Spark plugs ignite the air and fuel mixture in the cylinder as the piston reaches the top of the compression stroke. The controlled explosion that results forces the piston down, turning the crankshaft and the rest of the drive train.
The average life of a spark plug 30,000 miles (48,300 km). On the 2.2L (LN2),engine the life of the spark plug is 100,000 miles (166,000 km). Part of the reason for this extraordinarily long life is the exclusive use of unleaded fuel, which reduces the amount of deposits within the combustion chamber and on the spark plug electrodes themselves, compared with the deposits left by the leaded gasoline used in the past. An additional contribution to long life is made by the High Energy Ignition (HEI) system, which fires the spark plugs with over 35,000 volts of electricity. The high voltage serves to keep the electrodes clear, which suffer less pitting and wear compared to breaker point ignitions.
Fig. 1: Breakdown of a common spark plug
Fig. 2: Heat range on spark plugs explained
Nevertheless, the life of a spark plug is dependent on a number of factors, including the mechanical condition of the engine, driving conditions, and the driver's habits.
When you remove the plugs, check the condition of the electrodes, they are a good indicator of the internal state of the engine. Since the spark plug wires must be checked every 15,000 miles (24,000 km), the spark plugs can be removed and examined at the same time. This will allow you to keep an eye on the mechanical status of the engine.
A small deposit of light tan or rust/red material on a spark plug that has been used for any period of time is to be considered normal. Any other color, or abnormal amounts of wear or deposits, indicates that there is something amiss in the engine.
Fig. 3: Explanation of the spark plugs numbering system
Spark Plug Wires
INSPECTION & TESTING
Every 15,000 miles (24,000 km), inspect the spark plug wires for burns, cuts, or breaks in the insulation. Check the boots and the nipples on the distributor cap. Replace any damaged wiring. General Motors requests that you replace your spark plug wires every 30,000 miles (50,000 km).
The resistance of the wires should be checked with an ohmmeter. Wires with excessive resistance will cause misfiring, and may make the engine difficult to start in damp weather.
To check resistance, remove the distributor cap, leaving the wires in place. Connect one lead of an ohmmeter to an electrode within the cap; connect the other lead to the corresponding spark plug terminal (remove it from the spark plug for this test). The following chart gives resistance values as a function of length. Replace any wire which shows a resistance over 30,000 ohms.
- 0–15 in. (0–38cm): 3,000–10,000 ohms
- 15–25 in. (38–64cm): 4,000–15,000 ohms
- 25–35 in. (64–89cm): 6,000–20,000 ohms
- Over 35 in. (89cm): 25,000 ohms
It should be remembered that resistance is a function of length; the longer the wire, the greater the resistance. Thus, if the wires on your car are longer than the factory originals, resistance will be higher, quite possibly outside these limits.