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What happens if my spark plugs fail?

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Updated 10 Oct 2019

Zak Zuiderduin

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The spark plug is a vital component in a petrol driven engine.

Its job is to ignite the air/fuel mixture in the engine combustion chamber – and do so at precisely the right time.

It works by receiving electrical power from an ignition coil (which in turn is fed by the car’s battery) to create an electric spark that bridges a gap at the base of the plug between two electrodes - a central electrode and a ground electrode.

That spark is what ignites the air/fuel mixture.

That ignition then moves the engines’ pistons and connecting rods, which in turn move the crankshaft, which then turns the flywheel . . . you get the picture.

What are the symptoms of bad spark plugs?

    When a spark plug fails it is very noticeable.
    The drivability – the smoothness and steadiness of the driving experience - becomes rough and you will be able to feel the engine hesitate and stumble.
    This may be caused by a misfire of one or more spark plugs.
    Spark plugs are part of the ignition system of the engine.
    It seems obvious then, that if you’ve got dodgy spark plugs, you’ll have a problem starting your car.
    If a spark plug is worn or damaged – when, for instance, the gap between the plug’s electrodes changes – it can lead to less efficient combustion and an increase, often a very noticeable increase, in fuel consumption.
    A worn or damaged spark plug will lead to sluggish acceleration – a symptom that is, once again, very noticeable.
    A misfiring spark plug creates a rough ride when the car is being driven, and a similar feeling will be obvious when your car is at idle.
    A vibration through the car, a ‘rough’ sound, and the sensation that your engine might stall are signs of spark plug issues.

How often do you need to change spark plugs and why do you need to change them?

Your vehicle’s manufacturer will recommend at what mileage your spark plugs need to be replaced and what type to use during normal driving conditions.

You could expect low-end plugs to last from 15,000 to 30,000kms while high-quality iridium plugs might last up to 180,000kms or more.

However, factors such as low octane fuel, lack of engine maintenance, and rich or lean running conditions will all cause your spark plugs to be consumed quicker.

So it’s important to keep your car well maintained.

How much do spark plugs cost?

There are many different types of spark plugs available and the quality and construction will determine the prices.

Expect to pay between $2.50 - $25.00 per plug depending on what type you intend to use (some vehicle manufacturers will recommend the higher quality spark plugs).

Labour costs vary too - fitting them can be a minor task, but the design of some engines means that other components will have to be removed before the technician can gain access to the plugs.

That will increase the time required to do the job and, of course, the costs.

Should I try to maintain my vehicle’s spark plugs?

To maintain the spark plug involves some pretty technical work.

For instance, setting the ‘spark plug gap’ – which refers to the distance between the central electrode and ground electrode – requires the use of a special tool.

Because of labour costs verses the price of a new spark plug, it is probably uneconomical to clean and adjust old spark plugs.

Best to get a qualified technician to install some new ones.

What types of spark plugs are available?

There are many variations of the humble spark plug:

Copper/Nickel Spark Plugs

are low in cost and generally have a shorter lifespan, especially if fitted to modern vehicles with high energy distributor less ignition systems or coil on plug ignition systems.

Single Platinum Spark Plugs

have the advantage of longevity under normal driving conditions.

The lifespan of a set of platinum spark plugs is typically double that of copper ones, although some vehicle manufacturers using them specify change intervals of up to 160,000 km.

Double Platinum Spark Plugs

Although this type of plug represents a step up in price, double platinum spark plugs typically produce slightly higher performance, with the traditional long life, for which platinum plugs are known.

Iridium Spark Plugs

offer better power and a more complete combustion that leads to smooth running engines and a longer life span than copper/nickel plugs.

Iridium plugs are the most expensive spark plugs.

Silver Spark Plugs

have the best thermal conductivity.

However, they do not feature the longevity of platinum or Iridium.

Generally, they are specified in older European performance cars as well as some motorcycles.

Having troubles with your spark plugs?

Use AutoGuru to see how much you could save on a Spark Plug Replacement

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Written By

Zak Zuiderduin

Zak has been working in the auto industry for 40 years.

For the past decade, he has been in the education business, training apprentices and preparing them for a career in the light vehicle and auto electrical trades.