Using Spark Plugs to Investigate a Cylinder
3-01 Examine spark plugs for engine performance problems.
When diagnosing an engine performance complaint, one of the best identifiers of what is happening in the combustion chamber is the condition of the spark plugs. Looking at the color, electrode condition, and overall state of the spark plug can help determine whether they are causing the problem or are affected by the problem. Finding a plug that is overly wet would indicate either a no-fire condition or an overrich condition. If the plug is covered in coolant, that would direct the technician to a cooling system problem. Oil consumption is a major problem that can show up on spark plugs as a white buildup. Whatever the case, understanding the condition of the spark plug will point in a direction to diagnose what is causing the engine performance problem.
- Locate the spark plug. If the spark plug is not readily visible, it may be necessary to use service information to ascertain the location and procedures for removing it.
- Determine what type of ignition system is used in this application.
• If the ignition system uses spark plug wires, proceed with the following steps:
- Grip the spark plug boot at the base as close to the spark plug as possible.
- Twist slightly from side to side to break the seal of the boot to the spark plug before pulling the spark plug wire off of the plug.
- Prevent damage by not pulling on the spark plug wire.
• If a coil-on plug (COP) is used, disconnect the electrical connector.
• Removing the connector may require releasing the assurance position connector (APC) before attempting to disconnect the connector from the coil.
• Failure to remove the APC may damage the connector.
- Before removing the coil from a COP engine or removing spark plug wires, blow air around each coil/plug boot to prevent debris from falling into the spark plug well.
- Select the correct size of spark plug socket, and get an extension if it needs one (the two most common sizes are 5/8" and 13/16").
- Allow the engine to cool before removing the spark plugs. This step is especially critical on aluminum cylinder heads to help prevent damaging the plug threads.
- Place the spark plug socket over the spark plug and turn the socket counterclockwise.
- Most spark plugs come out relatively easily, but some manufacturers have a major problem with spark plugs breaking during removal. If a spark plug is too tight to remove, stop removing it before breaking the spark plug off in the cylinder head. At this point, making the service advisor aware allows them to alert the customer to the fact they may have a bigger problem than just spark plugs.
• For a hard-to-remove plug, break the spark loose and turn one-eighth to one-quarter turn. Squirt carbon removal (top engine) cleaner into the cylinder so that it can seep past the spark plugs threads and down into the insulator.
• The plugs usually stick due to carbon buildup on the insulator, which is not a problem with the threads in the cylinder head.
• Allow the cleaner to soak for several hours to remove the carbon.
• If the spark plug is still tight, slowly work the spark plug back and forth between tightening and loosening in small increments as it is removed.
• If the spark plug breaks in the cylinder head, plug removal and repair kits are available for specific manufacturers.
• Some manufacturers require that the cylinder head be removed in order to remove the broken spark plug.
• If a broken spark plug is being removed by using the special tool on the vehicle, crank the engine over several times with the plug removed to blow out any debris in the cylinder.
- After removal, place the spark plugs in cylinder order so that the spark plug electrodes and gap can be inspected.
- Inspect the spark plug boot or coil boot for signs of arching (white spots on the boot), damage, and proper fitting.
- Inspect and read the spark plug insulator for information on how the cylinder was operating.
- Examine the threads in the cylinder head, and if necessary, run a spark plug chaser over the threads to clean it.
- If the spark plug is being replaced, verify that the new spark plug is in the correct heat range and that it is the correct plug for the engine.
- Before installing the new spark plug, verify that the gap meets specification or set the gap to specification.
- Install the spark plugs by hand at least two full turns into a cold engine, preventing cross-threading.
- Tighten the spark plug to specification.
• If the spark plug gets over-tightened, its threads in the head can be damaged or it can crack or break off in the cylinder head from uneven stress being placed on the spark plug.
- When installing a spark plug boot, a slight click should be felt. A slight tug on the boot verifies that the plug wire has been installed correctly.
Reading Spark Plugs
The ability to read spark plugs offers insight into what is occurring in the combustion chamber. Spark plugs provide a good deal of insight into the overall engine’s operating condition. This information is valuable during diagnosis and is tuning aid. Most spark plug manufacturers provide charts (find it online with a search engine) that can identify the majority of spark plug problems. The information given includes visual aids of the condition and a description of its meaning (FIGURE 3-1).
FIGURE 3-1 Keep track of where the spark plugs are removed from and identifying those cylinders that you suspect are misfiring can help you the technician to diagnosis cylinder-related misfires.
Locating the spark plug in the combustion chamber is the best position for igniting the air-fuel mixture. Due to its placement, the spark plug is affected by what happens during the chemical-to-mechanical energy exchange during combustion. Closely inspect the firing end to garner diagnostic insight into a cylinder’s operation. If the insulator and electrodes are clean and display normal wear, all is well. Erosion and corrosion cause electrode wear. The electrode material gradually burns away due to normal arcing that increases due to the chemical and thermal events taking place in the combustion chamber. If a fault is present during the combustion process, the condition of the spark plug may offer an understanding of the cylinders operating conditions.
During operation, chemical components in the fuel may leave deposits on the spark plug, altering its ability to ignite the air-fuel mixture correctly. Deposits resulting from the combustion process include soot, carbon residue, ash from fuel and oil additives, and metallic deposits. Deposits can continue to collect even during the correct operating conditions. Over time, the buildup can become substantial enough to conduct electricity, causing an engine misfire. The electrical properties of deposits may change depending on the cylinder’s temperature. What is nonconductive at ambient temperatures may be entirely different from normal engine operating temperatures. Usually, the deposits that collect closer to the spark plug tip, near the electrodes, are responsible for causing a misfire. If the deposits become conductive, they offer an easier path to ground than jumping the electrode gap during compression.
Spark plugs are designed to be “self -cleaning” by running hot enough during normal driving conditions to burn off deposits. The ceramic shell surrounding the center electrode’s normal operating temperature is sufficient to burn off spark plug fouling fuel or ash deposits. The heat range of the spark plug specified by the manufacturer determines its operating temperature and therefore its antifouling resistance. Selecting the incorrect heat range can decrease the spark plug’s antifouling characteristics (too cold) or increase the possibility of pre-ignition (too hot) or detonation.
Keep the spark plugs in order by cylinder after removing them, for inspection. Inspecting the signs of spark plug fouling informs the technician of an overall engine concern or an individual cylinder problem (or problems with several cylinders). The usual color of a properly running engine is light gray or tan—an indication that the engine is operating correctly and that the spark plug is working at the ideal temperature. All the spark plugs should appear similar in wear and color. Dark deposits can appear as thick and wet or dry. A rich air-fuel mixture is the usual cause of dark deposits. The causes of dark deposits include low compression, too large of a spark plug gap, a possible vacuum leak, overly retarded ignition timing, or too cold of a spark plug heat range.
Wet deposits on the firing tip of the spark plug can indicate oil control problems from the valve train or piston oil control rings. Internal coolant leaks or an overrich running condition can also cause wet deposits. The cause of wet deposits depends on the liquid found on the spark plug tip. This information aids in diagnosis, as well as in the selection of what test(s) should be performed to isolate the fault. When the technician discovers signs of fouling or excessive heat, identifying the cause quickly can prevent further deterioration of performance and possible engine or catalytic converter damage.