Ignition system problems caused by faulty spark plugs and damaged leads are a common cause of engine performance issues, especially in petrol cars. A weakened spark leads to starting problems, rough running and higher fuel consumption. Recognising and replacing these parts in time prevents further damage and keeps your engine running smoothly.
What does the ignition system in your car do?
In a petrol engine, the ignition system ensures the air–fuel mixture ignites at exactly the right moment. The battery supplies low voltage, the coil converts this into high voltage and, via spark plug leads or coil-on-plug units, a spark is generated in the cylinder. If any part of this chain fails, the engine receives too little or irregular ignition. This is directly noticeable as symptoms while driving.
The ignition system consists of several components that must work together. Spark plugs form the end point where the spark jumps across. Spark plug leads carry the high voltage from the coil to the plug. In older cars with a distributor cap these leads often run visibly over the engine. More modern engines often use coil-on-plug units that sit directly on the spark plug, but even then leads and connections can still cause problems.
Typical symptoms of worn spark plugs and leads
Problems with the ignition system manifest themselves in different ways. The Ford Mondeo with a ZETEC engine is known for ignition problems caused by worn plugs and damaged leads. Recognisable symptoms include:
- Hard starting, especially in cold weather or high humidity
- Rough idle with vibrations and juddering
- Hesitation or stumbling when accelerating
- Increased fuel consumption without an obvious reason
- Popping noises from the exhaust or inlet manifold
- Smell of fuel or soot deposits at the exhaust
An engine that seems to be “running on three cylinders” often points to misfires caused by ignition problems. This occurs when one or more cylinders are not getting proper combustion. Under load a weak spark becomes noticeable more quickly, so symptoms when accelerating are more obvious than when cruising gently.
When should you replace spark plugs?
Most manufacturers recommend replacing spark plugs every 30,000 to 60,000 kilometres, depending on the type of plug. Platinum or iridium spark plugs last longer than standard copper electrodes. In practice, plugs wear gradually due to normal combustion. The electrodes become thinner, the insulator can become dirty and the spark gap changes.
Check your plugs in between if you have complaints or if the service history is unknown. A visual inspection often already tells you a lot:
- Black sooty deposits indicate a too-rich mixture or a weak spark
- White, dry deposits point to a too-lean mixture or combustion that is too hot
- A wet plug can indicate a misfire or a fuelling problem
- Heavy wear on the electrodes calls for immediate replacement
Always use the correct type of spark plug for your specific engine variant. An incorrect heat range can cause overheating or fouling even when the plug is new.
How can you recognise damaged spark plug leads?
Spark plug leads can break down internally without this being visible from the outside. Heat, moisture and movement of the engine cause wear to the insulation. A damaged lead allows high voltage to leak away before it reaches the plug, making the spark too weak for proper combustion.
Check leads regularly for visible signs of wear. Look for cracks in the insulation, green oxidation on connections and plugs that fit loosely or poorly. In a dark garage you can sometimes see whether leads are arcing while the engine is running. Small flashes or sparks along the lead indicate leakage.
Spark plug leads can also cause problems only under specific conditions. Many owners report that symptoms only occur in rain or high humidity. Moisture combined with weakened insulation promotes arcing. That is why a test in dry weather is not always reliable.
‘ Resistance measurements can help with diagnosis, but a coil can still fail under load while static measurements show no deviation. ’
Problems with coils and connections
A weak or faulty coil sometimes only causes problems under load. Old or damp coils can cause arcing to the bonnet or other metal parts. On some engine versions there are separate coil-on-plug units which can fail per cylinder. This leads to fault codes and misfires that can clearly be linked to one cylinder.
Check coils for cracks in the housing and traces of moisture or oil around the connections. A coil that feels very hot or makes strange noises in use should be investigated further. Resistance measurements can help with diagnosis, but a coil can still fail under load while static measurements show no deviation.
Practical checks you can do at home
Start with a visual inspection of the ignition system. Open the bonnet and check leads, coils and connections for visible damage. Look for cracks, oxidation, moisture and loose connections. Keep the engine bay dry and clean, as moisture intensifies ignition problems.
Test spark plugs by removing them and inspecting them. The colour and condition of the electrodes reveal a lot about the engine condition and the combustion process. Compare all spark plugs with each other. Large differences indicate uneven combustion between cylinders.
Check spark plug leads by gently bending them and looking for cracks. Measure resistance with a multimeter if you are unsure. Excessive resistance points to internal damage. Bear in mind that a lead can look fine visually and electrically, but still fail under load or in damp conditions.
