Replacing spark plugs: a step-by-step algorithm for beginners and professionals

Replacing spark plugs is one of those procedures that car owners often put off until the last minute. But in vain: worn spark plugs not only worsen the dynamics of the car, but can also cause increased fuel consumption (up to 30%!), unstable engine operation and even detonationfraught with serious damage. In this article, we will look at how to properly change spark plugs yourself - from choosing tools to the intricacies of installing new parts.

Spark plugs are the “heart” of the ignition system of a gasoline engine. They generate a spark that ignites the air-fuel mixture in the cylinders. Over time, the spark plug electrodes wear out, the gap increases, and the quality of the spark deteriorates. Manufacturers recommend changing spark plugs every 30–100 thousand km (depending on the type of spark plugs and car model), but the actual service life depends on the quality of the fuel, driving style and engine condition. For example, on GAZelle Next with engine UMZ-4216 candles NGK BPR6ES can last up to 60 thousand km, and Toyota Corolla with motor 1ZR-FE — all 120 thousand km when using iridium spark plugs.

If you have never replaced spark plugs, do not be alarmed: the procedure is simpler than it seems. The main thing is to be consistent, take your time, and avoid common mistakes (we’ll talk about them below). And for those who already have experience, the article will contain useful life hacks - for example, how to check spark plugs without special equipment or what to do if a spark plug is “stuck” to the block head.

1. When is it time to change spark plugs: 7 obvious signs

Many drivers focus only on mileage, but spark plugs can fail even earlier. Here key symptomsindicating the need for replacement:

• 🔥 Engine tripping — noticeable vibration at idle, especially in the morning or after a long stay. One or more cylinders are unstable.
• ⚡ Misfires — the engine “sneezes”, jerks during acceleration, loses power. The dashboard may light up Check Engine.
• 🛢️ Increased fuel consumption — if consumption increases by 10–15% for no apparent reason, the spark plugs (or oxygen sensor) may be to blame.
• 🚗 Poor engine starting — the starter turns, but the engine does not “catch” the first time, especially in cold weather.
• 💨 Black or white carbon deposits on the electrodes - indicates incorrect operation of the spark plug (over-rich or lean mixture).
• 🔧 Visible damage — cracks in the insulator, melted electrodes, traces of oil or fuel on the threads.
• ⏳ Mileage exceeded recommended interval - even if the spark plugs “seem to be working,” after 80–100 thousand km their service life is running out.

If you notice at least 2-3 signs from the list, do not delay diagnosis. To check the spark plugs, you don’t have to go to a service station: you can unscrew them and inspect them yourself (more on this in the next section).

2. Preparing for replacement: tools and safety precautions

Before you get started, make sure you have everything you need. Minimum set of tools:

• 🔧 Candle key with a rubber bushing (so as not to damage the insulator) or socket head with extension cord. Fits most cars size 16 mm or 21 mm.
• 🔩 Ratchet handle or crank - for ease of unscrewing.
• 📏 Gap gauge (if you install regular spark plugs, not iridium/platinum spark plugs with a fixed gap).
• 🧤 Dielectric grease (optional) - applied to the threads of new spark plugs to prevent sticking.
• 🧹 Compressor or can of compressed air - to blow out the wells before installing new spark plugs.
• 🔦 Flashlight - in some cars, access to spark plugs is difficult (for example, in VW Passat B6 or Ford Focus 3).

Also prepare new spark plugs. Their make and model can be found:

1. In the vehicle owner's manual.
2. On a sticker under the hood (sometimes recommended spare parts are indicated there).
3. Through online catalogs by VIN code (for example, Autodoc, Exist or Denso Cross Reference).

Security measures:

⚠️ Attention: Never change spark plugs on a hot engine! The metal of the cylinder head expands when heated, and you risk stripping the threads. Allow the engine to cool for at least 2-3 hours.

• Disable negative battery terminal - this will prevent a short circuit when accidentally touching high-voltage wires.
• Do not use WD-40 or other penetrating lubricants for unscrewing spark plugs! They can get into the cylinder and cause water hammer.
• If the candle “sticks”, do not use excessive force - it is better to use special puller or contact a service station.

3. Step-by-step instructions: how to unscrew old spark plugs

Now let's move on to practice. Algorithm of actions:

1. Clear the area around the candles. Dust and dirt can get into the cylinders when unscrewing the spark plugs. Use a compressor or brush.
2. Remove high voltage wires or ignition coils.
   • In older cars (eg. VAZ 2107) - simply pull the wires off the lugs by pulling the cap.
   • In modern machines (for example, Kia Rio 4 or Hyundai Solaris) - unscrew the bolts securing the coils (usually 10 mm) and carefully remove them.

