The Ford spark plugs are made so well, this ignition component is used on other makes of vehicles on the road and on the track. In fact, they are one of the official sponsors of NASCAR. The spark plug has to have the correct gap (set by the installer) which is its depth into the combustion chamber. It also needs to have the correct temperature range for it to operate properly in your Ford engine. For these reasons the correct spark plug must be in place in each cylinder for the engine to performance at its maximum potential on your Ford vehicle. The testing of a Ford spark plug is done by pulling the plug and reconnecting it to the wire. Then ground the end of the plug to the engine. When the engine is being turned over, but not started, a strong bluish white flash should be seen emanating from the electrode at the end of the spark plug. If the flash is either hard to see or non-existent, then the plug should be exchanged for another one and the test performed again. This will check to make sure there is a spark coming from the wire and distributor. The checking for spark on your Ford is one of the three main factors that must be present for an internal combustion engine to operate. The other two include compression in the combustion chamber and fuel being supplied to the engine. A tune-up on your engine is the replacement of all spark plugs. There is one for each cylinder. If one is missed and not replaced, not only will the efficiency of the engine be reduced but the engine will run rough. No matter how difficult pulling the plugs are, they must all be replaced even if you have to go through the wheel well to gain access to them.