Article 972 - Spark Plug Problem Diagnosis

TSB #57 - January 6, 1967

Analysis of spark plugs returned from the field indicate that unnecessary replacement is continuing. Generally, such replacement will provide a noticeable improvement in engine performance, however unless the basic problem component is corrected or replaced, this improvement will only be temporary. This article outlines the procedure recommended to detect and evaluate spark plug problems caused by related components and defines the types of problems which require spark plug replacement.

Spark plugs should be replaced which have internal components omitted or exhibit damage caused by manufacturing or assembly processes. The following spark plug problem areas are within this category:

    Electrode Weld
    A weak electrode weld is obviously identified by the absence of the electrode upon spark plug removal.
    Electrode Form
    A spark plug with an electrode not properly bent or trimmed should be replaced. This problem does not refer to improper gap adjustment.
    Shell Seal
    An improper shell seal can be detected by the insulator being loose in the shell or by signs of gas leakage around the shell crimp. Do not confuse this leakage with the tan or gray deposits on the insulator resulting from corona.
    Center Seal
    An inadequate internal center seal can be detected by checking the continuity from the center electrode to the terminal with an ohmmeter. A spark plug without this continuity should be replaced. Also, a terminal not fully pressed into the insulator will identify an inadequate center seal.
    Damaged external threads are self evident. Be sure the mating threads in the cylinder head did not cause this spark plug thread damage.
    Center Electrode Insulator
    A chipped or broken center electrode insulator can be detected by visually examining the spark plug at the firing end.
    External Insulator
    A cracked external insulator is most often caused by improper installation. Spark plugs which have not been used are not warranted for this installation damage.

Normal operating spark plugs are indicated by the presence of light tan to brown deposits and a normal gap increase of approximately 0.001 inch per thousand miles of operation. If electrodes do not show excessive wear, they can be cleaned and reinstalled. If replacement is necessary, use the same type spark plug.

Premature spark plug failures are most often caused by related component problems or unusual vehicle operating conditions. Service personnel are normally aware of the visual conditions which appear on spark plugs of this type but are unable to relate these conditions to the real problem cause. Unless the real problem cause is identified and corrected, reconditioned or replacement spark plugs will only provide temporary problem correction.
The following spark plug conditions represent those which are caused by related problems. Use of the included corrective information will insure the correction of a spark plug problem.

Lead Fouling
Lead fouled spark plugs are indicated by the presence of dark grey, black, yellow or tan cindery deposits or a shiny glaze coating of these same colored deposits.
CAUSE: These deposits are by-products of fuel combustion. The shiny glaze coating is formed during periods of high speed and heavy load conditions following extended periods of city type operation.
CORRECTION: Lead fouled spark plugs can be cleaned and reinstalled with good results. Spark plugs with a shiny glazed deposit can generally be cleaned, however plugs with heavy deposits lodged in the shell bore that cannot be removed by cleaning should be replaced. If this condition continues to occur after short periods of service it may be necessary to install spark plugs of a different heat range.

Fused Spot Deposits
Spark plugs with spot deposits have melted or fused spotty deposits on the insulator resembling, bubbles, or a blistered insulator.
CAUSE: During periods of sudden acceleration deposits are blown from the combustion chamber and thrown against the hot insulator - melting and fusing to form these globule type deposits.
CORRECTION: Spark plugs can usually be cleaned and reinstalled with good results. However, in cases where the fused deposits cannot be removed by cleaning, discard and replace with new plugs of the same type.

Carbon Fouling
Carbon fouled spark plugs are indicated by the presence of black dry fluffy carbon deposits.
CAUSE: If only one or two spark plugs are carbon fouled check the following components:
    A. High Tension Leads — open circuit, high resistance, deteriorated or hardened insulation.
    B. Valves — burnt or sticking.
If the complete set is carbon fouled, check the following component areas:
    A. Carburetor — rich air-fuel mixture.
    B. Improper automatic choke operation.
    C. Dirty or clogged air cleaner.
    D. Improper heat riser operation.
    E. Excessive idling — stop and go type operation.
CORRECTION: Correct defective components as required; spark plugs can then be cleaned and reinstalled with good results. If the fouling condition was caused by excessive idling and/or stop and go type operation, and this type of operation will be continued, install spark plugs of a higher heat range.

Carbon Fouling - Oil
Oil fouled spark plugs are indicated by the presence of black wet deposits.
CAUSE: This type of fouling can occur in new or rebuilt engines before the piston rings become properly seated and oil control is achieved. However, oil fouling in older engines generally indicates that one or more of the following conditions prevail:
    A. Worn piston rings.
    B. Worn Cylinders.
    C. Worn pistons.
    D. Worn valve guides.
    E. Worn valve guide seals.
CORRECTION: If this condition occurs on new or rebuilt engines, the spark plugs can be dried with an air hose, cleaned, and installed with good results. In older engines where excessive oil consumption is prevalent, the faulty condition must be corrected, otherwise the cleaning or replacement of spark plugs will only be a temporary cure. The use of a hotter spark plug in engines with excessive oil fouling will provide longer periods of satisfactory service.