Unscrew the spark plugs. Insert the spark plug wrench into the well and turn it counterclockwise. If the candle does not budge:

• Try screwing it back a little (1/4 turn), and then unscrewing it again - this will help “break” the thread.
• Use extension with universal joint, if access is limited.

The engine has cooled down (temperature no higher than 40°C)|

Negative battery terminal disconnected |

The spark plug wells have been cleared of dirt|

There are new candles and tools at hand-->

What to do if the candle breaks off?

⚠️ Attention: If, when unscrewing, the spark plug breaks and part of it remains in the cylinder head, do not try to remove the fragment using improvised means! Use candle extractor or contact specialists. Incorrect actions can damage the threads in the cylinder head, and its restoration will cost 10–15 thousand rubles.

After removing the spark plugs, inspect them:

• 🔍 Normal Wear: light brown or gray carbon deposits, evenly distributed over the electrodes.
• 🛑 Thread oil: indicates problems with valve stem seals or piston rings.
• ⚡ Melted electrodes: a sign of detonation or overheating (possibly incorrect glow number of spark plugs).

How to check spark plugs without a tester?

Remove the high-voltage wire and insert a known-good spark plug into it. Place its metal part against ground (for example, against a valve cover) and crank the starter. If the spark is blue and stable, the spark plug is working. If there is no spark or it is weak (orange), the spark plug requires replacement. Attention: Hold on to the insulated part of the wire to avoid electric shock!

4. Installing new spark plugs: nuances and mistakes

Before installing new spark plugs, check:

• 📏 Gap between electrodes. For most modern cars it is 0.8–1.1 mm. The exception is iridium spark plugs (for example, Denso Iridium Power), where the gap is fixed.
• 🔧 Carving. On new spark plugs it should be clean and free of burrs. If necessary, banish it tap (for example, M14x1.25 for most passenger cars).
• 💧 Availability of lubrication. For some candles (for example, Bosch Super Plus) the manufacturer applies a non-stick coating - it does not need to be washed off!

Installation sequence:

1. Carefully insert the candle into the well hands (without a key!) so as not to distort the thread.
2. Tighten the spark plug by hand all the way, then tighten with the key:
   • For aluminum cylinder head: 20–25 Nm (for example, Honda Civic or Mazda 3).
   • For cast iron cylinder head: 30–40 Nm (for example, UAZ Patriot or old Mercedes-Benz).

Reinstall the ignition coils or high-voltage wires. Check that they seat until they click (on coils) or press the contact tightly (on wires).

Critical error: over-tightening of candles! With a tightening torque of over 40 Nm, the risk of thread failure in an aluminum cylinder head is 80%. If you don’t tighten it enough, the spark plug will “shoot” and quickly fail.

Candle type	Tightening torque (Nm)	Examples of car models
Regular (nickel)	20–25	Lada Vesta, Renault Logan
Platinum	18–22	Toyota Camry, Honda CR-V
Iridium	15–20	Mazda CX-5, Subaru Forester
For cast iron cylinder head	30–40	UAZ Hunter, Chevrolet Niva

5. Checking after replacement: how to make sure everything is done correctly

After installing new spark plugs:

1. Connect battery and start the engine. It should start the first time and run smoothly (no vibrations).
2. Listen to the engine at idle speed - there should be no extraneous noise (knocks, claps).
3. Check Check Engine - if it was on before replacement, it should now go out (you may need to reset the error with the scanner).
4. Do test drive:
   • Accelerate to 60–80 km/h - acceleration should be smooth, without jerking.
   • Brake the engine (let off the gas in gear) - there should be no “twitching”.

If after replacing the spark plugs the problems remain (for example, the engine is running rough or fuel consumption has not decreased), check:

• 🔌 High voltage wires — may break through the insulation (especially important for older cars).
• 📊 Ignition coils - measure the resistance of the primary and secondary windings (standard: 0.5–2 Ohm and 6–15 kOhm respectively).
• 🛢️ Fuel system - clogged injectors or a faulty fuel pump also cause similar symptoms.

6. Common mistakes when replacing spark plugs (and how to avoid them)

Even experienced car owners sometimes make mistakes. Here top 5 most dangerous:

1. Using unsuitable candles.

   Candles differ in:

   • Heat number (for example, NGK BPR6ES - “hot” NGK BPR8ES - “cold”)
   • Thread size (M14x1.25 or M12x1.25).
   • Thread length (a short spark plug in a long well will lead to carbon deposits on the piston).

⚠️ Attention: Installing spark plugs with the wrong heat rating can lead to piston burnout (if the candle is too “hot”) or loss of power (if it’s too “cold”).

Dirt getting into the cylinders.

If the wells are not cleaned before removing the spark plugs, sand or dust will enter the combustion chamber and act as an abrasive, accelerating wear on the piston group.

