What Causes Spark Plugs to Go Bad? (Reasons to Fail)

Spark Plug

Spark plugs are essential for engines. Without a proper functioning spark plug, the fuel mixture will not ignite and combust completely to generate maximum power. If the spark plug goes bad, the engine will not generate the required power output.

So, what causes spark plugs to go bad? What are the reasons for the spark plug fouling?

The main reason for the spark plug to go bad include:

  • Too rich a fuel mixture
  • Engine oil leak
  • Coolant leak
  • Cracked distributor case
  • Dirty fuel injector, carburetor, or air filter
  • Incorrect heat range or wrong spark plug
  • Improper plug gapping

Let’s discuss each of these causes one by one in detail.

#1: Too rich a fuel mixture

Engine and Spark plug

Rich fuel mixture refers to more fuel than required for the given air quantity in the engine. The air-fuel mixture in the engine should be in the desired proportion for optimum combustion. This desired proportion of air to fuel is referred as the stoichiometric ratio.

To put it simply, a rich air-fuel mixture is not desirable. For that matter, a lean air-fuel mixture (air is more than required for the given fuel quantity) is not desired either. A rich fuel mixture will lead to lots of carbon deposits on the electrode.

Spark fouling caused by rich fuel mixture can be identified from the electrodes’ gray or matte black finish. In addition, the deposit will most likely be evenly distributed over the electrode.

#2: Engine oil leak

Another major cause of spark plug fouling is engine oil leaking into the combustion chamber.

Engine oil flows within the crankcase to lubricate and cool down the engine components. The piston and the piston rings separate the combustion chamber and the crankcase and hence, prevent the oil flowing in the crankcase from moving into the combustion chamber.

But, if there is a leak, most possibly from piston rings, the oil will flow into the combustion chamber. The oil will burn and flow out as exhaust gas. But, it will also start forming carbon deposits by sticking onto the heated electrode in the spark plug.

As a result, the oil leak will start carbon depositing on the plug leading to the spark plug failure. A spark plug gone bad because of the oil leak will usually have a shiny black carbon deposit on its electrodes. The deposit won’t be evenly distributed like fuel carbon deposits. Rather, it would be blotchy and inconsistent.

#3: Coolant leak

Coolant is another thing that can foul a spark plug.

A leaking head gasket or any of the other gasket leaks can allow the coolant to enter the engine combustion chamber. The coolant droplets can stick to the spark plug itself and cause the plug to foul.

The deposit on the spark plug will be gray and ashy and can vary in texture depending on the type of coolant you are using.

Coolant leaks apart from spark plug fouling can also cause multiple problems. Mainly, engine overheating and wear and tear of the engine components. A coolant leak is a headache you want to get rid of as soon as possible.

#4: Cracked distributor case

A spark plug can go bad even apart from carbon deposit formation on the electrodes. One such way is a cracked distributor case.

The distributor case routes high voltage current from the ignition coil to the spark plugs. If the distributor case is cracked or damaged, the spark plug will not be able to generate the spark and ignite the fuel regularly.

While this will not affect the spark plug functioning, if left unchecked, it is going to have an impact. As the spark plug is not receiving the high voltage current continuously, the irregular electric transmission will take a toll.

Over time, the spark plug will stop functioning properly as well.

#5: Dirty fuel injector, carburetor, or air filter

Other major sources of carbon deposits on the spark plug include dirty fuel injectors, carburetors, or air filters.

A motorcycle will either have a carburetor or a fuel injector. Here is a detailed post explaining their differences.

If these components are not clean, the fuel or the air which passes through these parts and enters the engine is contaminated as well.

As a result, the dirt in the air-fuel mixture will start accumulating on the spark plug electrodes. With increasing dirt accumulation on the electrodes, the spark plug will eventually go foul and stop working properly.

#6: Incorrect heat range or wrong spark plug

Spark Plug

Another reason why a spark plug has gone bad can be because of the incorrect heat range the spark plug is exposed to.

This is the case when you have replaced the OEM spark plug with a new one during your motorcycle servicing. If the spark plug is not of the right type suitable for the engine conditions in your motorcycle, the spark plug will fail.

Incorrect heat ranges are usually faced by copper spark plugs when fitted in motorcycle engines operating at high temperatures. At such times, it is advised to go for a platinum or an iridium spark plug for your vehicle.

#7: Improper plug gapping

The spark plug gap is the gap between the center electrode and the ground electrode. The gap needs to be optimum with neither too big a gap nor too small a gap.

If the spark plug gap is too small, the spark will not have enough contact with the fuel mixture for ignition. There will be incomplete combustion of the fuel due to such inadequate ignition.

On the other hand, if the spark plug gap is too big, the electrode draws too much current to produce the electric spark. The spark travels longer and has large contact, but is an inefficient ignition process.

Both the small gap and the big gap are undesirable. This improper plug gapping will again lead to the poor working of the spark plug over time.

Symptoms of a bad spark plug

Bad spark plug symptoms are plenty. You can start noticing rough idling and hard-to-start problems in the engine. Couple that with poor acceleration and low mileage, it’s not tough to say there is something fishy with the spark plugs.

Other symptoms of a fouled spark plug include engine misfiring, rattling noise from the engine, lack of power, and poor engine performance.

For a more detailed view, here is our post on the symptoms of bad spark plugs.


Of course, the solution for a fouled spark lug is to replace it with a new spark plug.

But, it doesn’t end there!

Replacing the spark plug is the short-term fix. We need to understand the reason why the spark plug was fouled in the first place.

If it is because of too rich of a fuel mixture, carburetor tuning along with spark plug replacement is the solution. If it is because of the engine oil leak, it’s best to check the condition of the piston rings as well.

Similarly, for coolant leaks and the cracked distributor case, you should check the cause along with the spark plug change. Dirty carburetors, injectors, or air filters should be cleaned if they are reasons for the spark plug foul.

Lastly, if the spark plug went bad because of improper gapping or incorrect heat range, make sure to replace it with the right spark plug with the correct gapping.