Contents: Leak test ↓ Electrical test ↓
Before testing, spark plugs with carbon deposits or contaminated ones are cleaned on a special installation with a sand jet and blown with compressed air. If the carbon deposits are light brown, then it is not necessary to remove them, since they appear on a serviceable engine and do not interfere with the operation of the ignition system.
After cleaning, inspect the spark plugs and adjust the gap between the electrodes. If there are chips, cracks on the spark plug insulator or the weld on the side electrode is damaged, then replace the spark plug.
The gap (0.7-0.8 mm) between the spark plug electrodes is checked with a round wire feeler gauge. It is impossible to check the gap with a flat feeler gauge, since this does not take into account the notch on the side electrode that is formed during the spark plug operation. The gap is adjusted by bending only the side electrode of the spark plug. The central electrode is not bent, since this can cause breakage of the ceramic insulator.
Leak test
Screw the spark plug into the corresponding socket on the stand and tighten it with a torque wrench to a torque of 3.2-4 kgf·m. Then create a pressure of 20 kgf/cm² in the stand chamber. Drop a few drops of oil or kerosene onto the spark plug; if the seal is broken, air bubbles will come out, usually between the insulator and the metal body of the spark plug.
Electrical test
Screw the spark plug into the socket on the stand and tighten it with the torque specified above. Adjust the gap between the spark gap electrodes to 12 mm, which corresponds to a voltage of 18 kV, and then use the pump to create a pressure of 6 kgf/cm² in the chamber. Install the tip of the high-voltage wire on the spark plug and apply high-voltage pulses to it.
If a full spark is observed in the stand eyepiece, the spark plug is considered to be in good condition. Irregular sparks on the spark gap are allowed. If sparking occurs only between the spark gap electrodes, then the pressure in the chamber is reduced and the pressure at which sparking occurs between the spark plug electrodes is checked. If it begins at a pressure below 3 kgf/cm², the spark plug is defective.
If there is no spark formation on the spark plug and on the spark gap, then most likely there are cracks on the spark plug insulator and the discharge occurs inside between the body and the electrodes. Such a spark plug is considered defective.