BUY WITH CARE

The 5.4L Triton's headline failure is a spark plug blowing clean out of the cylinder head. The aluminum heads hold each plug with only a few threads, so a plug can strip those threads and eject under compression -- owners report a loud pop while driving, sudden rough running and a misfire, and a repair that runs from a steel thread insert (Time-Sert / Heli-Coil) per hole up to head removal, commonly quoted between several hundred and a few thousand dollars. Ford never recalled it; the fix is documented only in TSBs on repairing spark-plug hole threads in aluminum heads (609650, 6090435). The same modular-V8 architecture and defect are shared with the 4.6L and 6.8L, so on the engine-mixed 2004-2010 F-150 the complaint texts that name a blown or ejected plug are the reliable signal, not the engine badge.

Check — Ask whether any head has already had a steel thread insert installed (a prior one-hole blowout repair is common and not disqualifying if done properly) and whether all eight plugs seat and come out clean. If the original plugs have never been serviced near the ~100k interval, budget for the repair and treat an open history of "sounds fine" with caution.

The 3-valve's variable cam timing is the engine's other weak spot. The cam phasers and their VCT actuators wear and the timing-chain tensioners and guides loosen, producing a loud rattle or tick that is worst on a cold start and at idle -- owners repeatedly describe the engine as "sounding like a diesel." Left alone it progresses to phaser or tensioner failure and, in the worst cases, a replacement engine. Dealers typically diagnose cam phasers, VCT solenoids, and lifters, and the repair is a multi-thousand-dollar top-end job. Like the spark-plug defects this was handled as a known-issue / TSB matter, not a recall.

Check — Start the engine stone cold and listen for a rattle or diesel-like tick in the first few seconds and at idle; a persistent noise points to worn phasers or timing components. Ask whether the phasers, tensioners, or timing chains have already been replaced. A 5.4L that starts quiet and has documented maintenance is fine -- but this repair is expensive once the noise sets in.