Hi everyone;

Hoping for some help diagnosing a misfire / fluctuating idle in my 2005 4.6L.



Misfire first occurred while driving, CEL code was for a misfire in cylinder 1.

I changed the spark plug in that cylinder and swapped the coil with another coil.

Misfire never went away, but it was a few weeks until the CEL came back on.



This time CEL was a P0506 code; Idle Air Control (IAC) System RPM Lower Than Expected.

I removed the throttle body and thoroughly cleaned it - was very, very gunked up. Noticed a crack where the needle valve screws into throttle body, so I put a new one on.

I cleaned the Mass Airflow sensor.

I pulled the PCV valve and it has very good suction.

I've tried looking for vacuum leaks and so far haven't found any, but I’m far from experienced in finding them.

Engine definitely fluctuates rpm at idle, but I would say it's brief fluctuations.


While I'm running significantly better than before, but still getting a misfire-like shake though and the rpms definitely fluctuate though I don't have a tach.

There is a distinct thumping of exhaust. Seems to be loudest from the muffler, maybe some noise from the catalytic converters, but it’s not consistent and not a loud rattle.

Catalytic converter issue? Muffler issue? Something engine wise that I am completely missing? Please point me in the right direction!



Any help would be greatly appreciated. I’m very inexperienced in doing my own work, but I enjoy it. If you need some more information, I'll do my best to find out.
Thanks kindly for your time.

 

https://www.ford-trucks.com/forums/1...-problems.html

mines an 99 but simular, maybe something there will help?