Candles in a car: what they are, what they are needed for and how to maintain them

When it comes to spark plugs in a car, most drivers immediately think of spark plugs - and this is correct, but not completely. In fact, a modern car can have up to three different types of spark plugs, each performing a critical function. Not only engine starting in cold weather, but also fuel consumption, acceleration dynamics, and in some cases even traffic safety depend on their condition.

In this article we will look at all types of candles, which are installed in cars: from classic spark plugs to specialized glow plugs for diesel engines and rare preheating systems. You will learn how they work, what signs indicate they are faulty, and whether their service life can be extended. And also - why do some cars have twice as many glow plugs as cylinders, and how does this relate to Euro 6 environmental standards?.

1. Spark plugs: the heart of a gasoline engine

Spark plugs are the most common type of spark plugs that are installed in gasoline engines (including hybrids and cars with gas equipment). Their main task is to ignite the fuel-air mixture in the cylinders at a strictly defined moment. A spark jumping between the electrodes initiates an explosion of the mixture, which pushes the piston down and causes the crankshaft to rotate.

Structurally, the spark plug consists of:

• 🔌 Contact pin - connects to a high-voltage wire or ignition coil.
• 🛡️ Isolator — ceramic case that protects against breakdown and overheating.
• ⚡ Central and side electrodes — a spark jumps between them.
• 🔧 Threaded part — Screwed into the cylinder head.

Modern spark plugs are divided into several types based on electrode material:

• 🔹 Nickel - a budget option, lasts up to 30-50 thousand km.
• 🔹 Platinum - more durable (up to 100 thousand km), better resistant to erosion.
• 🔹 Iridium — premium segment, resource up to 120–150 thousand km, thin central electrode for a more powerful spark.

⚠️ Attention: If your car has iridium spark plugs, never clean them with a sandblaster or wire brush. Iridium is a soft metal, and these cleaning methods will destroy the electrode in a matter of seconds.

2. Glow plugs: why diesel doesn’t like cold

Unlike gasoline engines, where the fuel is ignited by a spark, diesel engines the mixture ignites when compressed. But at low temperatures (below +5°C), compression alone is not enough - the air in the cylinders does not warm up to the required temperature. This is where they come to the rescue glow plugs.

Their task is to heat the air in the combustion chamber to 800–1000°C in a matter of seconds, facilitating cold starts. In modern diesel engines, glow plugs work not only during startup, but also for 3–5 minutes after it, reducing exhaust toxicity and improving fuel combustion.

By design, glow plugs are:

• 🔥 Pin (rod) - a classic version with a metal spiral inside.
• 🔥 Ceramic - heat up faster (in 2-3 seconds), but are more expensive and fragile.
• 🔥 Self-heating with open spiral - used in old diesel engines (until the 2000s).

Interesting fact: some diesel engines with a BlueTec (for example, Mercedes-Benz OM642) glow plugs may be twice as many cylinders. This is due to the need to quickly warm up the catalyst to comply with the standards. Euro 6.

Candle type	Heating time	Service life	Application
Pin metal	5–10 seconds	60–100 thousand km	Old diesels, trucks
Ceramic	2–3 seconds	100–150 thousand km	Modern passenger diesels
Self-heating	10–15 seconds	30–50 thousand km	Diesels before 2000, tractors

3. Glow plugs: a forgotten but useful attribute

Few people know, but some cars (especially older ones or those intended for extremely cold climates) are equipped with glow plugs. Unlike glow plugs, they heat not the air in the cylinders, but coolant or oil in the pallet.

Such systems are found:

• ❄️ B military vehicles (for example, UAZ-469 or GAZ-66).
• ❄️ B northern modifications auto (for example, Toyota Hilux Arctic Trucks).
• ❄️ B old diesel trucks (for example, ZIL-130).

Glow plugs are:

• 🔌 Electrical - run on battery power, heat the liquid in the cylinder block.
• 🔥 Liquid - use hot gases from fuel combustion (for example, Webasto or Eberspächer).

