Check the spark plug:
Remove the spark plug and inspect it for damage or excessive wear. If the spark plug is damaged or worn, replace it.
Ensure the spark plug wire is properly connected to the spark plug and the distributor cap.
Check the spark plug gap and adjust it if necessary. The spark plug gap for a 1991 Mustang should be .044-.054 inches.
Check the spark plug wire:
Inspect the spark plug wire for any damage or cracks. If the spark plug wire is damaged or cracked, replace it.
Ensure the spark plug wire is securely connected to the distributor cap and the spark plug.
Check the distributor cap and rotor:
Remove the distributor cap and inspect it for cracks or carbon buildup. If the distributor cap is cracked or has carbon buildup, replace it.
Check that the rotor is not worn and replace it if needed.
Check the ignition coil:
If the spark, plug, plug wires, cap, and rotor are all in good condition, the next step is to check the ignition coil.
Test the ignition coil to see if it is providing a strong spark. If the coil is not providing a strong spark, replace it.
Check the fuel injector:
Remove the fuel injector and inspect it for any damage or clogs. If the fuel injector is damaged or clogged, replace it.
Check the fuel injector wiring and ensure it is properly connected.
Perform a compression test:
A compression test will help determine the condition of the piston rings and valves. If the compression is low in cylinder 1, it could indicate a problem with the piston rings or valves.
Check for vacuum leaks:
If the fuel system and ignition system are both in good condition, a vacuum leak could cause a misfire. Inspect all of the vacuum lines for cracks or damage and replace any that are damaged.
Retighten the cylinder head bolts:
Loose cylinder head bolts can cause a gasket to leak, leading to a misfire. Remove the cylinder head bolts and tighten them to the specified torque.
Check the camshaft:
If the camshaft is worn, it can cause the valves to open or close at the wrong time. This can lead to a misfire. Remove the camshaft and inspect it for any wear or damage. Replace or recondition the camshaft as needed.
If you have checked all these and still have a misfire, you may need to take your Mustang to a professional mechanic for further diagnosis and repair.
