Symptoms Of Bad Spark Plug And How To Diagnose It

1 What Does A Spark Plug Do?

In a gasoline or petrol engine, three basic elements are required for combustion to take place and for the engine to operate: air, fuel, and spark. The supply of air is done through the intake manifold and fuel is delivered using the fuel injection system. The spark is provided by the ignition system and consists mainly of the spark plug and the ignition coil. The spark plug has two electrodes separated by an air gap and as a high voltage current passes through one electrode, it jumps the gap to reach the other electrode which creates a spark. This spark is enough to ignite the pressurized fuel/air mixture and cause combustion. This is why a spark is essential for igniting the fuel-air mixture in a gasoline engine. 

The voltage is produced by the ignition coil which is essentially a step-up transformer that converts the battery’s 12v to more than 20,000 volts necessary to create a spark strong enough to ignite the fuel-air mixture.

The job of a spark plug is to ignite the fuel-air mix present in the combustion chamber to create power. It also helps dissipate some of the engine heat by absorbing it through the shell. If the spark plug fails or works improperly, the entire engine operation is compromised. Engine misfiring, inability to start, and reduced acceleration are a few common results of spark plug failure. 

2 What Are The Symptoms Of Bad Spark Plugs?

Failing to replace the spark plugs in their specified service interval will cause many issues with your engine and vehicle. These signs must not be ignored as this can damage the engine and its components and require more expensive repairs. These are signs of spark plug failure:

Car Starting Trouble

The first and most obvious sign of spark plug failure is the inability to cold start the car. Worn out spark plug will prevent combustion and therefore the engine will not start. Hence, bad spark plugs can be the reason for starting issues, and replacing them can lead to easy and quick engine starts.  

Engine Misfiring

If one or more spark plugs are broken then there is potential for engine misfiring and knocking. This is bad for various engine components like the connecting rod, crankshaft, piston, etc. as they undergo erratic stresses and forces that can cause premature failure of the components. Misfiring engines also increases the pollutants produced which can damage the catalytic converter. 

Rough Idle

When idling If you hear knocking or irregular sounds from the engine then the spark plug may have given out. A rough idle is a vital indication of spark plug failure.

Reduced Gas Mileage

If the fuel and air mixture is left un-ignited by the spark plug, the fuel simply goes to waste without producing any power which can eventually result in drastically increased fuel consumption and lowered engine performance. 

Lousy Acceleration

Another telltale sign of worn-out spark plugs is poor acceleration as the engine creates no power in the power stroke due to the lack of spark. If you are experiencing sluggish acceleration with the gas pedal floored, then you most likely have bad spark plugs in need of immediate replacement. 

It is always advised to stick to the manufacturer-recommended service intervals for spark plugs to get the best performance and gas mileage from your engine. Many external factors can also be responsible for premature spark plug failure like bad ignition coil, frayed spark plug wires, low battery voltage, etc. This is why it is necessary to know the common symptoms of spark plug failure so as to prevent any major damage to the engine and other components. 

Since ignition coil failure also shows similar symptoms, it is possible that you may instead have bad ignition coils. Refer to this guide to identify and replace bad ignition coils.

3 How To Find A Bad Spark Plug? Diagnosing Bad Spark Plugs

If you are experiencing any of the above symptoms then having the spark plugs checked is the best way to find the real cause of the issue – plus it is quite simple to diagnose or find a faulty spark plugs.

Visual Inspection

The most common failure point for a spark plug is a worn-out electrode tip. The high-voltage current jumps the gap from the tip to the side or ground electrode. This is repeated several thousand times per engine run and so eventually the tips wear out and lose their edge and also increase the air gap which causes plug failure. Upon inspection of the plug electrode if you see that it is worn down then you can assume the spark plug will not work effectively. Seeing a red deposit on the plug is normal and comes from low-quality gasoline. Having black soot on the plug can indicate a rich fuel mixture being supplied or the plug is simply very old. 

Use A Multimeter

You will have to remove the spark plug before you attempt to test it using a multimeter. This test checks the resistance across the central electrode from the tip to the lead. To test it, set the multimeter to 20k ohms or anything above that limit. Now place one probe on the tip of the plug and one at the metal lead or top end of the plug where the wire connects to. The reading on the multimeter should be within 5 to 15 (or 5000 to 15000 ohms). Make sure the centre electrode is clean and does not have any carbon deposits that can hinder the test reading. If the readings are off considerably then the spark plug should be replaced. 

Trial & Error

You can easily isolate a bad spark plug from a good one by going through each plug as the engine is running. To do so, start your vehicle and first listen to the sound the engine makes. Now remove one of the spark plug wires and if you hear knocking or engine misfiring then that plug is fine and if no change occurs when a plug wire is pulled out then it’s likely the faulty spark plug. Repeat the process for every spark plug and you will identify any and all spark plugs. You can also check the spark working with the spark plug removed and a spark tester attached. Run the engine and If the tester illuminates and the plug fires a bright blue spark then the plug is working properly. If the tester does not illuminate then the issue actually lies with the ignition system. 

Make sure to follow necessary safety precautions when dealing with running engines and spark plugs as this can be extremely dangerous. If in doubt, simply replace the spark plugs as they are relatively cheap components. If the problem still persists then you can be sure that the spark plugs are not the issue and move on to further investigate the problems you are facing.

4 FAQs

What does a bad spark plug sound like?

A bad spark plug can produce a variety of sounds, depending on the nature of the problem. Some common sounds include:

• Sputtering: If the spark plug is not producing enough spark, the engine may sputter or hesitate when you try to accelerate.
• Rattling: A loose or damaged spark plug can cause a rattling sound in the engine.
• Popping: If the spark plug is not firing at the right time, you may hear a popping sound from the engine.
• Backfiring: A backfire occurs when the spark plug ignites the fuel in the cylinder at the wrong time. This can cause a loud bang or pop from the engine.

What makes a spark plug go bad?

Spark plugs are crucial components of an engine that generate electricity to ignite the fuel-air mixture inside the vehicle for producing energy. They put the engine’s pistons in motion, enabling your car to power up, stay powered up, and produce a smooth burn of the compressed air-fuel mixture. Spark plugs have a limited lifespan and can malfunction over time. Here are some common reasons that lead spark plugs to go bad fast:

• Bad Fuel Filters: Bad fuel filters can cause the tip of the plug to get oily and dirty, resulting in buildup and damage to the plug, which will ultimately cause it to fail.
• Unsuitable Spark Plugs Gap: The gap between the metals on the tip has to be accurately measured for your engine when the spark plug is installed. If the gap is incorrect, there will be extra stress that can happen on the tip and that leads to wear out faster.
• Leaky Head Gasket: Coolant leaks into the combustion chamber can cause spark plug fouling. A fouled spark plug may be an early sign that a head gasket is starting to leak.
• Overheating: Overheating of the tip of the spark plug can cause it to wear out prematurely. This can arise from a number of different causes. If the timing of the engine is off and pre-ignition occurs, this can cause excess heat to build up in the combustion chamber.
• Wet Spark Plugs: A slow steady flow of oil into the combustion chamber is one of the leading causes of spark plug problems. It can cause the tip of the plug to get oily and dirty, resulting in buildup and damage to the plug.

Can you mix spark plugs?

It is generally not recommended to mix spark plugs. Different types of spark plugs have different heat ranges, and mixing them can cause uneven wear and performance issues. However, some sources suggest that it is possible to mix spark plugs as long as they have the same heat range and are compatible with your vehicle’s engine.