⚠️ Attention: If your car has a spark plug preheating system, never start it while the engine is running. This can lead to overheating and cracks in the cylinder block.

How to check the operation of the glow plugs?

To check electric glow plugs, disconnect the wires from them and measure the resistance with a multimeter. The normal value is 0.5–6 Ohm (depending on the model). If the resistance tends to infinity, the candle has burned out. For liquid systems, check the operation of the pump and the integrity of the pipes.

4. How to understand that it’s time to change the spark plugs: symptoms and diagnosis

Faulty spark plugs (of any type) manifest themselves in different ways, but there are common signs:

• 🚗 Difficult start — the engine spins for a long time with the starter, especially when cold.
• 💨 Dips during acceleration — the car jerks, does not pull.
• ⛽ Increased fuel consumption - 10–20% higher than usual.
• 🔊 Unstable idle — the revolutions “float”, the engine troits.

To diagnose spark plugs:

1. Unscrew the spark plug and inspect it:
• 🔴 Black soot - rich mixture or oil in the combustion chamber.
• ⚪ White soot - lean mixture or overheating.
• 🟠 Red plaque - low-quality fuel with additives.

Check the gap between the electrodes (must comply with manufacturer's recommendations).

Test the spark on a special stand or “ground” (carefully!).

For glow plugs:

• Use the multimeter in resistance test mode.
• On some models (for example, VW TDI) you can hear the spark plug relay clicking when you turn on the ignition - their absence indicates a malfunction.

Unscrew the spark plugs (only on a cold engine!) |

Inspect the deposits and condition of the electrodes|

Check the gap with a feeler gauge|

Test the spark on a stand or to ground |

Compare with a new spark plug (if possible) -->

5. Replacing spark plugs: step-by-step instructions and nuances

You can replace the spark plugs yourself, but there are important nuances. To get started, prepare your tools:

• 🔧 Spark plug wrench (preferably with a magnetic grip).
• 🔧 Socket wrench with extension (for some models).
• 🔧 Torque wrench (so as not to overtighten).
• 🔧 Feeler gauge for checking the gap.

Procedure for replacing spark plugs:

1. Allow the engine to cool (working on a hot engine can damage the threads).
2. Disconnect high voltage wires or coil connectors.
3. Unscrew the old spark plugs, clean the wells from dirt (you can use compressed air).
4. Check the gap on new spark plugs (if adjustment is required).
5. Screw in new spark plugs by hand, then tighten with a torque wrench (the tightening torque is indicated in the manual).
6. Connect the wires/coils in the correct order (observe the cylinder numbering!).

For glow plugs:

• Disconnect the battery (required!).
• Remove the protective cover (if equipped).
• Disconnect the power wires.
• Unscrew the spark plugs (they often stick - use penetrating lubricant).
• Install new ones, observing the tightening torque (usually 15–20 Nm).

⚠️ Attention: When replacing glow plugs on diesel engines Common Rail (for example, BMW M57 or Audi 2.7 TDI) Never use copper grease on threads. It can enter the combustion chamber and cause detonation.

6. Myths and misconceptions about candles

Candles are one of those parts that have a lot of myths surrounding them. Let's look at the most common ones:

Myth 1: "The more expensive the candles, the better." Reality: Iridium spark plugs are indeed more durable, but for an old carburetor engine (for example, VAZ 2106) they will not give any increase in power. Choose spark plugs according to the car manufacturer's recommendations, and not based on price.

Myth 2: "Glow plugs are only needed in winter." Reality: In modern diesel engines (for example, Volkswagen 2.0 TDI) glow plugs also turn on at positive temperatures to reduce exhaust toxicity. Their malfunction may lead to an error P0380 (heat circuits) even in summer.

Myth 3: "If the engine is running normally, you don't need to change the spark plugs." Reality: Spark plugs wear out gradually and their degradation may not be noticeable. For example, increasing the gap between the electrodes by 0.1 mm increases the breakdown voltage by 1000–1500 V, which increases the load on the ignition coils.

