- Engines: Gas
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The proper maintenance of spark plugs will provide the following benefits: maximum life for the spark plug and optimum performance of the engine. Refer to the applicable Operation and Maintenance Manual for the correct Maintenance Intervals for the spark plugs.
Removing the Spark Plug
- Before removing a spark plug, ensure that the area around the spark plug is clean and free of dirt and debris.
- Always use a spark plug socket to remove spark plugs. Use a 9S-5031 Spark Plug Socket for 14 mm spark plugs. Use a 4C-4601 Spark Plug Socket for 18 mm spark plugs. After the spark plug has been loosened, try to remove the spark plug by hand in order to detect problems with the threads.
- If it is difficult to remove the spark plug by hand, use the appropriate tool to clean the threads in the cylinder head that are for the spark plug. Use the 9U-7511 Spark Plug Seat Cleaner for spark plugs with 18 mm thread. Use the 154-9521 Spark Plug Seat Cleaner for spark plugs with 14 mm thread. Be sure to clean any debris from the cylinder.
Note: Do not use a thread tap to clean the threads in the cylinder head that are for the spark plug. A thread tap will remove metal unnecessarily. This practice will eventually result in the failure of the threads in the cylinder head that are for the spark plug.
Inspecting the Spark Plug
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| Illustration 1 | g00778942 |
(1) Terminal post (2) Insulator (3) Shell (4) Gasket (5) Electrode | |
Inspect the spark plug closely for damage. The condition of the spark plug can indicate the operating condition of the engine.
Terminal post (1) must not move. If the terminal post can be moved by hand, carefully tighten the post into the threads of the insulator. If the post cannot be tightened, discard the spark plug.
Inspect insulator (2) for cracks. If a crack is found, discard the spark plug.
Faint marks may extend from shell (3) onto the insulator. The marks may be a result of a corona that forms at the top of the shell. The conductor will develop a corona when a very high voltage potential ionizes the air. This is a normal condition. This is not an indication of leakage between the shell and the insulator. See Engine News, SEBD9262, July 1998, "Effects Of Corona On Spark Plugs" for additional information.
Inspect shell (3) for damage. Cracks can be caused by overtightening the spark plug. Overtightening can also loosen the shell. Discard any spark plug that has a shell that is cracked or loose.
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| Illustration 2 | g00778946 |
(6) Precious metal tips on the electrode and the ground strap | |
Caterpillar spark plugs have precious metal tips (6) on the ends of the electrode and the ground strap. This material will gradually erode. Use extreme care when you set the electrode gap. Maintain even spacing and proper alignment between the two precious metal surfaces.
Replace the spark plug if the precious metal is worn off.
A light brown deposit or a beige deposit is produced by normal operation.
Deposits that are gray or black may be caused by the following substances:
- Excessive oil
- Use of the wrong oil
- A substance that is introduced through the fuel system or the air system
A spark plug can operate despite a buildup of ash. However, a buildup of ash can cover the electrode gap. This will cause misfire. Large deposits may retain heat which can cause premature fuel ignition. This can lead to uncontrollable detonation.
Cleaning the Spark Plug
Thoroughly clean the spark plug. Do not use a wire brush. Glass beads are the preferred method for cleaning.
Follow these guidelines for using glass beads:
- Always use clean glass beads. Do not reuse the glass beads.
- Use care in order to clean only the electrode and the insulator near the electrode.
- Do not use glass beads on the threads or on the outside of the shell or on the upper part of the insulator.
Checking the Spark Plug
After the spark plug is clean, measure the spark plug's resistance.
Note: Do not use a standard multimeter for measuring the resistance of a spark plug. The oxidation of the spark plug's internal components may distort the resistance that is measured by a standard multimeter.
Use the 142-5055 Insulation Testing Group (Electrical) to measure the resistance of the spark plug. Follow the warnings and instructions that are provided with the tool. A resistance measurement of more than 1000 ohms usually indicates a failure of the spark plug's internal resistor. If the resistance is greater than 1000 ohms, discard the spark plug and obtain a new spark plug.
If the resistance is correct, make sure that the electrode gap is correct before you install the spark plug. It is important for the electrode gap to be correct for the optimum performance of the engine. Use a 9U-7516 Spark Plug Gauge to measure the electrode gap. Do not use a flat feeler gauge for measuring the electrode gap of used spark plugs. A feeler gauge will incorrectly measure the actual electrode gap because the used precious metal tips are curved.
Adjust the electrode gap, if necessary. See Engine News, SEBD9264, September 1998, "Fabricate Spark Plug Gapping Tool" for information about making a tool for adjusting the electrode gap. Bend the ground strap at the existing bend. Then bend the strap near the weld in order to achieve proper alignment and even spacing between the two precious metal surfaces. It is very important for the two precious metal surfaces to remain parallel after adjusting the electrode gap. Measure the electrode gap after the alignment. Correct the electrode gap, if necessary.
Installing the Spark Plug
- Ensure that the spark plug is clean and free of dirt and oil.
Note: Do not use anti-seize compound on threads of the spark plugs. Most of the heat is transferred from the spark plug through the threads and the seat area of the spark plug into the cylinder head. Those surfaces must be in direct contact in order to provide the maximum heat transfer from the spark plug to the cylinder head.
- Always use a new spark plug gasket when a spark plug is installed. If a used spark plug is installed, place a new spark plug gasket on the spark plug. The 108-3515 Spark Plug Gasket is available for 18 mm spark plugs. The 9Y-6792 Spark Plug Gasket is available for 14 mm spark plugs. Orient the tabs on the inside diameter of the spark plug gasket toward the electrode.
- Install the spark plug by hand until the spark plug contacts the gasket. Use a torque wrench to tighten the spark plug to specified torque.
Note: Do not overtighten the spark plugs. If the spark plug is overtightened, the shell of the spark plug can be stretched and deformed. This will loosen the seal between the shell and the insulator.
- Ensure that the ignition transformer and the spark plug extension are clean and free from dirt and oil. Lubricate the O-ring seals with one of the following lubricants:
- Be sure that the seal on the ignition transformer for an Electronic Ignition System (EIS) is not damaged. Also, be sure that the screws that fasten the spring plate to the top of the transformer are tight.
- Install the spark plug extension and/or the ignition transformer. The spark plug extension must fit snugly on the spark plug terminal and the spark plug extension must make clean contact with the terminal on the ignition transformer. Orient the ignition transformer toward the wiring harness. Carefully align the socket of the transformer with the wiring harness connector. Connect the wiring harness.