Preliminary check
1. Before testing, clean spark plugs with carbon deposits or dirt using a special installation with a sand jet and blow them with compressed air.
2. If the carbon deposit is light brown, it does not need to be removed, as it appears on a serviceable engine and does not interfere with the operation of the ignition system.
3. After cleaning, inspect the spark plugs and adjust the gap between the electrodes. If the spark plug insulator has chips, cracks, or the weld on the side electrode is damaged, replace the spark plug.
4. Check the gap (0.5–0.6 mm) between the spark plug electrodes with a round wire feeler gauge. You cannot check the gap with a flat feeler gauge, since this does not take into account the notch on the side electrode that forms when the spark plug is operating. Adjust the gap by bending only the side electrode of the spark plug.
Leak test
1. Screw the spark plug into the corresponding socket on the stand, and then create a pressure of 2 MPa in the stand chamber (20 kgf/cm²).
2. Drip a few drops of oil or kerosene from an oil can onto the candle; if the seal is broken, air bubbles will escape, usually between the insulator and the spark plug body.
Electrical test
1. Having adjusted the gap between the spark plug electrodes to 0.6 mm, screw it into the socket on the stand and tighten it with a torque wrench to 31–39 N·m (3.2–4 kgf·m). The seal is ensured by the elastic gasket of the socket nipple.
2. Place the high voltage lead tip on the spark plug and apply high voltage pulses to it.
3. In this case, the following may be observed.
4. A full spark between the spark plug electrodes is observed in the stand's eyepiece; in this case, the candle is considered excellent.
5. Sparking occurs between the electrodes of the spark gap; the pressure in the device should be reduced and the pressure at which sparking occurs between the spark plug electrodes should be checked. If it begins at a pressure below 0.3 MPa (3 kgf/cm²), then the spark plug is defective.
6. Several sparks on the spark gap are allowed. If there is no spark formation on the spark plug and on the spark gap, then it must be assumed that there are cracks on the spark plug insulator and that the discharge occurs inside, between the mass and the electrodes. Such a spark plug is rejected.