As it is in all internal combustion engines, it is the Eagle spark plug that ignites the air-fuel mixture in the combustion chamber so the engine will function as expected on your vehicle. There is one spark plug per cylinder on all models from this manufacturer. Each of the Eagle spark plugs are located in the cylinder head, right at the end of the spark plug wires that receive their power from the distributor or ignition coil. Each one must be changed periodically at the same time for the correct level of spark can be introduced to the cylinder on your Eagle. This is what a periodic tune up accomplishes, the changing of the spark plugs along with an inspection of the ignition system including the wires and distributor cap. Without all of them functioning properly, an insufficient level of spark will be reaching the combustion chamber and the engine will not run smoothly or at all in some instances. The testing of the spark plug is a simple procedure the owner of the Eagle can perform. With the removal of the plug from the engine the wire must be reattached and the end of the electrode grounded to the engine. A strong blue spark should be seen at the electrode if it is functioning properly when the engine is running. If it is not, then change the plug in the wire and retest. If you now see a blue spark then the previous plug is faulty. If there is still not a spark, the wires and distributor cap could possibly be faulty. It is important that a tune up on your Eagle be performed every 24,000 miles or at least once every couple years. This will make it possible for your engine to perform when called upon by the owner of the vehicle.