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How and why to check the ignition system on a used car

Dec 01, 2023

Strong spark plugs and ignition coils contribute to healthy running, but can spell pain if neglected.

Many shoppers find buying a used car to be stressful, with a key pain-point being uncertainty about that car's history. Was it used as a taxi? Well cared for? Maintained properly?

Understanding the basic major components of a car or truck — and the different variations and needs of those components — can help used-car shoppers fend off some uncertainty.

We’ve previously covered used-car topics like the continuously variable transmission (CVT), used hybrid cars, the used-car steering-system checkup, and used-car air-conditioner checks, explaining how each system or component works, how they’re maintained, and how to spot potential trouble when shopping for a used car or truck.

Below, we’re covering another major component of the new ride you’re considering: its ignition system.

Ignition systems come in many shapes and sizes depending on the age and make of the used vehicle you’re considering. Some work differently than others, but all share the goal of sending electricity to spark-plug electrodes inside of your engine, which ultimately ignite the air-fuel mixture within.

Every cylinder in your gasoline engine has a spark plug threaded precisely into the cylinder head above. Some engines have two spark plugs per cylinder, but it's rare.

The spark plug threads into the cylinder, sealing combustion gasses and heat in, while positioning the electrode tip within the cylinder for optimal combustion. The other end of the plug is connected to an electrical source (a large coil pack or small ignition lead, which sends the correct amount of electricity to the plug at the correct time to make it spark.

A properly functioning ignition system has plenty to do with the performance, efficiency, and refinement of your ride. This is the foundation of life within your car's engine.

Thankfully for used-car shoppers, ignition trouble can give itself away easily, and many common problems can be quickly identified if you know where and how to look.

Shoppers seeking maximum confidence from an upcoming used-car purchase may find the cost of an ignition system overhaul minor in comparison to the peace of mind that comes running a known-healthy system.

Spark plugs are like belts, filters, and brake pads: they require periodic replacement at pre-determined intervals. These vary by vehicle age and type, with many modern cars going 100,000 kilometres or more before any attention is required.

Older cars, specialty performance models, and turbocharged engines may have more frequent spark plug change requirements, perhaps every 50,000 kilometres. Hybrid car engines have spark plugs, too. Diesels do not.

To find out how often the spark plugs need changing on the used car or truck you’re considering, check the owner's manual or online in a maintenance calendar accessed via the manufacturer website. Be sure to key in the specific year, make, model, and engine you’re considering to get the right figure.

Like engine oil, some drivers prefer to change their spark plugs early, accelerating the intervals outlined in the owner's manual. In this way, they choose to spend a few bucks extra on pre-emptive maintenance and peace of mind. Others don't.

Accelerating your spark plug change intervals can't hurt your engine, and it's a good idea if your engine will frequently see severe-use situations like extreme cold, stop-and-go driving, or a variety of other very Canadian driving conditions.

If you’re a Canadian buying a used car with over 50,000 kilometres, and if that used car's service history is unclear, budgeting for a spark-plug change and ignition system checkup alongside your purchase is a good way to ensure you’re starting your ownership experience off confidently and reliably.

Yes, but you probably shouldn't.

Changing the spark plugs in some vehicles is easy and fast. Changing the spark plugs in others requires removal of body parts or wheels, speciality equipment, and special training.

To be clear, some vehicles are great candidates for a low-cost, DIY spark plug change provided you know exactly what you’re doing and have the necessary tools and know-how.

If that's not the case, a lot can go wrong. Get the plug size, type, gap, or installation torque wrong, and you could cause serious damage to your engine that's not covered by remaining warranty. Ditto if you improperly disconnect the car's battery (or fail to).

A lot can go wrong when it comes to spark plug selection and installation in a modern engine, and there's no room for error. Leaving this job to the pros means you’re off the hook if anything goes wrong.

Many modern engines use direct injection (DI) technology, turbocharging, or both.

