Why is there fuel on spark plugs?

Sometimes car owners face a bizarre problem with spark plugs, respectively the presence of fuel on the spark plugs. This problem is often caused by using low-quality fuel.

Other problems might be with the ignition system, a bad starter that is rotating slowly, reduced compression in one or more engine cylinders, sensors incorrectly adjusting the air-flow mixture.

Most of the time the fuel appears on the spark plug due to a failed engine start. When it fails to start, fuel is injected into the combustion chamber, and the longer you turn the starter, the more difficult it will be to start the car and the worse it will be for the engine.

What are the symptoms of a spark plug with fuel on it?

  • Difficult engine start. Often, this is due to a bad or failing starter, but also the reasons may be other elements of the motor.
  • Unstable engine operation most often when the engine is cold. In this case, there may be a decrease in its dynamic characteristics.
  • The smoke from the exhaust is black and often it smells like fuel.

If, during this operation of the engine, the spark plugs are unscrewed directly, then most likely they will be wet, since they were filled with fuel. 

Causes of fuel on spark plugs

Low battery charge. The car battery is in a poor state and there are large leaks of current. If the starter will not function properly, rotates slowly, then most likely, if the fuel comes in, ignition will not occur. As a result, the spark plugs will be wet. Each unsuccessful attempt will only worsen the situation because a new portion of fuel will be supplied, which will fill the spark plugs.

You should check the battery state, especially if the temperatures outside are low. If the battery gives signs of failure, it’s better to change it.

  • Problems with the ignition system. A weak spark can be caused by problems with high-voltage wires in case of insulation failure, and also if the entire ignition system or one of the coils is damaged.
  • Bad or failing engine sensors. Failing sensors can also cause fuel on the spark plugs, although in rare cases. The electronic control unit (ECU) is responsible for the amount of fuel supplied, based on signals from a number of sensors.
  • Bad or failing spark plugs. If you have worn or damaged spark plugs, then it will more likely cause this issue, especially if the spark is poor, it will not ignite the fuel.
  • Low-quality fuel. Often the most common cause why fuel is present on the spark plug, which will lead also to increased consumption. If you see this issue on a cold engine then most likely the octane number does not match. In the warm season, low-quality fuel is easier to ignite than in the colder season.

For example, the amount of fuel needed when starting a cold engine and its operation as a whole is controlled by the coolant temperature sensor, the throttle position sensor, the MAF sensor, and the crankshaft position sensor.

So, if ECU receives the wrong info from those sensors, then it is very likely that too much fuel will be supplied to the engine cylinders, which will fill the spark plugs with fuel. To solve the problem, it is necessary to check the sensors, starting with the coolant sensor, being the first to tell the ECU to supply more fuel, continuing with the MAF sensor.

However, then in addition to the fuel correction data, it is advisable to look for errors in the ECU memory.

How to start the engine that has fuel on spark plugs?

To start the car if you have this issue:

  • Remove the spark plugs and after that, they need to be dried and calcined. This can be done at home on a gas stove. It is recommended to always carry with you another set with working spark plugs.
  • Blow out the cylinders with air. For this, an air compressor should be used whenever possible. If you don’t have one, then to dry it is necessary to completely press the gas pedal to the floor and turn the starter for 10 seconds. In this case, you also need to remove the connectors from the fuel injectors.
  • Unscrew the spark plugs and put the caps back on, and then try to start the engine again. You may need to repeat the drying steps several times, but usually, two or three times is enough.


If you have fuel on the spark plugs, then you will first need to check the battery for low charge voltage. Do that after you have high-quality fuel in your tank.

Further, if the problem is on the injection side, then it makes sense to connect a diagnostic device to the ECU and check the memory for errors associated with incorrect operation of the sensors.

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