Signs of failing spark plugs

The car electric system is one of the most complicated things to repair yourself. Photo | Kelvin Atuhaire

What you need to know:

  • Spark plugs do not wear out at the same time or at once. For instance, if you have a four cylinder engine, one spark or two spark plugs can wear out while the other two are still functioning normally.

Spark plugs are small but powerful devices that deliver electric current from an ignition system to a combustion chamber of engines.

In cars that are started with push-to-start buttons, the ignition system is still connected to the spark plugs to start the engine upon pressing the button.
Like most car parts, spark plugs reach a time when they have aged and need replacement. However, this aging is not an abrupt event, it takes a while and gives off tell-tale signs that they are due for replacement such as:

Delay to start the engine
A fully functional spark plug ought to fire up the engine immediately as you turn the key in the ignition switch. However, when your plugs are beginning to give way, you will notice that at times you have to hold the key in the start engine position, as though you are forcing the engine to start.

According to Alex Kadoli, a mechanic at Bunga, when you find yourself in such a position, which in mechanical terms is also referred to as a hard start, it could be a pointer that your spark plugs are worn out and can no longer start the engine. Sometimes the car will completely fail to start until the spark plugs have been replaced.

Slow car acceleration
Often, when your spark plugs are worn out, you will have a situation where you accelerate and the vehicle does not respond accordingly. Kadoli explains that when the spark plugs are worn out, it means they are unable to produce enough firepower to allow air effectively mix with fuel (air to fuel mixture) for the car to pick up speed when you step on the accelerator.

Black exhaust smoke
Peter Amadi, a mechanic at Dalas Auto Limited, explains that under normal circumstances when your spark plugs are functioning normally, the colour from your exhaust pipe ought to be clear, but not thick black. This is an aspect that takes place across all car brands regardless of whether your car runs on diesel or petrol.

“When your exhaust pipe gives off black smoke, one of the things to get worried about are bad or worn out spark plugs. It means fuel burning in the combustion chamber is not being done 100 percent or burnt fully. It also means that air is not being well mixed with fuel to burn effectively. The excess fuel that will not be burned fully will be emitted in form of black smoke,” Amadi explains.

Poor fuel economy
When a spark plug works correctly, it helps burn fuel efficiently in the combustion cycle. When this occurs, your car can achieve better than average fuel economy. When the plug is not functioning optimally, it is frequently due to the fact that the gap between the spark plug electrodes is either too close or too far apart.

Many mechanics will take out spark plugs, examine them, and adjust the gap to factory settings as opposed to replacing the spark plug entirely. If your vehicle has a reduction in fuel economy, it could be attributed to a worn out spark plug.

Engine misfiring
If the engine misfires, it is typically due to an issue in the ignition system. Often, in modern cars misfiring is due to a sensor malfunction. However, it may also be caused by a spark plug wire or the tip of the spark plug that connects to the wire being damaged. An engine misfire can be noticed by intermittent stumbling or sputtering sounds from the engine.

“If the engine is allowed to keep misfiring, exhaust emissions will increase, engine power will decrease, and fuel economy will drop,” the mechanic says. Because of all the potential problems associated with engine misfiring issues, the mechanics advise that you contact a mechanic as soon as you notice an engine misfire. A mechanic can inspect the issue and determine the right course of action to repair the problem.

Care and service for spark plugs
Like windscreen wiper blades, spark plugs cannot be serviced but rather replaced overt ime. This is normally after you have used them for approximately 5000 kilometers. However, Amadi notes that there are certain types of spark plugs that can serve you for approximately 20,000 kilometres. When they fail and have served you for this mileage, there is no way out or shortcut except replacement.

“If your mechanic advises you to carry out service after driving for 5000 kilometres, do not replace crucial parts and ignore spark plugs because they have not failed you yet. It is better to have them replaced than wait and first get stuck on the road because you do not know when, where and what time they will stop functioning,” Amadi advises.

“The thing about car parts such as spark plugs is that they can fail you at the worst time of day when the money you have in your pockets cannot even save you. You could be in a place where there are no spare part shops or no mechanic to come to your rescue,” he adds. The cost of original long-lasting spark plugs is approximately Shs80,000.

When to replace 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 spark plug to misfire.