Gap Bridging
Gap bridged spark plugs are indicated by the presence of deposits lodged between the side and center electrodes.
CAUSE: Improper fuel combustion with the accumulation of unburned carbon molecules forms a thread like chain which lodges between the center and side electrodes; or combustion chamber deposits may be shed during rapid acceleration or high speed operation and thrown against the hot center and side electrodes, fusing on contact to short out the plug.
CORRECTION: The bridged deposit can be removed and the plug cleaned and reinstalled with good results.

Overheated spark plugs are indicated by an extremely white insulator tip with small black deposits, bluish-burnt cast on center or side electrodes, general lack of insulator deposits, and premature electrode erosion.
CAUSE: Incorrect spark plug heat range or a related problem such as:
    A. Distributor — over advanced ignition timing.
    B. Carburetion — very lean air-fuel mixture.
    C. Cooling system — inoperative or partially clogged.
    D. Improper spark plug seating.
CORRECTION: Be sure correct heat range spark plugs were in use. If the condition was caused by a related component, replace or repair as required and install new spark plugs if necessary.

Pre-ignition spark plugs are indicated by melted or very severely burnt center and/or side electrodes, blistered insulator and aluminum or metallic deposits on the insulator.
CAUSE: Incorrect spark plug heat range or a related problem such as:
    A. Burnt valves — low compression.
    B. Distributor — Over advanced ignition timing.
    C. Cooling system — inoperative or partially clogged.
    D. Carburetion — lean air-fuel mixture.
    E. Detonation — improper octane fuel, low grade fuel.
    F. Improper spark plug seating.
CORRECTION: Be sure recommended spark plugs were in use. If this condition was caused by a related component, make necessary repairs and replace the spark plugs as required.

Split Insulator Tip
Spark plugs with a split insulator tip are indicated by a cracked or chipped insulator at the firing tip of the spark plug.
CAUSE: Cracked or split insulators resulting from improper timing or use of improper fuel can result from excessive center electrode temperatures or severe engine detonation. However, defects of this nature are usually caused by incorrect gapping tools (plier type) and/or procedure.
CORRECTION: Replace with new spark plugs. If the cracked or chipped insulator has resulted during engine operation, check and set ignition timing to the recommended specification. Be sure the proper octane fuel is being used. If this condition continues to occur, it may be necessary to replace with spark plugs of a colder heat range.

Spark plug testers operate with dry air whereas the atmosphere in the engine utilizes a volatile air-fuel mixture. This mixture acts as an ionization agent between the electrodes and promotes spark discharge.

Engine ignition normally occurs before the piston reaches top center in the cylinder, thus the spark actually occurs before peak cylinder pressure is reached.
Competitive spark plug testers employ a vibrator type ignition source that produces a voltage of an entirely different wave form than that produced by automotive ignition coils. The voltage of a vibrator type ignition source decreases after the first few minutes of use and the voltage pulses vary in amplitude to cause intermittent misfiring.

One of the phenomena observed in analyzing spark plugs in the tester is that a spark plug with sharp cornered electrodes will fire under an appreciably greater pressure than one with rounded electrodes. The compression tester will therefore indicate that a plug with rounded electrodes is faulty. Since such a plug may function satisfactorily in the engine, the correlation between the spark plug tester and actual operating conditions is not realistic. The following procedure will help provide a more honest evaluation of spark plug capabilities when checked in a compression tester.
    1. Be sure all reconditioned spark plugs electrodes have been filed properly so that the electrodes have sharp corners.
    2. Clean and dry all spark plug insulators.
    3. Check the operation of a new spark plug in the tester and note the pressure at which the plug no longer sparks.
    4. Check the pressure at which the reconditioned spark plug no longer sparks.
    5. If the difference in the two readings does not exceed 30%, the reconditioned spark plug is suitable for reinstallation and will provide good service.
Voltage Flashover
Many spark plugs checked in compression testers are condemned because of voltage flashover. (Sparking from the plug terminal down the insulator to ground on the shell). This flashover condition does not indicate a faulty spark plug. In fact, it proves that the plug insulator is in good condition, meaning the plug has no cracks or pin hole punctures that could cause misfire. Installed in an engine, the spark plug insulator is covered by an insulating boot to insure against flashover. A similar boot should be installed during a spark plug compression test to accurately duplicate actual operating conditions.
In many instances "corona" is mistaken as spark plug flashover, however, corona is the glowing that appears above the plug shell around the base of the insulator. This condition (more readily seen in the dark) is caused by the electrical stress in the air adjacent to the insulator - corona is in no way detrimental to spark plug operation, and will cause a brown or grey deposit on the insulator top just above the shell.