These are what ignite the fuel within the engine, they consist of a small metal tip that can over time wear down from heat and pressure and the continual sparks which they produce. Once this gap is bigger than the recommended space, the spark plug will either spark intermittently or not spark at all.
This is what you would know as a misfire and happens when the fuel is not being ignited in that cylinder. When spark plugs are replaced during a tune up, it is recommended to use original equipment as recommended by your manufacturer.
Over time these can break down, either the connection with the spark plug or the ignition coil fails to make contact, and when this happens it would result in a cylinder not firing at all. It is recommended to change all of these in sets so they are all of an equal gauge, they are also wired in a certain configuration to match the rotation of the cylinders, if these are crossed wired, your engine would run but sound very rough as the engine would be trying to spark out of time.
Distributor Cap & Rotor (if applicable)
Inside the distributor cap is a rotating arm with a metal tip, this spins at a high speed and makes contact with metal points that allow the spark to travel down the cables and to the spark plugs. Over time as with a spark plug, these contacts can wear and will not make a sure contact on every rotation. The symptoms of this are random firing or no firing at all. During a car engine tune up it is recommended to change both of these at the same time as they will be perfectly matched.
A car has many filters on an engine and it is their duty to prevent any contaminants from entering into the engine or the actual body of the car through the ventilation system. These filters when blocked can have a direct impact on engine performance.
Whilst a car is running, air is constantly being sucked into the engine, over time an air filter will become clogged with dust and other particles that get sucked along with the air whilst you are driving around. A dirty air filter can reduce MPG’s by up to 14% in carbureted engines, and also performance. More modern cars are likely to have fuel injected engines so the MPG’s will not be affected by a dirty air filter, but performance will still suffer.