In a DI engine, the intake valves are only used to control the flow of air into the cylinder while fuel is blasted directly inside at extreme pressure.

In a turbocharged engine, exhaust gasses drive an air compressor which blasts additional air into the engine, allowing it to burn more gasoline and make more power. Most modern engines have at least one of these features, and many have both.

High-efficiency engines like these rely on clean and precise combustion process to do their jobs properly. They’re also sometimes prone to accumulation of ‘valve gunk’, a contaminant that can build up in DI engines (turbocharged or not) over time.

Keeping the ignition system in this type of engine healthy is a very good idea. When used alongside on-time oil changes and high-quality fuel, fresh spark plugs help form the bedrock of the clean combustion process that's required to keep valve gunk and other deposits at bay.

Starting your used-car ownership experience off with a fresh set of spark plugs and accelerating the plug change intervals, is a good idea for any car or truck, and an even better idea if a DI or DI turbo engine is installed.

Depending on the used vehicle you’re considering, it may be easy to pop the hood and remove the spark plug tubes. These are basically electrical connectors insulated with rubber that slip into the cylinder head and pop onto the spark plugs below.

With this setup, the tubes can be removed by hand for inspection. In other setups, coil packs may be fastened to the top of the spark plug tubes, requiring removal of a few fasteners to free up each.

Whether on your own or with the help of a professional, removing these tubes can be a useful check for potential trouble.

When removing the tube, listen closely for a good slurpy, liquid-suction sound. Look down inside of the opening, which should be dry, clean, and have a spark plug screwed in at the bottom. If you see liquid, debris and (especially) engine oil in one or more of these openings, the vehicle requires professional attention and repair before you buy — likely to replace a valve cover gasket.

Some engines (V6, V8, H4, H6) have multiple valve cover gaskets.

With the spark plug tube / ignition coil pack removed, the electrical wiring, contacts, and insulation can be checked for damage as well.

Scorching, melting, or cracking of the rubber insulation indicates a problem that could cost you money and make for a poor driving experience. Spark plug wires that have been exposed to oil (such as from a leak found in the previous step) may be damaged as well.

Replacing damaged or decayed ignition components is a great way to make sure your engine doesn't suddenly die (or run like it's about to), when you least expect it.

If you’re considering this check but are unclear on whether or how to do it yourself, have a licensed technician do it for you. It’ll cost you a few bucks, but could save you hundreds or more.

What if you buy a high-mileage used car that's never had its plugs changed, and then never bother to change the plugs either?

Likely nothing good for your wallet.

In addition to missing out on an opportunity to inspect wires, coil packs, and plug tubes for signs of trouble and damage, old spark plugs tend to degrade and dirty quickly once past their prime, making it harder and harder for your engine to ignite its fuel-air mixture efficiently. This can result in poor performance, misfires, carbon buildup, incomplete combustion, a check-engine light (CEL), and accelerated wear to other ignition components.

Old plugs can also cause collateral damage to other parts of your engine, elevating repair bills. As combustion becomes less efficient, it becomes dirtier. This can lead to accelerated buildup of gunk and deposits within your engine. This contributes further to the dirtying of your neglected plugs.

If a spark plug totally loses its ability to spark, your engine will run very poorly, waste a lot of fuel, wear out quickly, and may even damage its expensive catalytic converter.

Old plugs may physically fail, dropping bits or chunks of degraded metal into your engine's cylinder(s). Next, the offending metal debris will chew away at cylinder walls and damage pistons and valves, causing permanent and likely-fatal engine damage.

It's much cheaper to change a set of plugs than it is to change an engine.

Justin Pritchard is a Sudbury, Ontario based automotive journalist and award-winning presenter, photographer, videographer and technical writer. Every week, Justin uses his keyboard, voice and cameras to share his latest automotive reviews and discoveries with his audience, via multiple Canadian television programs, print and online publications.

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