

• 

plug/s overheating due to insufficient tightening (caused by combustion gases leaking past the threads). 

Broken Insulator 

Broken insulators are usually the result of improper installation or carelessness. 

Breaks in the upper insulator can result from a poor fitting spark plug socket or impact. The cracked insulator may not 
show up until oil or moisture penetrates the crack. The crack is often just below the crimped part of the shell and may not 
be visible. 

Breaks in the lower insulator often result from careless re-gapping and are usually visible.  

This can also result from the plug operating too hot. For example, in periods of high speed operation or under heavy 
loads. 

Engine Management – V6 – Service Operations 

Page 6C1-3–51 

 

Analysis of Spark Plug Condition 

 

Figure 6C1-3 – 69 

Legend 

1 Normal 

2 Carbon 

Fouled 

3 Oil 

Fouled 

4 

Deposit Fouling A  

5 

Deposit Fouling B  

6 

Deposit Fouling C 

7 Detonation 

8 Pre-ignition 

9 

Heat Shock Failure 

10 

Insufficient Installation Torque 

 

Normal Operation (1) 

Brown or greyish-tan deposits and slight electrode wear indicate correct spark plug heat range and mixed periods of high 
and low speed driving. 

Carbon Fouled (2) 

Dry, fluffy black carbon deposits possibly due to poor ignition output, a weak coil, excessive idling or slow speeds under 
light load. If spark plug temperatures remain too low for normal combustion, the deposits are not burned off.