Spark plug blowout on Ford engines—where the spark plug threads strip and the plug ejects from the cylinder head—has been a documented issue on certain Ford modular engines. Understanding this problem and how to prevent it applies to proper maintenance of your Expedition.
Historical Context
Spark plug blowout was primarily associated with Ford's older 4.6L and 5.4L modular V8 engines (1997-2008) that used only 4 threads to secure the spark plug—an insufficient number that made thread stripping more likely. The 2018 Expedition uses the 3.5L EcoBoost V6, which has a different cylinder head design with more robust spark plug threads. However, understanding proper procedures remains important.
What Causes Blowout
On engines susceptible to the problem, blowout occurs when the aluminum cylinder head threads strip, unable to contain combustion pressure. Contributing factors include over-torquing during installation (which damages threads), cross-threading, improper anti-seize application, and removal/reinstallation cycles that wear threads. On any engine, these same factors can damage threads over time.
Signs of Impending Problems
A loose spark plug may cause misfires, rough running, and a ticking or hissing sound from the engine bay. If caught early, the plug can be properly re-torqued. If the plug has been running loose, thread damage may have already occurred. Any unusual sounds from the engine after spark plug service should be investigated immediately.
Proper Spark Plug Procedure
Always torque spark plugs to specification—typically 11-15 lb-ft for the EcoBoost. Use a torque wrench, not feel. Apply anti-seize sparingly only to the threads, keeping it off the electrode. Never install plugs in a hot engine—aluminum heads expand when hot. Inspect threads before installation and chase with a tap if any corrosion exists. Replace spark plugs at recommended intervals to avoid seized plugs requiring excess force.
Repair Options if Blowout Occurs
If threads strip, repair options include Time-Sert or Helicoil thread inserts that restore proper thread engagement. These repairs cost $200-$500 per cylinder when done properly. In severe cases where the head is damaged beyond insert repair, cylinder head replacement may be necessary at $2,000+. Prevention through proper procedure is far more economical.