Your Guide To Servicing: Ignition Systems

Posted on December 2nd, 2013 by GEM Motoring Assist

All petrol engines require a healthy ignition system to run efficiently, therefore Rob Marshall advises on which components are likely to need attention.

 ignition system

The petrol engine ignition system has developed immeasurably over the past twenty years. Older cars, many of which are now regarded as classics, needed regular tinkering to their contact breaker points every three to six thousand miles, to ensure that their limited efficiency was at its peak.

Ignition systemMost pre-1980 cars were fitted with contact breaker ignition systems, which require regular adjustment. Some modern replacement parts, including the rotor arm, can be of a poor quality and you should buy new parts with care. Condensers can also fail without warning, causing the engine to simply stop.


Nowadays, the demand for longer service intervals, enhanced reliability and better efficiency has seen most separate ignition components being integrated with the fuel system, via a host of electronic sensors and mysterious ‘black boxes’.

However, rough running, increased fuel consumption, an illuminated dashboard light or even a non-starting engine can be caused by a neglected ignition system. Should you decide to check out a modern electrical system, note that extremely high voltages are employed. Never touch any part of the circuitry either when the ignition is switched on or the engine is running.

Ignition lamp

An illuminated engine management lamp, coupled with erratic engine running, might indicate a fault within the ignition circuit but it could also denote a problem within the emission or fuel systems as well. Even with the problem cured, the lamp may have to be extinguished using specialist diagnostic tools. To reduce the risk of total ignition failure, never allow any component to become wet


Spark plugs lie at the heart of the ignition system. Dirty or worn parts will result in a weak spark and incomplete combustion within the engine, resulting in higher fuel consumption and tailpipe emissions. The colour of the plugs’ exposed tips will also reveal clues about the engine’s internal health. For example, if one is covered in oil, you know that lubricant is entering the combustion chamber, possibly due to excessive wear or damage.

Spark plug

 The break between the electrodes should be set by checking it with a feeler gauge and bending the top electrode to the intended gap. This spark plug is covered in soot, denoting incomplete combustion

 Correct spark plug gap is essential to smooth engine running and the tolerances vary between engines. Yet, some plugs are of the multi-electrode type that eliminates the need for any adjustment. Always ensure that spark plugs are renewed in accordance with the manufacturer’s service schedule.


Check that any wires or thick leads, which run to the spark plugs, are in good condition and are not vulnerable to damage from a neighbouring component. The spark plugs are supplied with an extremely high voltage by one or more coils. Several different layouts exist but all modern coils are not repairable; they must be discarded and replaced if faulty.


Ensure that any leads that might run to the spark plugs are protected from being damaged by other components, including hot exhausts

Some ignition systems rely on a single coil, which feeds the high voltage to a distributor, which dispenses the power to each plug via a rotor arm that rotates within the distributor cap. As the electrical current leaps from the rotor am to the cap’s contacts, prior to flowing to the relevant spark plug, an easy DIY check is to make sure that the metal contacts have not burnt away. Worn rotor arms and distributor caps can result in a weak spark at the plug.

The distributor cap is a service item, which should be renewed periodically. Fortunately, they are often easy to remove.

distributer capInspect inside the distributor cap, for any burning or pitting of the contacts

The rotor arm

The rotor arm can often be pulled from the shaft and renewed

 Later models tend to have separate coils bolted directly to the engine, beneath which lies the spark plugs. Alternatively, a single coil may power one or even a pair of spark plugs. Individual coil packs can fail, leading to the engine running unevenly and causing the engine management warning lamp to illuminate. If your car suffers from running on less than its full complement of cylinders, do not be tempted to drive the car, because unburnt fuel can enter the catalytic converter and damage it.

Classic car engine

Classic cars tend to have large, cylindrical coils, bolted to the bodywork

 Ignition amplifiers

Ignition amplifiers replaced coils on models fitted with electronic ignition

 Individual coil packs

Individual coil packs can power one or a pair of spark plugs on the latest models, such as this Peugeot