Over-tightening or under-tightening of candles.

Tightening too tightly deforms the threads in the aluminum cylinder head, while tightening too weakly leads to loss of compression and “firing” of the spark plugs.

Ignoring the order of operation of the cylinders.

When connecting high-voltage wires, it is easy to mix them up. For example, on VAZ 2114 cylinder operating order: 1–3–4–2. If you mix it up, the engine will trip.

Replacement of only part of the spark plugs.

If you change spark plugs, then all at once! Installing 2-3 new spark plugs and leaving the old ones will lead to an imbalance in the operation of the cylinders.

To avoid mistakes, follow a simple rule: “Measure 7 times, unscrew once”. Before starting work, check:

• Compatibility of spark plugs with your engine (according to catalog or VIN code).
• Availability of all tools and spare parts.
• Engine temperature (it must be cold!).

7. How to extend the life of spark plugs

Even the most expensive iridium spark plugs will not last long if you do not follow simple rules:

• ⛽ Refuel with quality fuel. Bad gasoline with a high sulfur content or additives forms carbon deposits on the electrodes. Optimal choice - AI-95 or AI-98 from fuel networks with a good reputation.
• 🔧 Monitor the condition of the engine.
  • Check compression (norm: 12–14 bar for gasoline internal combustion engines).
  • Change valve stem seals at the first sign of oil on the spark plug threads.
• 🚗 Avoid short trips. If you often drive for distances less than 5 km, the engine does not have time to warm up and carbon deposits form on the spark plugs. Once a month, let the engine idle for 10–15 minutes or drive 20–30 km along the highway.
• 💡 Check the spark plugs every 15–20 thousand km. Even if they “seem to be working,” a visual inspection will help identify problems early.

For iridium and platinum spark plugs (Denso IK20, NGK Iridium IX) the same rules apply, but their resource is longer - up to 100–120 thousand km. However, they are sensitive to fuel quality: if you fill with cheap gasoline, even iridium will quickly become covered with soot.

8. Top 5 questions about spark plugs (FAQ)

❓ Can spark plugs be cleaned and reused?

Theoretically yes, but not recommended. Mechanical cleaning (with sandpaper or a wire brush) damages the electrode coating, while chemical cleaning (for example, Cleaner for carburetors) does not remove carbon deposits completely. Exception - sandblasting, but it requires special equipment and is justified only for expensive spark plugs (for example, iridium).

If the candle has already used up more than 50% of its resource, it is better to replace it - cleaning will extend its life by a maximum of 5-10 thousand km.

❓ Which candles are better: iridium, platinum or regular?

The choice depends on budget and driving style:

• Regular (nickel): cheap (from 100 rubles/piece), but last 30–50 thousand km. Suitable for older cars (eg VAZ 2109).
• Platinum: resource 80–100 thousand km, self-cleaning better. The best option for most modern cars (Toyota, Honda).
• Iridium: the most durable (up to 120 thousand km), give a more powerful spark. Ideal for turbocharged and forced engines (Subaru WRX, Mitsubishi Lancer Evo).

For gas equipment (GBO) it is better to choose candles with small gap (0.7–0.9 mm) and high heat rating (for example, NGK LPG1 or Denso W20EPR-U11).

❓ Why do new candles quickly become covered with soot?

There are several reasons:

1. Rich air/fuel mixture - check oxygen sensor (lambda probe) and injectors.
2. Oil entering the combustion chamber — oil seals or piston rings are worn out.
3. Low fuel quality - change the gas station.
4. Incorrect heat rating - spark plugs that are too cold will burn out faster.

If carbon deposits appear after 1–2 thousand km, diagnose the engine - the problem lies deeper than just the candles.

❓ Do I need to lubricate the candle threads before installation?

This optional, but recommended for:

• Aluminum cylinder heads (for example, on Nissan Almera or Renault Duster).
• Spark plugs with nickel-plated threads.
• Cars operated in high humidity conditions.

Use special dielectric grease (for example, Permatex 22058) or copper paste. Do not use WD-40, Litol or graphite lubricant! They cannot withstand high temperatures and can cause burning.

❓ Is it possible to replace spark plugs yourself if you have no experience?

Yes, but start with a simple car (for example, Lada Granta or Kia Rio, where access to candles is free). Difficulties may arise with:

• Engines V6/V8, where the candles are located in two rows (for example, Ford Explorer or Toyota Land Cruiser 200).
• Cars with individual ignition coils (for example, BMW N46), where you need to carefully remove the connectors.
• Models where access to spark plugs requires remove the intake manifold (for example, Opel Astra H).

If you're not sure, take a look video instructions for your model or contact your service station. Mistakes when replacing spark plugs can cost more than the work itself.