A car spark plug is not just a consumable item, but an essential diagnostic tool available to any vehicle owner. Internal combustion of fuel is a complex chemical process, and the condition of the electrodes directly reflects the “health” of the cylinders. Many motorists, when unscrewing a part during routine maintenance, notice various digital markings or changes in the color of the ceramic insulator, but do not always understand what exactly they are talking about.
Numerical values can occur in two main contexts: as part of a factory marking indicating technical characteristics (heat number), or as a visual indicator of condition (a conventional “number” of mixture quality). Misinterpretation of this data may result in the installation of inappropriate components or the neglect of serious engine problems. In this article we will look in detail at how to read manufacturer codes and what the color of the soot will tell you about the operation of the engine.
Factory marking and heat rating
The first thing a buyer encounters in a store is the alphanumeric code on the product body. In the context of spark plugs figure most often denotes the heat number. This is a parameter that determines the ability of a part to remove heat from the working area to the cylinder head. The higher the number in the markings of domestic and many European manufacturers (for example, Bosch, NGK for some markets), the “colder” the spark plug.
So-called “hot” spark plugs have a low heat rating and are designed for engines with low compression ratios or for city driving at low speeds. They release heat slowly to maintain a high temperature and self-clean from carbon deposits. On the contrary, “cold” analogues with a high numerical value are necessary for forced engines and racing cars, where it is critical to avoid glow ignition — spontaneous ignition of the mixture from a hot electrode.
If you replace standard spark plugs with products with the wrong heat rating, the consequences can be fatal to the engine. A part that is too “hot” will cause detonation and burnout of the pistons, and a part that is too “cold” will quickly become covered with conductive carbon deposits, which will lead to misfires and engine tripping.
⚠️ Attention: Never rely solely on thread length when purchasing. The heat rating number is more important than geometry, since an error in heat transfer is not visible visually until the piston group fails.
To understand the difference, let’s look at the heat number correspondence table for popular brands, since each manufacturer has its own scale:
| Brand | Low number (Hot) | Average | High number (Cold) |
|---|---|---|---|
| NGK | 2 - 4 | 5 - 7 | 8 - 11 |
| Bosch | 7 - 9 | 10 | 11 - 14 |
| Denso | 16 - 20 | 22 - 24 | 27 - 31 |
| Brisk | 10 - 13 | 14 - 15 | 16 - 19 |
Why do different brands have different numbers?
Each manufacturer has its own test method and scale for measuring heat transfer. Bosch's number 7 is not equal to NGK's number 7. Always check cross-tabulations or applicability catalogs for your specific engine, rather than just looking for a similar one based on appearance.
Color code: diagnostics by soot
When it comes to the “number” or color on a spark plug in the context of diagnosing a running engine, we move on to a visual assessment of carbon deposits. An experienced mechanic will determine the composition of the fuel-air mixture at first glance. The ideal condition is characterized by a light brown or gray-beige tint of the insulator. This suggests that mixture formation normal, the ignition system is working correctly, and the thermal value is selected correctly.
However, deviations are more common. A black, velvety coating (similar to soot) indicates an over-enriched mixture. There is more fuel supplied than air, and it does not have time to burn completely. This could be caused by faulty injectors, a dirty air filter, or problems with the oxygen sensor. In such cases, fuel consumption increases sharply and power decreases.
The opposite problem is a mixture that is too lean, when there is more air than needed. In this case, the insulator becomes almost white, sometimes with a slight glossy sheen. The danger is overheating of the combustion chamber, which can lead to burnout of the valves. Also, black dots are often visible on the white insulator - traces of spark breakdowns.
Oil deposits and mechanical problems
Particular attention is required when a black, wet, oily coating is observed on the threads and electrodes. This is no longer a question of tuning the carburetor or flashing the ECU firmware. Such a “number” in the list of engine problems puts a bold cross on the condition of the cylinder-piston group (CPG). Oil enters the combustion chamber through worn valve stem seals or stuck piston rings.
If you notice this deposit on only one or two spark plugs, it may indicate a local problem in a particular cylinder, such as a damaged valve seal. If the entire set becomes oily, then the engine’s service life, alas, is coming to an end and a major overhaul is required. Ignoring this symptom will lead to the oil clogging the catalyst, disabling the expensive environmental system.
