The diagnosis of a misfire in an engine can be more difficult than figuring out why the engine won't start at all; misfire problems are normally minor and are often overlooked. The diagnostic procedure recommended by the Ford Motor Company starts with looking at electrical faults. It starts with unscrewing and visually checking each of the spark plugs for wear, corrosion, carbon buildup or residue preventing a spark. Then each is checked to see if it's capable of producing a spark. If it can, then the spark plug's wiring to the distributor must be checked with an ohm meter. The distributor caps must be inspected to make sure they contain no water or other obstructions and fit onto the distributor securely. The wires leading from the distributor to the ignition coil must be checked with an ohm meter, and then the ignition coil itself must be tested as well. If the charge created by all the wires, plugs and the ignition coil is weak, then the car battery must be checked to make sure that it's charged, the leads connected and not corroded, and finally the terminal of the alternator must be checked to make sure it's functioning correctly and providing the entire system with current.
Should no fault be found with the electrical system, the fuel system must be evaluated. Oxygen sensors should be used to look for leaks in the vacuum lines in newer model Fords, and in older model Fords, the air filter in the carburetor should be cleaned, and then the carburetor itself inspected. The inspection can only be done by eye with someone else revving the engine to ensure that the engine is being provided with a solid fuel/air mix. Should it not be, then the carburetor must be disassembled and cleaned. Should this prove ineffective in resolving the problem, the constituent fuel filter and fuel lines leading into the carburetor need to be either flushed out or replaced entirely.
Should the inspection of the fuel system reveal no abnormalities, the engine itself must be checked. This requires that the engine be partially disassembled in order to reveal the combustion cylinders and fuel injectors. If it's a modern Ford, the fuel injectors are removed and tested to make sure they are not clogged and work normally. Simply looking into the cylinders should be enough to discern whether or not they have become dirty with carbon deposits and other residue from fuel combustion. They will need to be cleaned if such is the case. Finally, should no other cause be evident, the engine must undergo a compression check. This involves removing the spark plugs from each of the engine's cylinders and cranking the engine with a specialized pressure gauge in place. Average Ford engine compression is between 140 and 160 psi. Any less than this indicates worn rings, cylinder and pistons of that specific cylinder, and indicates that an engine overhaul is required.
