Notorious Ford V8 spark plugs need an extended warranty

Replacing spark plugs has been a no-brainer for most passenger vehicles for, say, the last 100 years. But for millions of owners of V8-powered Fords from the 2004 to 2008 model years, a spark-plug change can lead to an expensive, hours-long ordeal.

Why? Because the original spark plugs can weld themselves to the cylinder head and break into pieces when someone tries to unscrew them, leaving the bottom of the plug stuck inside the cylinder head. Fishing out the broken section requires special tools and a trained technician.

The engines in question are three-valve V8s, mostly 4.6- and 5.4-liter powerplants, found in many Ford Explorers, Explorer Sport Tracs, Expeditions, and F-Series pickups, and Mustangs, Mercury Mountaineers, and Lincoln Navigators and Mark LTs.

At the heart of the problem is a unique Motorcraft spark plug with a two-piece metal shell that's vulnerable to corrosion over time and to breaking apart on removal. The original service interval was 100,000 miles, meaning that the first recommended service would take place long after the vehicle's basic warranty expired.

Ford has known about this problem for a long time, and in 2008 issued an 11-page technical service bulletin (TSB 08-7-6) with exacting instructions for handling three different ways technicians could expect the spark plugs break on removal.

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A further aggravation is that mechanics can't predict how long the service will take. Anywhere from none to all eight plugs could shatter, running the bill to many hundreds of dollars. If things really go wrong, it could mean removing or replacing one or both cylinder heads, which can run to thousands of dollars.

In the meantime, Motorcraft, Ford's parts brand, has designed a one-piece spark plug that's less likely to fall apart. If you own one of these vehicles, you should certainly demand the most up-to-date plug. Our technical advisers say it's also a good idea to have the plugs changed every 30,000 miles, regardless of what the owner's manual may say, even if they're the newer type.

Since this is a known problem related to what we see as a design defect, we think Ford should cover this service under an extended warranty. It's not fair to make consumers pick up the tab for a known defect.

—Seung Min Yu and Gordon Hard