A spark plug in a car is a key element of the engine: structure, functions and fault diagnosis

When it comes to “candle in the car”, 99% of drivers mean spark plug - a small but critical element, without which a gasoline engine simply will not start. This is not just a “part”, but a precise device responsible for igniting the air-fuel mixture in the cylinders. The stability of the engine, fuel consumption and even acceleration dynamics depend on its condition.

However, few people know that diesel engines also have spark plugs - glow plugs, performing a different, but no less important function. But in hybrid and electric vehicles, this term can mean completely different components. In this article we will figure out what it is candle in the context of a car, how it works, what types there are and how to recognize its malfunction in time to avoid serious damage.

What is a spark plug and how does it work?

A spark plug is an electrical device that converts high voltage electrical energy (from 20,000 to 45,000 V) in spark dischargeignites the fuel mixture in the cylinder. The process occurs in fractions of a second and is repeated thousands of times per minute, ensuring continuous operation of the engine.

Structurally, a candle consists of several key elements:

• 🔌 Contact pin - connects to a high-voltage wire or ignition coil.
• 🛡️ Isolator (usually ceramic) - protects against breakdown and overheating.
• 🔥 Central and side electrodes — a spark jumps between them.
• 🔧 Ribbed part of the insulator — prevents current leakage along the surface.
• 📏 Threaded body — Screwed into the cylinder head.

When voltage is applied from the ignition coil, a discharge is formed between the electrodes, the temperature of which reaches 2500–3000°C. This spark ignites the compressed air-fuel mixture, causing a micro-explosion that pushes the piston down and starts the engine cycle.

Types of spark plugs: what they are and how they differ

Candles are classified according to several criteria: electrode material, number of electrodes, heat rating and design features. The service life of the part and the stability of the motor depend on the correct choice.

By material of the central electrode highlight:

• 🔹 Copper/Nickel - budget option, resource up to 30,000 km.
• 🔹 Platinum - increased wear resistance (up to 90,000 km), are better at self-cleaning.
• 🔹 Iridium — premium segment, resource up to 120,000 km, thin electrode for a more powerful spark.

By designs:

• 🔧 Single electrode - classic version with one side electrode.
• 🔧 Multielectrode (2–4 side electrodes) - increase the service life and reliability of sparking.
• 🔧 flare - with a remote spark gap, used in some modern engines.

Also, candles differ in heat number — a parameter indicating the thermal operating mode. The higher the number, the “cooler” the candle (it removes heat better). Turbocharged engines usually require “cold” spark plugs, while naturally aspirated engines require “hot” spark plugs.

Glow plugs in diesel engines: what is the difference

Unlike gasoline engines, where spark plugs ignite mixture, used in diesel engines glow plugs, which warm up air in the combustion chamber. This is necessary for reliable engine starting, especially in cold weather, when air compression is not enough for diesel fuel to ignite spontaneously.

Structurally, a diesel spark plug is a metal rod with a heating coil inside. When the ignition is turned on, the coil heats up to 800–1000°C in a few seconds, making it easier to start the engine. In modern diesel engines, glow plugs can operate after starting, optimizing fuel combustion and reducing emissions.

Signs of faulty glow plugs:

• ❄️ The engine starts poorly when cold (especially at temperatures below +5°C).
• 🔊 White smoke appears from the exhaust pipe during startup.
• ⚠️ The indicator lights up on the dashboard Check Engine or spiral symbol (on some models).

⚠️ Attention: In diesel engines with a system Common Rail faulty glow plugs can lead to problems with the operation of injectors and increased fuel consumption by 10–15%.

Signs of faulty spark plugs: when is it time to change them?

Spark plugs wear out gradually, and their malfunction does not appear immediately. However, there are a number of symptoms that should alert the driver:

1. Problems starting the engine

• 🚗 The motor does not “catch” the first time or requires a long rotation of the starter.
• 🔋 The battery drains faster than usual due to prolonged scrolling.

2. Rough engine operation

• 🎢 “Trippling” - vibrations at idle, especially noticeable on the steering wheel or gearshift lever.
• 📉 Dips during acceleration or jerks when moving.

3. Increased fuel consumption

Inefficient combustion due to a weak spark causes the ECU (electronic control unit) to richen the mixture in an attempt to compensate for the loss of power. Consumption may increase by 5–20%.

4. Visual defects of spark plugs

When inspecting the spark plugs (if you have unscrewed them), pay attention to:

• 🔥 Carbon deposits on the electrodes - black (rich mixture), white (lean mixture) or red (fuel additives).
• 💥 Electrode melting or erosion - indicates overheating or detonation.
• 🛢️ Oil deposits — a sign of oil getting into the combustion chamber (wear of piston rings or seals).

How to check spark plugs yourself

Diagnostics of spark plugs can be carried out without special equipment. Here are the step-by-step instructions:

1. Visual inspection

Remove the spark plug (use spark plug wrench with a rubber seal so as not to damage the insulator) and assess its condition:

• 🔍 Normal candle - light gray or brown soot, no damage.
• 🚨 Faulty spark plug - cracks, melting, a thick layer of soot or wet traces of fuel.

2. Checking the spark (on a unscrewed spark plug)

Carefully insert the spark plug into the high-voltage wire, place it on a metal part of the engine (for example, a valve cover) and have an assistant crank the starter. Should slip between the electrodes bright blue spark. If the spark is weak, red or absent, the spark plug is faulty.

⚠️ Attention: When checking for spark, hold only the insulated part of the wire or use insulated pliers. Voltage in the ignition system can be deadly!

3. Check with a multimeter

Set the device to resistance measurement mode (20 kOhm) and connect the probes to the central electrode and the contact terminal. The resistance of a working spark plug should be within 5–15 kOhm (check the manual for exact values). No resistance or too high a value indicates a malfunction.

