Dan is an experienced motoring journalist who has more than 20 years of experience. He has been the editor of titles such as Fast Ford and Redline, and his latest project was converting an old Renault Trafic into a family campervan.
Have you ever wondered what a spark plug's job is? How to find them, and how they are tested? Well read on and we'll give you the answers...
The spark plug's job is to deliver an electric current into the engine's combustion chamber. This current causes a spark to jump across the plug's electrodes which ignites the compressed fuel/air mixture within the cylinder.
In general each cylinder has one spark plug (there are cars with two spark plugs per cylinder, but these are in the minority). So a four cylinder car will have four spark plugs.
They can generally be found on the top, or side of the engine, sometimes under an engine cover, and sometimes not.
Each spark plug will have an HT lead, (on older cars) or a coil on plug attached to the top of it. Removing the HT lead, or coil on plug will reveal the base of the spark plug.
Removing and replacing spark plugs is very simple – getting to them, on some engines, can be a little tricky.
As the name suggests this tests the spark plug, or more accurately it tests whether there is an electrical current capable of generating a spark. The devices are very simple to operate, and safe.
You simply remove the HT lead and plug the device in-line, so it connects to the end of the HT lead and the base of the spark plug. Operate the engine as usual, and the tester will illuminate if it is receiving a strong electrical input.
If no light is illuminated further diagnostic work is needed. First swap the plug, then the HT lead. If you get no joj, then work back down the ignition system until you find the fault.
You can test a plug for a spark by 'earthing it'. To do this removing the plug, but reconnect the HT lead – hold the HT rubber boot with insulted pliers and almost touch the engine block with the spark plug.
Get someone to attempt to start the engine and you should see a strong blue spark jump from the plug to the engine. If this is a weak spark it may not be sufficient to overcome the immense cylinder pressures – which is why an in-line tester is a more reliable method of diagnosis.
A spark plug generates huge voltages anywhere between 25-45,000 volts! Measuring the voltage is not really necessary as a visual check will confirm whether it is sparking strongly or not – either using the earthing method or a spark plug tester.
In short – yes. But you'll generally feel the car misfiring (lumpy engine note, and lack of power) as it brings on the engine management light. Reading the fault codes will help you pinpoint the cause. It's not uncommon for the coil-on-plug units to fail causing the car to misfire, and the dash light to come on.