I have a 2008 Expedition EL (munchkin transport) with 148,000 miles.
A couple of months ago we started having a mild misfire / loss of power / rough idle. Nothing much. I investigated and found that the heater hose was leaking into the passenger side spark plug wells.
I replaced the heater hose and changed all of the spark plugs (broke #'s 6, 7 and
8). After that, there was a new problem... a LOUD popping/rattle and shuddering misfire starting at about 2500-3000 RPM, and only under load (won't happen at idle or if revving it in park or neutral- making it very hard to troubleshoot). Still have low power - it will get to 75 MPH or so, and downshift on small hills at that speed, causing the popping to start. The misfire is now throwing a code.
I have an Auto Enginuity scanner, and it is giving a #3 misfire code every time the popping happens.
I have replaced the #3 COP and boot for a new one.
I switched the #2 and #3 injectors - they looked clean, though - I changed the fuel filter about 10k miles ago.
Air filter is good.
Compression test is good (#3 tied for lowest @ 152, but also got that on 4 and 5; highest was 167 psi on #7 and #8).
Verified that there is a spark from #3 COP, at least at idle.
Ran with #3 injector and COP unplugged and still had the popping noise, so it's not fuel getting into the exhaust header (at least, not from #3).
Also, still have a rough idle, but it got better after the spark plug change. I did run some seafoam in the vacuum line, but no improvement from that.
What could cause the popping under load?
Could a cam phaser / timing chain problem do this?