7.1.8. Spark plugs - the general information

General description

The candle serves for transfer of the high-tension current developed by the ignition coil to the electrodes lowered in the combustion chamber where this current makes the spark category providing ignition of working mix.

In the combustion chamber the candle is exposed to cyclic thermal and barometric influences, besides, not the last role the climatic factor plays.

One of the main requirements imposed to a spark plug are its working capacity at a voltage over 30 kV and ability of its isolation to resist to destruction at influence of temperatures to 1000 °C. Besides, the part of a candle placed in the combustion chamber has to have the good resistance of high-temperature corrosion.

One more important parameter providing reliability of functioning of a spark plug is its heat-removing ability. As the candle is exposed to the bystry periodic changes of temperature condition determined by serial its contact with the cold working mix given to the cylinder, with the combustion products warmed up to the huge temperatures, its ceramic insulator has to be extremely steady against influence of cyclic thermal loadings.

Typical design of a spark plug

1 — the Contact head with a nut
2 — the Ceramic insulator
3 — the Case
4 — the Thermoshrinkable flute

5 — Conducting стеклогерметик
6 — Laying
7 — the Bimetallic central electrode (Ni/Cu)
8 - Weight electrode

The recommended type of spark plugs is defined among other things by the kalilny number characterizing ability of a candle to resist to thermal loadings. At the same time, the kalilny number is higher, the temperature characteristic of a candle is lower.

Temperature characteristic of a spark plug

1 — Low kalilny number (a hot candle)
2 — Average kalilny number
3 - High kalilny number (cold candle)

The correct selection of candles for kalilny number allows to avoid admissions of ignition both during warming up of the engine, and in the state the mode of its functioning. Besides increase in stability of turns of the engine and improvement of indicators of its profitability prevention of interruptions of ignition allows to reduce significantly concentration of emission in the atmosphere of hydrocarbons (NANOSECONDS).

Stability and intensity of sparking of a candle is defined by the size of its interelectrode gap. It is at first sight preferable to have the maximum gap between the central and side electrode of a candle as increase in the size of a spark provides reliability of combustion of air-fuel mix due to increase in initial volume of its ignition. On the other hand, excessiveness of size of a gap increases probability that the penetrative tension developed by the coil it will be insufficient for obtaining the steady spark category under adverse circumstances, especially at the end of candle service life.

On modern engines, differing in high extents of compression, for providing steady ignition and an exception of admissions of ignition of working mix during all service life of a candle it is necessary to treat observance of a rated interelectrode gap especially attentively.

Replacement of spark plugs should be made strictly according to the Schedule of routine maintenance of the car (see the Section Schedule of Routine Maintenance of the Head the Current leaving and service), - the description of the procedure is provided in the same place, in the Section Replacement of Candles of an Incandescence.

On appearance of a candle it is possible to draw a conclusion on work of candles, quality management of mix and a condition of the engine (pistons, piston rings, etc.).

Coal deposits

Symptoms: Availability of soot indicates reenrichment of fuel-air mix or weak intensity of a spark. Causes admissions of ignition, complicates start and results in instability of operation of the engine.
Recommendations: Whether check the air cleaner is hammered whether fuel level in the float-operated camera is not too high, whether the air gate is jammed and whether contacts are not too worn-out. Try to use candles with longer insulator that increases the resilience to pollution.


Symptoms: Oiling of a candle is caused by wear of maslootrazhatelny caps. Oil gets to the combustion chamber through worn-out guides of valves or piston rings. Causes admissions of ignition, complicates start and results in instability of operation of the engine.
Recommendations: Perform mechanical recovery work and replace candles.


Symptoms: Porous, white insulator, erosion of electrodes and lack of any deposits. Leads to reduction of service life of a candle.
Recommendations: Check whether the kalilny number of the established candles conforms to requirements of Specifications of the Head Sistema of electric equipment of the engine, whether the ignition advancing corner is correctly exposed, whether too poor fuel-air mix moves, whether there is no leak of vacuum of the inlet pipeline and whether valves are jammed. Check also the level of a cooler and whether the radiator is corked.

Too early ignition

Symptoms: Electrodes are melted off. The insulator has white color, but maybe is polluted owing to admissions of ignition or hit in combustion chambers of foreign particles. Can lead to an engine exit out of operation.
Recommendations: Check kalilny number of the established candles, an ignition advancing corner, quality of mix (whether it is not too grown poor) whether the cooling system is corked and whether the lubrication system normally functions.

Electroconductive luster

Symptoms: The insulator has yellowish color and the polished appearance. Speaks about sudden temperature increase in combustion chambers at sharp acceleration. Usual deposits at the same time are melted off, taking a form of a varnish covering. Leads to admissions of ignition at high speeds of the movement.
Recommendations: Replace candles (colder, at preservation of a manner of driving).

Short circuit of electrodes

Symptoms: Waste of products of combustion gets to interelectrode space. Firm deposits accumulate, forming a crossing point between electrodes. Leads to refusal of ignition in the cylinder.
Recommendations: Remove deposits from interelectrode space.

Normal state of a candle

Symptoms: Gray-brown color and easy wear of electrodes. The Kalilny number of candles corresponds to type of the engine and its general state.
Recommendations: When replacing candles establish candles of the same type.


Symptoms: Soft deposits of brownish color on one or both electrodes of a candle. A source of their education are the applied additives to oil and/or fuel. Excessive accumulation can lead to isolation of electrodes and cause admissions of ignition and unstable operation of the engine at acceleration.
Recommendations: At bystry accumulation of deposits change maslootrazhatelny caps that will prevent hit of oil in combustion chambers. Try to replace fuel brand.


Symptoms: A rounding off of electrodes with a small congestion of deposits on the working end. Color is normal. Leads to difficulty of start of the engine in cold damp weather and to increase in fuel consumption.
Recommendations: Change candles on new, the same type.


Symptoms: Insulators can be chopped off or burst. Also inaccurate technology of adjustment of a candle gap can lead to damage of the insulator. Can lead to damage of pistons.


Symptoms: After admissions of ignition during a long period of adjournment can be loosened at preservation of working temperature in the combustion chamber. At high speeds of adjournment in flakes come off the piston and stick to the hot isolation center, causing admissions of ignition.
Recommendations: Replace candles or smooth out and establish into place old.

Mechanical damages

Symptoms: Can be caused by hit of foreign materials in the combustion chamber or arise at blow of the piston about too long candle. Lead to refusal of functioning of the cylinder and to damage of the piston.
Recommendations: Remove foreign particles from the engine and/or replace sveya.

At a vvorachivaniye of candles do not exceed the extreme admissible effort of tightening specified in Specifications of the Head Sistema of electric equipment of the engine.