6 Symptoms of Bad or Fouled Spark Plugs

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Rough idling engines, lack of power, poor fuel economy, and even check engine lights are all possible causes of bad spark plugs. Spark plugs are a key ingredient to any proper functioning gasoline combustion engine.

The spark plug’s function is rather self explanatory, its job is to ignite the fuel in the combustion chamber. The engine control unit ECU sends an electrical signal to the ignition coil. The ignition coil jumps the 12 volts it receives into several thousand volts. This higher voltage causes a “spark” in the spark plug’s gap, igniting the air fuel mixture.

How Long Do Spark Plugs Last?

Spark plug life varies depending on the application. Plugs have a shorter lifespan in a motorcycle versus a car. This is due to the higher RPM a motorcycle engine sees.

In a four stroke engine, a spark plug will ignite every four revolutions. Since a motorcycles regularly operate at 8,000 rpm or higher, the spark plug is igniting 2,000 times per minute. Whereas a car usually operates in the 2,000 – 3,000 rpm giving 500 to 750 spark plug ignitions per minute.

Spark plug life can vary drastically by the material used. Copper plugs will be good for 10,000 to 20,000 miles, while platinum or iridium plugs can be good for 60,000 miles or more. In motorcycles, plugs typically only last 15,000 miles, while car and truck spark plugs can last from 30,000 miles to even 100,000 miles.

As you can see the replacement intervals can vary by 70,000 miles in different applications. Its very important that you look at your specific vehicle and see what interval the manufacturer recommends.

Bad Spark Plug Symptoms

Common signs of spark plug issues include: rough idling, poor acceleration, reduced fuel economy, misfiring, hard starting, and even the dreaded check engine light. The large variance in issues that can be caused by faulty spark plugs just goes to show how critical these parts are to your engine.

1. Check Engine Light

Faulty spark plugs can trigger a check engine light. In serious cases, it can trigger a flashing check engine light. I’ve actually experienced this myself.

oil pressure check engine light
Check engine lights can be caused by malfunctioning spark plugs

Upon starting my truck for work I noticed a little more vibration than normal. When the engine rpm climbed above 1,000 rpm it smoothed out. However, I had barely made it down the road and my check engine light started flashing indicating a major engine problem! The issue turned out to be a bad spark plug that was causing a misfire.

Check engine lights will trigger a certain code. Be sure to check this code and determine what it means. A P0300 code indicates a cylinder misfire and can be related to the spark plugs, while a P0171 would be related to PCV valve issues.

2. Engine Misfires

While engine misfires can trigger check engine lights, they don’t always trigger them. Sometimes the engine can misfire without any warning sign on your dash. When this happens, you will notice increased vibration, rough running, and the vibration can disappear as the engine speed increases.

A misfire is what happens when the spark plug fails to ignite the air-fuel mixture in the combustion chamber. When this happens the forces in the engine become unbalanced causing vibration. Other symptoms include rough idling, poor fuel economy, and slower acceleration. The last indicator of a misfire would be the engine sound. If the engine isn’t running smoothly you should hear an abnormal exhaust sound.

3. Rough Idle

A rough engine idle can be caused by bad spark plugs. What happens is the spark plug fails to ignite the gasoline in the cylinder causing more fuel than normal to leave the engine through the exhaust. The O2 sensor is then tricked into thinking that the engine is getting too much fuel when actually the engine is getting the proper fuel amount.

This faulty data is then sent to the ECU and the ECU adjusts the air-fuel mixture to a lean mixture. The engine then struggles to run the lean mixture and this is what causes a rough idle.

The soot on this spark plug indicates a rich air-fuel mixture

An easy way to tell if your engine is running lean is to look at your spark plugs. If your spark plugs are clean and white after being ran in your engine, the air-fuel mixture is lean.

4. Reduce Gas Mileage

A lot of these symptoms go hand in hand. If you notice your engine misfires, you’ll likely also notice you have poor fuel economy. This happens because your engine doesn’t ignite the fuel in the cylinder on each combustion cycle. The wasted fuel exits out the exhaust reducing fuel economy.

You likely won’t see a huge loss of fuel economy. When my truck had a bad spark plug and was misfiring, my fuel economy dropped to 15 mpg while it usually hovers around 17.5-18.5 mpg.

5. Slow Acceleration

If your spark plugs are bad and misfiring, slow acceleration could be another symptom that you notice. Basically whats happening is your car’s engine has lost a functional cylinder. So if you have a v8, you’d only have seven functioning cylinders (and a lot of vibration), but if you had a smaller 4 cylinder engine, you’d be down to just three.

This reduction in cylinders causes a loss of power and consequently a loss of acceleration. If your engine is a V configuration (v6, v8, etc), you’ll probably notice more vibration than if it was an inline 4 cylinder. This is because the V configuration engines will more off balanced with a misfire than an engine with all its cylinders in one line.

6. Hard Starting

The last sign that you have bad spark plugs is that the engine is hard to start. Now there are other causes for hard starting vehicles, for instance if you have a carbureted motorcycle check out our article here on why they can be hard to start. However, spark plugs can be the cause of a hard start.

If the spark plugs are worn or dirty, it my take a lot of energy to spark the plugs and ignite the fuel. The engine is also cold making starting a little harder than if it was up to operating temperature.


Spark plugs are essential to engine operation. They can cause all sorts of problems, but these problems occasionally go hand in hand. Thankfully spark plugs are not incredibly expensive as evidenced by these Crown Automotive Plugs on Four Wheel Parts. Be sure to follow your manufacturers manual on replacement intervals for spark plugs. These intervals can range from as little as 15,000 miles to 100,000 miles depending on the spark plug material.