Symptoms of Bad Spark Plugs


We all know that the engine in our cars burn gasoline. And everything that burns, from campfires to cigars, needs a source of ignition. In the internal combustion engine, that ignition source is the spark plugs.

How Many Spark Plugs Are There?

All the gasoline that enters your engine is burned inside of cylinders. Each cylinder has a spark plug that burns an air-fuel mixture of about 14.7 parts air to 1 part fuel. So the number of spark plugs your engine uses is the same as the number of cylinders your engine is made of. So if your car is equipped with a 6 cylinder engine, your ignition system is firing 6 spark plugs.

It Takes an Ignition System to Fire a Spark Plug

Spark plugs produce an electrical spark that can be measured between 20,000 and 50,000 volts. That’s a pretty good trick considering that the main source of power is a 12 volt battery. This abra cadabra moment happens inside of an ignition coil. An ignition coil is capable of storing voltage, then releasing it all at once.

Since the spark plugs need to fire in time with the engine, the ignition system determines engine position with data received from a crankshaft position sensor. An electronic module uses this information, plus inputs from other sensors, to determine when to fire the coils.

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The coils send a high voltage spike to the spark plugs which produces the spark that burns the fuel.

Spark Plugs Live in a Violent World

The reason that spark plugs don’t last forever, is that they work so hard. Depending on the situation, spark plugs can be asked to fire well over 1,000 times a minute, into a combustion chamber that can easily reach temperatures of about 4,500 degrees F. Considering these conditions, it’s impressive that today’s platinum spark plugs can go 90,000 or even 100,000 miles before needing to be replaced.

Spark Plugs Usually Die a Slow Death

Sometimes a good spark plug will just go bad. All of a sudden a spark plug will just stop producing a spark. When this happens one cylinder will stop working. This is called a misfire.

So your six cylinder engine would then be running on only five cylinders. You’ll know when this happens. The engine will lose power, vibrate, and idle poorly. Your check engine light will illuminate, and possibly start to blink. This is not the usual way for a spark plug to die.

As a spark plug gets older, it begins to produce a weaker spark. This weaker spark affects the efficiency of combustion. If the air-fuel mixture isn’t completely burned, the engine will still run, but not as well.

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Over time the electrode on the end of the spark plug deteriorates, causing the gap to increase. This, plus the buildup of combustion by products that contaminate the spark plug are usually the beginning of the end.

How Do I Know When To Replace My Spark Plugs?

Symptoms of Bad Spark Plugs

The first sign that you might be due for a new set of spark plugs is a decline in fuel economy. Good fuel economy is all about efficient engine operation, and weak spark plugs hinder efficiency.

As spark plugs get tired they could start to intermittently misfire. Intermittent misfires usually happen more frequently when the engine is under a load. The most common time for this to happen is during light acceleration at highway speed.

Accelerating while the transmission is in high gear is a high engine load situation, and is the most likely time for a borderline spark plug to misfire. During this misfire event you’ll feel the engine shake, and the shake will go away as soon as you let off the gas pedal.

Another symptom of old and tired spark plugs is long crank time during a cold start. A cold engine needs a good strong spark to start. Weak spark plugs just won’t do it. So the engine will have to crank until compression slightly warms up the combustion chamber so it can start.

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Check your owner’s manual for the mileage that the manufacturer suggests replacing your spark plugs. You should replace the spark plugs at this mileage, whether or not you’re having any symptoms.

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