Often seen on major forums，I saw a netizen asking，"Is my spark plug leaking?" "The auto mechanic said my spark plug is leaking，So I changed the spark plugs。”Some auto mechanics also call it leakage，Pictures like the one below。

1. Spark plug leaking，It has become a classic rumor in the auto repair industry，Maybe even they themselves can’t tell whether it’s a rumor or the truth。

If this is really a leaking spark plug，Then，All brands of spark plugs in the world can be used in your car，In a few days，They are all leaking!

This yellow, brown dirt is called corona.

[Explanation] Spark plug corona - because the center electrode inside the spark plug conducts high-voltage electricity，High-voltage electricity has an adsorption effect on oil particles floating in the air，Adsorbed on the surface of white insulator。

Just like plastic bags with static electricity will attract people、Old TV high-voltage components attract dust、High-voltage electrical boxes can attract people，Same reason。

Also，Due to the blocking effect of the ignition coil (cylinder wire) rubber sleeve，Corona is only present in the section close to the metal shell。

Corona does not affect spark plug performance，Not directly related to the service life of the spark plug。Corona is not used as a basis for judging whether spark plugs need to be replaced。

2. Where do the corona oil particles come from?

The engine cylinder head is filled with oil，Engine oil plays a role in heat dissipation and lubrication，There is usually a rubber seal between the cylinder head and valve cover，called valve chamber gasket。With the engine working，The sealing performance of the valve chamber gasket deteriorates as it ages，Vapor particles from the oil will disperse seabet iointo the spark plug hole，As shown by the arrow below，Adsorbed on the surface of the white insulator of the spark plug，Corona formation。

Oil leaks into the spark plug hole after the valve chamber gasket has aged to a certain extent，It will soak the spark plug in oil，Let the ignition coil (cylinder wire) be immersed in oil，This will affect the normal operation of the spark plug。As shown in the picture below。

3. What does a real spark plug leak look like? Is there any leakage?

National Standard of the People's Republic of China GB 7825-1987 The thermal sealing performance of the spark plug is specified in the spark plug:

When the air medium temperature around the spark plug electrode is 190-220℃、When the air pressure difference is 41.2x105Pa (42kg/cm2)，and kept warm for 4-5min，The total air leakage of all parts of the spark plug should not exceed 5mL/min。

In other words，Spark plug is working，The air leakage cannot exceed 5ml per minute，Considered qualified。

Whether the spark plug itself is leaking，Scientific testing without professional equipment，It is impossible to know。

Generally speaking，All spark plugs from regular manufacturers，All are qualified，Very small air leakage，We can all roughly think that it is airtight。

4. When installing the spark plug，Man-made air leakage caused by loose installation，That’s what you should be concerned about。

The compression ratio of modern automobile engines is generally 8 to 11，When the spark plug is working，The pressure is from about 1 atmosphere to 8 to 11 atmospheres at the top dead center of the piston，Then to 30-40 atmospheres during the power stroke，What goes around comes around，The pressure on the spark plug，Roughly circulates between about 1-40 atmospheres。

If the spark plug is not tightened according to the standard torque specified by the manufacturer，There will be a real air leak，High temperature and high pressure gas slowly leaks out，The pressure in the cylinder will immediately decrease。

Leakage caused by the spark plug not being tightened，Will cause engine weakness，Unsteady idle，Easy to stall at seabet iohigh speed，Increased fuel consumption，Even the engine cannot start，Seriously causing spark plug failure、Serious consequences of engine damage。

The picture below is a schematic diagram，The gap between the spark plug threads and the cylinder head threads is intentionally shown to be large，Specially to show how air leakage is caused。

Tighten most spark plugs with gaskets to the standard torque specified by the manufacturer，The gasket will naturally be flattened，As shown in the picture below。

Spark plugs without gaskets also need to be tightened.

5. The spark plug is not tightened，After serious air leakage，What will happen to the spark plug?

The one on the right side of the picture below is because it is not tightened，Air leakage，Spark plug blown black by high temperature and high pressure gas。

The picture below is a set due to air leakage，High temperature persists，The temperature is estimated to be around 300℃，The thinnest part of the shell is baked into a blue spark plug。

and，Ignition coil rubber sleeve for this set of spark plugs，Also burnt，The temperature is estimated to be around 200-300℃。

The picture below shows one that is not tightened、Air leakage，High temperature persists，The temperature is estimated to be around 260℃，Spark plug baked into yellow-purple color。

6. Why is the metal shell of the spark plug silver、Black、Yellow、What about blue and other colors?

The main component of the metal shell of the spark plug (except for the electrode and gasket parts) is steel，A thin layer of anti-corrosion coating is electroplated on the outside。

When steel is heated，Due to the effect of steel and oxygen in the air，An extremely thin layer of oxide film (below 500℃) is produced immediately，The color of the oxide film changes with temperature。After the temperature gets higher，The temperature of steel also changes。In this way, you can roughly estimate the approximate temperature of each part of the spark plug。

The color of the spark plug housing is evidence of whether the spark plug is leaking or overheating，Of course，Don’t be dogmatic，Also exclude gasoline components、Gasoline additive ingredients、The influence seabet myand change of engine oil composition on the color of spark plug housing。

The relationship between the color and temperature of heated steel.

Summary: The spark plug "pretends" to leak，Almost no impact on vehicles。When installing the spark plug，Because of too much strength、Air leakage caused by installation that is too tight and due to insufficient strength、Air leakage installed too loosely，This is what auto repair practitioners should pay attention to。