Spark plugs are responsible for delivering the high voltage spark that will ignite the air-fuel mixture to kickstart the combustion process.
Several issues can arise with one or more spark plugs. One of these is black deposits.
When a new spark plug turns black, that doesn’t necessarily mean that you received a faulty unit. This phenomenon is called carbon fouling, which can be caused by four things:
Let’s take a closer look:
When the engine runs too cold (thermostat stuck open or opening far too soon), it runs richer and may slightly soot the plugs with hydrocarbon fouling if the engine is running entirely too cold, especially in the winter.
An overly rich fuel condition means there isn’t enough oxygen to match the amount of fuel.
Too much fuel can cause carbon soot to form on the spark plugs, which may not cause misfiring until it reaches a certain point. If the rich condition is bad enough to wet the spark plugs, the engine may not start.
An overly rich air-fuel mixture may be caused by one of the following:
The oxygen (O2) sensor measures the oxygen content of the exhaust between the exhaust manifold and the light-off converter.
There is another O2 sensor behind the light off converter to measure the efficiency of that converter. The oxidation catalyst is farther downstream in the exhaust and isn’t monitored by any O2 sensor.
If the powertrain control module (PCM) is unable to get accurate readings from the O2 sensor, the computer won’t be able to adjust fuel levels accordingly, which can result in an overly rich mixture.
A mass airflow sensor helps the PCM determine the amount of fuel needed based on the temperature and volume of incoming air. It’s mounted between the air filter and the throttle body.
The sensor also backs up the throttle position sensor in case it fails, and the throttle position sensor backs up the MAF sensor if it fails out of range (it can fail in range, which the PCM can’t detect).
If a problem with the mass airflow sensor has been detected, the engine computer will use the readings from the throttle position sensor and engine speed to calculate fuel delivery.
Over time, dirt, oil, silicon, and other contaminants can coat the sensing wire, but note that this doesn’t cause the engine to run rich. It’s more likely to simply make the MAF sensor slow to respond on acceleration, which makes the engine feel sluggish.
The PCM relies on the manifold pressure sensor to get the following information:
Most modern vehicles have a MAP sensor AND a MAF. Some vehicles only have a MAP sensor. This is more common on vehicles that ONLY have a MAP sensor and no MAF sensor.
Similar to other sensors, a problem with the manifold pressure sensor can result in inaccurate fuel delivery calculations. In the case of the manifold pressure sensor, it might report the wrong air pressure numbers to the computer, resulting in a rich fuel mixture.
The engine coolant temperature sensor monitors the coolant temperature so that the PCM can make the necessary adjustments in the fuel mixture. If the ECT indicates that the engine is too cold when it isn’t, the PCM may never enter closed loop fuel control. However, in most cases, this won’t cause soot on the plugs because of the efficiency of the PCM algorithms.
Typically, a cold engine will require a rich fuel mixture to prevent stalling. This may not be sensor related. If the engine is actually running cold or is frequently operated very briefly without allowing it to warm up, cold enrichment will foul the spark plugs and can even wash down the cylinder walls, causing the engine to lose compression so that it sounds like the timing belt has jumped on modern engines with low tension piston rings.
The PCM needs to determine the engine temperature to properly control the spark timing and fuel mixture.
Problems with the coolant temperature sensor can result in an overly rich or lean fuel mixture.
The fuel pressure regulator works with the fuel pump to maintain the appropriate amount of pressure drop at the injector tips.
If the fuel pressure regulator sticks, it can cause the pressure to max out. This can happen even on fuel systems that have the fuel pressure regulator mounted in the tank with the pump. If the fuel pressure regulator is mounted on the fuel rail, it’ll have a vacuum hose connected to it, and if the diaphragm ruptures, fuel will go through the vacuum hose and into the manifold.
A weak ignition system output can be attributed to the ignition coil. This usually causes no-starts rather than sooty plugs, however.
Spark plugs are often the root cause of engine performance problems. The spark plug is prone to developing soot, which can be caused by other parts from various systems.
Spark plugs can also have wet, oily deposits with little electrode wear, which is commonly referred to as oil fouling.
Spark plugs covered in oil are an indication of oil entering the combustion chamber. In most cases, this issue can be traced to the following:
In some cases, a weak ignition wire will still run fine under cold temperatures. When temperatures rise, however, the wire insulation can break down and cause the engine to misfire. If the ignition wire’s carbon core fails at the end of the wire the wire may have rust or soot inside the boot when you disconnect it from the plug.
A compromised insulation material will usually have its molecules farther apart, creating a new path for electricity to flow instead of jumping the gap of the spark plug.
A black spark plug is usually an indicator of fuel and ignition system problems. Also known as carbon fouling, this issue can affect even newly installed spark plugs.
Excessive idling and a defective thermostat are also potential reasons why the spark plug can turn black.
When resolving this problem, a thorough inspection of the engine is important to accurately determine the part that’s causing the problem. Common solutions typically involve replacing the faulty parts or cleaning them to keep them functional.
If you still need to replace the spark plugs, it’s better to get high-quality replacements to avoid expensive repairs down the line. Shop at CarParts.com or download our mobile app to access a wide variety of auto parts, accessories, and tools that are built to last.
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