Spent 4 hours chasing a phantom misfire on a '17 F-150 EcoBoost

Had this truck come in with a rough idle and a flashing check engine light. Code said cylinder 3 misfire, so I figured easy fix, swap the coil pack and plug. Did that, cleared the code, and it came right back. Started checking wiring, compression, even swapped injectors between cylinders. Nothing. Finally, after pulling my hair out, I found the real problem. The PCV valve diaphragm had a tiny tear, letting unmetered air into the intake and leaning out that specific cylinder. The part cost maybe 15 bucks, but the hunt for it ate up my whole morning. It's one of those things you don't think to check until you've tried everything else. Anyone else run into weird vacuum leaks causing misfires on these newer trucks?

2 comments

2 Comments

elliot_shah11d ago

Those PCV valves are a real weak spot on the 3.5s... the diaphragm just gets brittle. It's a cheap part that causes a really expensive-sounding problem. Always check that hose for oil sweat first, it's a dead giveaway.

kim.stella11d ago

My buddy's Accord started making this awful whistling noise last year, and his mechanic found a cracked PCV hose covered in oil. Total nightmare. There was a whole thread on a forum about how the factory ones just don't last in certain climates. Replacing the whole assembly seems to be the move, not just the valve.