Our technicians tell us the spark plugs in these engines can be very difficult to remove. Ford has issued a service bulletin (08-7-6) to address this issue, which includes a recommended procedure on how to remove the plugs. Failure to follow the recommended procedure can result in the one or more spark plugs breaking off in the cylinder head.
The recommend procedure is as follows. Using this procedure will greatly reduce the chance of breaking a spark plug.
1. Break the spark plugs loose when the engine is warm.
2. Turn each plug 1/8 to 1/4 turn and soak the treads with 1/2 to 3/4 teaspoon of Motorcraft Carburetor Tune-Up Cleaner or a quality penetrating oil. Do not try to remove the plugs at this time.
3. Let the threads soak for at least 15 minutes.
4. After the soak period, tighten and loosen each spark plug, working it back and forth until the turning effort is reduced. Then, you can remove the spark plug.
There are special tools available to remove the broken spark plugs and repair shops will often charge additional labor time, over and above the quoted spark plug replacement cost, for the removal of each broken spark plug.