Myth 4: "All glow plugs are the same." Reality: Candles differ in voltage (5V, 12V), heating time, length and diameter. For example, candles for Mercedes OM617 (old diesel) and BMW N47 (modern turbodiesel) are not interchangeable.

7. How to extend the life of candles: care tips

The service life of spark plugs depends not only on their type, but also on operating conditions. Here are some tips to help extend their life:

• ⛽ Use quality fuel. Additives in cheap gasoline or diesel fuel form carbon deposits that “eat” the electrodes.
• 🔧 Monitor the condition of the valve stem seals. If oil gets into the combustion chamber, the spark plugs quickly coke.
• 🔥 Avoid running the engine at idle for long periods of time. This leads to incomplete combustion of fuel and the formation of soot.
• 💨 Check compression in cylinders regularly. Low compression increases the load on the spark plugs.

For glow plugs:

• 🔌 Before winter, check their performance (especially if the car is more than 5 years old).
• 🔥 Do not turn on glow plugs more than 3 times in a row without starting the engine - this reduces their resource.
• 💧 Monitor the antifreeze level. Its leakage can lead to overheating of the spark plugs.

If you frequently drive short distances (especially in winter), the glow plugs will wear out faster due to frequent heating/cooling cycles. In this case, it is worth considering installing ceramic candles, which are more resistant to such loads.

FAQ: Frequently asked questions about spark plugs in cars

Is it possible to drive with a faulty spark plug?

Technically possible, but not advisable. A broken spark plug leads to:

• Loss of power (up to 25%).
• Increased fuel consumption (by 10–15%).
• Unburned gasoline gets into the catalyst, which shortens its service life.
• Vibrations and discomfort when moving.

On some machines (for example, with a COP — Coil On Plug) Ignoring the problem for a long time can damage the ignition coil.

How many glow plugs are there in a diesel engine?

In most diesel engines, the number of glow plugs is equal to the number of cylinders (for example, 4 plugs on a 4-cylinder engine). However, in some modern diesel engines (for example, Mercedes OM654 or Audi 3.0 TDI) could be:

• Two spark plugs per cylinder (for quick warm-up).
• Additional spark plugs in the exhaust manifold (to heat the catalyst).

The exact quantity is indicated in the repair manual for your model.

How do spark plugs for gas differ from regular spark plugs?

Spark plugs for cars with gas cylinder equipment (GBO) have several features:

• Narrower gap between the electrodes (0.7–0.9 mm versus 1.0–1.3 mm for gasoline).
• High-quality electrode materials (most often iridium or platinum), since the gas burns at a higher temperature.
• Increased heat index (plugs must be “cooler” to avoid hot ignition).

Examples of suitable candles for HBO: NGK LPG2, Denso IW20, Bosch WR7DPX.

Why do glow plugs stop working all at once?

If all the glow plugs fail at the same time, the problem is most likely not with them, but with:

• Glow plug relay (common problem on VW Golf IV or Opel Astra H).
• Spark plug control unit (on modern diesel engines).
• Wiring or fuses.
• Discharged battery (spark plugs require high current, especially in cold weather).

Check the voltage at the spark plugs with a multimeter with the ignition on. If there is no voltage, the problem is in the power circuit.

Can spark plugs be cleaned and reused?

Technically yes, but with caveats:

• ✅ Can be cleaned: nickel spark plugs with slight carbon deposits (sandblasting or ultrasound).
• ❌ Cannot be cleaned: iridium or platinum spark plugs - their electrodes are too thin and will be damaged.
• ⚠️ Limitations: even after cleaning, the life of the spark plug is reduced, since the wear of the electrodes is irreversible.

It is better to consider cleaning as a temporary measure. For example, if the spark plugs are filled with oil due to faulty caps, they can be cleaned and taken to a service center.