Diagnosis should be carried out comprehensively. In addition to the spark plugs, measure the compression. Low readings combined with oil deposits confirm ring wear. Also pay attention to the color of the exhaust: blue smoke from the pipe is a sure sign of oil combustion.
⚠️ Attention: Do not try to solve the problem of oil deposits by installing spark plugs with a higher heat rating. This will not eliminate the cause of oil leakage, but will only worsen engine overheating.
Red soot and fuel quality
Sometimes motorists encounter an unusual brick-red or orange coating on the electrodes. Many people mistake this for rust or a sign of a rich mixture, but it is actually a chemical reaction. This color indicates the presence of a large amount of metal-containing additives in the fuel, in particular ferrocene.
Ferrocene was added to gasoline to increase the octane number, but it is extremely harmful to modern engines. Deposits on the spark plugs conduct current, which leads to failures in sparking. The spark plug stops working normally, misfires appear, and the engine begins to jerk and jerk. In addition, these deposits are very difficult to remove by mechanical cleaning.
The only way to combat the “Red Death” is to change gas stations and buy quality fuel. If carbon deposits have appeared recently, sometimes a long trip on the highway at high speeds helps, but most often the set of spark plugs has to be changed, since the insulator has already lost its dielectric properties.
When purchasing gasoline, pay attention to the color and smell. Too bright yellow tint of fuel may indicate an excess of additives, which will subsequently turn the spark plugs red.
Algorithm for checking and replacing spark plugs
To obtain reliable information about the condition of the engine, it is necessary to remove the spark plugs correctly. The procedure seems simple, but requires compliance with certain safety rules and sequence of actions. Violation of technology can lead to damage to the threads in the block head or chipping of the ceramics.
Make sure the engine is completely cool before starting work. Unscrewing spark plugs on a hot engine can result in thread deformation and damage to the aluminum cylinder head. It is also necessary to thoroughly clean the space around the spark plug wells from dust and dirt so that debris does not get inside the cylinder.
☑️ Checklist for checking candles
When installing new elements, it is critical to maintain the tightening torque. Insufficient tightening will lead to overheating of the spark plug and possible scoring, and excessive tightening will damage the threads or destroy the o-ring. Use a torque wrench to control the force.
Typical tightening torques (Nm):M14 (standard): 20-30 Nm
M12: 15-20 Nm
M18: 30-40 Nm
(Always check the car manufacturer's manual!)
Resource and replacement intervals
The service life of spark plugs directly depends on their type and operating conditions. Standard nickel spark plugs require replacement every 20–30 thousand kilometers. They are the most affordable, but they lose their properties faster, the electrodes burn out, and the gap increases.
More durable are platinum and iridium analogues. Thanks to the use of precious metals on the electrodes, they can withstand higher temperatures and loads. The service life of such products can reach 60–100 thousand kilometers. However, their high price and sensitivity to fuel quality make their use not justified in every car.
Don't wait for the part to completely fail. Even if the engine starts, old spark plugs with too much gap force the ignition system to work harder, which can damage the coils or ignition module. Regular preventative maintenance is cheaper than repairing the high-voltage part.
Change spark plugs strictly according to the regulations or condition, without waiting for the engine to start running. Preventative replacement costs less than repairing ignition coils or converter.
Is it possible to clean old candles and put them back?
Mechanical cleaning (with a brush or sandblasting) is permissible only as a temporary diagnostic measure. During the cleaning process, the special coating of the electrodes is often damaged, and there is also a risk of changing the gap. For continuous operation, it is recommended to use only new spark plugs, since the cost of the part is not commensurate with the risk to the engine.
What does spark plug gap mean and how to check it?
The gap is the distance between the central and side electrodes through which the spark jumps. It must strictly comply with the engine manufacturer's specifications (usually 0.7–1.1 mm). A gap that is too small will produce a weak spark, while a gap that is too large can lead to coil breakdowns. The check is carried out with a special probe.
Why can spark plugs be wet from gasoline?
If the plug smells like gasoline and is wet to the touch, the mixture is not flammable. Reasons: lack of spark (malfunction of the coil, explosive wire) or “flooding” during a cold start. In winter, a short-term “flood” is acceptable, but constant humidity indicates a problem in the ignition or compression system.