In january I purchased my 2006 f150 with the 3v 5.4. The truck currently has 130,000km. When I bought the truck, nothing was amiss. Shortly afterwards the engine developed a slight stumble at idle, and under very light load at 70km/h and again at around 110km/h. It is more apparent at 110km/h.

Now not knowing the history of the truck, aside from needing an oil change at 130,000km, which I did with 5w20 and the motorcraft filter, I decided to tackle the plugs, as I figured they were original. 4 broke, and once the job was done, with the help of the napa removal tool, things haven't improved with the engine.

It still has the stumble at idle, but as far as I can tell the stumbling at speed has almost completely disappeared. Now here is what I don't understand. I went from getting an average of 18mpg at highway speed, to 14mpg after the plug swap. The truck is not throwing any codes, nor does it have any lack of power under wot, or even part throttle. It doesn't appear to be making any phaser noises.

I am stumped. Any ideas as to what could possibly be causing these issues?

 

http://www.ford-trucks.com/forums/72...mble-help.html

 

Did you use Motorcraft SP-515 plugs? Did you replace the coil boots?