Spark plugs — bench test

Before testing, spark plugs with carbon deposits or contaminated ones are cleaned in a special installation with a jet of sand and blown with compressed air. If the soot is light brown in color, then it can not be removed, since it appears on a working engine and does not disrupt the operation of the ignition system.

After cleaning, inspect the candles and adjust the gap between the electrodes. If there are chips, cracks on the spark plug insulator, or the side electrode welding is damaged, then the spark plug is replaced.

Gap (0.7—0.8 mm) between the electrodes of the candle is checked with a round wire probe. It is impossible to check the gap with a flat probe, since this does not take into account the recess on the side electrode, which is formed during the operation of the candle. The gap is adjusted by bending only the side electrode of the spark plug. The central electrode is not bent, as this can cause breakage of the ceramic insulator.

Leak test

The spark plug is screwed into the corresponding socket on the stand and tightened with a torque wrench to a torque of 3.2-4 kgf-m. Then a pressure of 20 kgf/cm is created in the stand chamber². Drop a few drops of oil or kerosene on the candle; if the tightness is broken, then air bubbles will usually come out between the insulator and the metal body of the candle.

Electrical test

Screw the candle into the socket on the stand and tighten to the above torque. The gap between the electrodes of the arrester is adjusted by 12 mm, which corresponds to a voltage of 18 kV, and then a pressure of 6 kgf/cm is created in the chamber by a pump². Install the tip of the high voltage wire on the candle and apply high voltage pulses to it.

If a full-fledged spark is observed in the eyepiece of the stand, then the candle is considered serviceable. In this case, irregular sparks are allowed on the arrester. If sparking occurs only between the electrodes of the spark gap, then the pressure in the chamber is lowered and the pressure at which sparking occurs between the spark plug electrodes is checked. If it starts at a pressure below 3 kgf / cm², the spark plug is defective.

If there is no sparking 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 candle is considered defective.