Symptom	Probable Cause	Solution
Black dry soot	Rich fuel mixture, late ignition	Check the oxygen sensor, injectors, ignition timing
White soot	Lean mixture, spark plug overheating	Check the tightness of the intake tract, the glow number of the spark plugs
Oil coating	Worn piston rings or valve seals	Engine diagnostics for oil waste
Melted electrodes	Detonation, overheating, incorrect heat rating	Replace spark plugs, check cooling system
No spark	Malfunction of the coil, wire or spark plug itself	Check the ignition circuit, replace the spark plug

When and how often to change spark plugs

The service life of spark plugs depends on their type, fuel quality and operating conditions. General recommendations:

• 🔧 Copper/Nickel - every 20,000–30,000 km.
• 🔧 Platinum - every 60,000–90,000 km.
• 🔧 Iridium - every 100,000–120,000 km.

However, these figures are approximate. The resource is affected by:

• ⛽ Fuel quality (additives and impurities accelerate wear of the electrodes).
• 🔥 Operating mode (frequent short trips lead to the formation of soot).
• 🛠️ Engine condition (oil burns or overheating reduce the life of the spark plugs).

Critical information: If your vehicle is equipped with natural gas equipment (LPG), spark plugs wear out 30-40% faster due to the higher combustion temperature of the gas. In this case it is recommended to use iridium spark plugs with reduced clearance or specialized “gas” spark plugs (for example, NGK LPG or Denso Iridium TT).

When replacing spark plugs, observe the following rules:

• 🔧 Use torque wrench for tightening (tightening torque is usually 20–30 Nm, exact data is in the manual).
• 🛠️ Apply to thread copper grease or special composition (for example, Liqui Moly Kupfer-Spray) to prevent sticking.
• 🚫 Never use candles with a different heat rating - this will lead to overheating or underheating.

Myths and misconceptions about spark plugs

There are many myths surrounding candles that can mislead the driver. Let's look at the most common ones:

Myth 1: “Cleaning spark plugs with sandblasting or sanding will prolong their life.”

In fact, mechanical cleaning damages the protective coating of the electrodes and the ceramic insulator, accelerating wear. The only safe way is ultrasonic cleaning at the service center.

Myth 2: “If the engine is running normally, you don’t need to change the spark plugs”

Spark plugs wear out gradually, and their degradation may not be noticeable until a critical moment. For example, increasing the gap between the electrodes by 0.1 mm increases the breakdown voltage by 3–5 kV, which leads to overload of the ignition coil.

Myth 3: “All candles are the same, you can use any”

This is a big mistake. Candles differ in:

• 📏 Thread size (for example, M14×1,25 or M12×1,25).
• 🔧 Thread length (short/long).
• 🔥 Heat number (cold/hot).
• 🛠️ Gap between electrodes (for example, 0.8 mm for gasoline and 0.6 mm for gas).

Myth 4: “Platinum and iridium spark plugs are marketing”

In fact, these materials have objective advantages:

• 🔹 Thinner central electrode → more powerful spark and better ignition.
• 🔹 High melting point → last longer no erosion.
• 🔹 The best self-cleaning from soot.

What happens if you don't change the spark plugs for a long time?

Misfire will lead to incomplete combustion of fuel, which will cause carbon deposits to form on the valves and pistons. In advanced cases, this can lead to detonation (micro-explosions in the cylinders), which destroys the pistons and connecting rods. Repairs will cost tens of thousands of rubles.

FAQ: Frequently asked questions about spark plugs in cars

Is it possible to drive with a faulty spark plug?

Technically possible, but highly undesirable. A faulty spark plug leads to:

• 🔥 Increased load on the ignition coil (may burn out).
• ⛽ Increased fuel consumption by 10–30%.
• 🚗 Loss of power and jerking during acceleration.
• 💥 Risk of detonation and engine damage.

If the spark plug fails along the way, it is better to replace it as soon as possible.

What gap should be between the spark plug electrodes?

The gap depends on the type of engine and ignition system:

• 🔧 Carburetor engines: 0.7–0.9 mm.
• 🔧 Injection (gasoline): 1.0–1.3 mm.
• 🔧 Engines with LPG: 0.6–0.7 mm (due to higher gas breakdown voltage).

The exact values are indicated in the vehicle's owner's manual.

What is the difference between spark plugs for gas and gasoline?

Candles for gas equipment (GBO) have:

• 🔥 More narrow gap (0.6–0.7 mm against 1.0–1.3 mm for gasoline), since gas requires a more powerful spark.
• 🛡️ Improved heat transfer (gas burns at a higher temperature).
• 💎 More often used iridium or platinum to increase resource.

Example models: NGK LPG LaserLine, Denso Iridium TT, Bosch Platinum WR7DPX.

Is it possible to adjust the gap on the spark plugs yourself?

Yes, but with caution. To do this:

1. Use special probe to measure the current gap.
2. If the gap is too large, carefully bend it side electrode pliers (not the central one!).
3. If the gap is too small, bend the electrode using a screwdriver or special tool.

⚠️ Do not use excessive force - ceramic insulator is fragile!

Why do new spark plugs quickly become covered with soot?

The reasons may be as follows:

• ⛽ Low fuel quality (impurities, high sulfur content).
• 🔧 Ignition system malfunction (problems with the coil or wires).
• 🚗 Rich fuel mixture (the oxygen sensor or air flow meter is faulty).
• 💨 Maslozhor (oil enters the combustion chamber through worn rings or seals).

If carbon deposits appear after 1000–2000 km, it is necessary to diagnose the engine.