Mercury Cougar, 1996–1997 Ford Thunderbird, 1996–1997 Ford Mustang, June 24, 1997 – 2001 (some vehicles) Ford Explorer, early 2002 Spark plug issues 2-valve 4.6 L, 5.4 L, and 6.8 L engines found in many 1997–2008 Ford, Lincoln, and Mercury vehicles may have aluminum cylinder heads with threads for spark plugs that are stripped, missing, or otherwise insufficiently bored out. Ford acknowledges this issue in Technical Service Bulletin 07-21-2 as well as earlier TSBs. Ford's TSB does not state that this issue is caused by owner neglect. Ford's only authorized repair procedure for out-of-warranty vehicles is to use the LOCK-N-STITCH aluminum insert and tool kit. For vehicles under the New Vehicle Limited Warranty, Ford will only cover the replacement of the entire cylinder head; however, the Ford recommended spark plug service interval extends beyond the duration of the New Vehicle Limited Warranty. 3-valve 5.4 L and 6.8 L engines built before 10/9/07 and 3-valve 4.6 Ls built before 11/30/07 found in many 2004–2008 Ford, Lincoln, and Mercury vehicles have an issue with difficult-to-remove spark plugs, which can cause part of the spark plug to become seized in the cylinder head. The source of the problem is a unique plug design that uses a 2-piece shell, which often separates, leaving the lower portion of the spark plug stuck deep in the engine's cylinder head. The 2-piece OE spark plug design is intrinsically flawed, thus making it susceptible to this problem. Ford acknowledges this issue in TSB 08-7-6 as well as earlier TSBs. Ford's TSB does not state that this issue is caused by owner neglect. The TSB provides a special procedure for spark plug removal on these engines. For situations in which the spark plug has partially broken off in the cylinder head, Ford distributes multiple special tools for removing the seized portion of the plug. Their TSB explains the multiple procedures required for handling the different cases/situations that occur when parts of plugs are seized in these engines. This repair is covered for vehicles under warranty; however, the Ford recommended spark plug service interval extends beyond the duration of the New Vehicle Limited Warranty. See also List of Ford engines List of Ford factories References Modular Goods manufactured in Canada Motor vehicles manufactured in the United States V8 engines V10 engines Gasoline engines by model