Specifications

1. Optimal Spark Plug Appearance

The ceramic center is gray to light-brown The electrode gap is correct. The heat-range is correct.

2. Soot Deposit (Velvet-like, dull black soot deposits.)

Cause: Air/Fuel Mixture too rich; Air cleaner dirty; Short distance driving and engine does not reach

operating temperature; Wrong heat-range, spark plug is too cold; Can cause missfire.

Repairs: Correct problem. Install new spark plugs.

3. Oil Deposits (Wet oil film or deposits.)

Cause: Oil reaches the combustion chamber; Oil level too high; Cylinder worn out; Piston rings or valve

guides bad; Will cause missfire.

Repairs: Replace worn out parts. Install new spark plugs.

4. Badly Eroded Electrodes (Abnormal wear.)

Cause: Corrosive gasoline additives; Heavy pinging or ignition knock; Ignition timing too much advanced;

Thermal overload; Will cause missfire.

Repairs: Stop using additive. Perform Tune-up. Install new spark plugs.

5. Molten Electrode (Severe wear.)

Cause: Thermal overload caused by knocking from over-advanced ignition timing, or from carbon deposits

in the combustion chamber (glowing hot spots); Spark plug loose; Low grade gasoline; Will cause

missfire. Could lead to catastrophic engine failure.

Repairs: Perform Tune-up. Torque spark plugs. Install new spark plugs.

6. Overglazing (Spark plug looks glazed.)

Cause: Gasoline additives; High thermal load from loose spark plug; Wrong heat-range; Can cause missfire.

Repairs: Perform Tune-up. Torque spark plugs. Install new spark plugs of correct heat-range.

7. Molten Center Electrode (Severely damaged center electrode.)

Cause: Possibly wrong heat-range (too hot); Thermal overload caused by knocking by too much ignition

advance, or from carbon deposits in the combustion chamber (glowing hot spots); Low-grade

gasoline; Sparkplug loose; Will cause missfire. Could lead to catastrophic engine failure.

Repairs: Perform Tune-up. Torque spark plugs. Install new spark plugs of correct heat-range.

8. Deposits (Brownish, cinder-like deposits.)

Cause: Gasoline additives, or oil additives; Can cause missfire.

Repairs: Install new spark plugs of correct heat-range.

9. Ceramic Center Shattered (Visible damage, or hairline crack.)

Cause: Mechanically damaged, probably was dropped; Putting pressure on electrode during gapping; Shock

load caused by knocking; Will cause missfire. Could lead to catastrophic engine failure.

Repairs: Install new spark plugs of correct heat-range.

10. Center Electrode Erosion (Normal wear.)

Cause: Spark plug not replace as scheduled, but shows normal wear. May cause missfire.

Repairs: Install new spark plugs of correct heat-range.

Attention: All examples are only applicable to 4-stroke engine.

Spark Plug Diagnosis