Costs and DIY work
A set of new spark plugs typically costs between 20 and 80 euros, depending on type and brand. Standard copper plugs are cheaper, while platinum or iridium versions are more expensive but last longer. Spark plug leads usually cost between 30 and 100 euros for a complete set. Coil-on-plug units are more expensive and range from 30 to 150 euros each.
Many car enthusiasts can replace spark plugs themselves with basic tools. You will need a spark plug socket, a torque wrench and possibly an extension. Observe the specified tightening torques to avoid damage. Always screw plugs in by hand first to prevent cross-threading.
Order is important when replacing leads. Route one lead at a time to avoid confusion. Leads connected in the wrong order cause an incorrect firing sequence and serious engine problems. Take a photo before you start or mark the leads.
‘ An incorrect diagnosis leads to unnecessary costs for parts that are not actually faulty, while the real problem remains. ’
When should you go straight to the garage?
Have the ignition system checked as soon as possible if the engine warning light is flashing. A flashing light indicates active misfires that can damage the catalytic converter. Immediate action is also needed if the engine is clearly running on fewer cylinders or hesitates badly when accelerating.
Fuel smell, popping noises or black smoke from the exhaust require professional diagnosis. These symptoms point to incomplete combustion that can cause further engine damage. A garage can read fault codes and use measuring equipment to identify the exact cause.
If you are unsure about the condition of coils, leads or spark plugs, it is wise to seek professional advice. An incorrect diagnosis leads to unnecessary costs for parts that are not faulty, while the real problem remains.
Preventive maintenance on the ignition system
Replace spark plugs according to the service interval in your owner’s manual or earlier if you have complaints. Keep a logbook of replacements so you know when components are due. At every service, check the condition of leads, coils and connections.
Avoid jet-washing the engine bay with high pressure unnecessarily. Water can penetrate coils and lead connections and cause ignition problems. If you do clean the engine bay, do so carefully and let everything dry thoroughly before starting the engine.
Pay attention if fuel consumption suddenly increases without an obvious reason. This can be an early sign of ignition problems. Do not wait until the symptoms get worse, because a weak spark puts extra strain on other engine components and increases the risk of expensive damage to the catalytic converter.
On the Carnews website you will find more practical information on engine problems, maintenance tips and car-specific complaints. Discover which problems are common with your car brand and how best to deal with them.
Frequently asked questions
Symptoms of problems with spark plugs or plug leads include hard starting and an irregular or jerky idle speed, sometimes as if the engine is “running on three cylinders”. While driving you may notice hesitation or juddering when accelerating, loss of power and higher fuel consumption. Popping noises or afterburn in the exhaust or inlet can also occur.
Worn spark plugs or defective plug leads cause a weak or irregular spark, so the air–fuel mixture does not ignite properly. This leads to loss of power, hesitation under acceleration and rough idling because the engine is (temporarily) running on fewer cylinders. Because combustion is incomplete, the engine management system will often inject more fuel to compensate. The result is higher fuel consumption and sometimes extra soot or a smell of fuel from the exhaust.
The recommended replacement interval for spark plugs is usually according to the car’s service schedule (often around 30,000–60,000 km for standard plugs, up to about 100,000 km for longlife/iridium), while spark plug leads are generally only replaced when there is clear wear, arcing or ageing. DIY: always work on a cold engine, replace spark plugs one by one so you do not mix up the leads, use the specified plug type and the correct tightening torque (or a torque wrench). Visually check plug leads for cracks, oxidation and signs of arcing, and if in doubt perform the “dark-room test” to spot stray sparks. Clean plugs and connections, and avoid spraying coils and leads with a pressure washer to prevent future faults.
For performance improvement people usually choose high-quality iridium or platinum spark plugs that can better withstand high temperatures and provide a more stable spark. In addition, quality plug leads with low resistance and good shielding are recommended, as they carry the high voltage to the plugs more efficiently and with fewer interference problems. Always choose plugs with the correct heat range and specifications for your engine variant to prevent damage. Under tougher driving conditions (for example many short trips, high loads or sporty driving) it is wise to check and replace plugs and leads a little more frequently.
First carry out a visual check of spark plugs, plug leads and the coil for cracks, discolouration, oxidation, oil/moisture and loose connectors. If necessary, remove the spark plugs and assess them: heavily worn electrodes, thick black soot or wet plugs indicate a problem. Do a test in a dark place: with the engine running you can sometimes see sparks or light traces arcing along leads or the coil. Note all symptoms (hard starting, rough running, hesitation when accelerating, higher fuel consumption) and take these to the mechanic so they can carry out more targeted measurements.