Hi all, I have a 2004 F-150 Supercrew 5.4 FX4, with 111k new body style.
Last night my truck was parked idle, the rpms dropped to 500-600 and started revving up and down, then died. I started it up again and it ran strong, let it sit for a minute or two and it died again. It seems to run fine while driving around, but if i let off the gas i can feel it rocking and I'm afraid it will die again. As for gas, I filled it full from nearly empty last time, and is currently at a half tank, so I don't think it's bad gas as I've burned a half tank of this gas without seeing any problems. It threw a CEL so I stopped in to the auto parts store and borrowed their code reader. The codes were 'System Running too Lean', 'O2 sensor system running too lean' (x2). I didn't write down the exact code numbers as I was just borrowing it.

The fuel pump and filter were replaced in late May/early June of this year. I drive a lot so I bought another fuel filter and will replace it tomorrow, just to be sure. I bought some CRC MAF cleaner and cleaned the MAF sensor really well today. Also bought a can of SeaFoam and dumped it into the gas tank, no difference from either of those. The truck has sounded like a diesel during idle since I bought it, sort of a knock I guess. Oil was changed 3500 miles ago but is synthetic with a high mileage filter.

I haven't checked the throttle body but acceleration seems fine to me. I also haven't checked the plugs, as all the threads about them have me afraid. I'm hoping the new fuel filter will fix the problem. Do you guys have any other suggestions or things to look for?

 

I have the same truck and about the same mileage. I recently had the exact same problem! If your truck is stalling when it idles suspect a vacuum leak! In my case the vacuum line going to the back of the motor for the 4x4 had come off! To see if that's your problem start the truck and crawl under the front of the truck and try turning your front axle with your hand, if it doesn't turn freely with your 4x4 off then that is likely the source of your vacuum leak! To get the line back on is a pain! I had to remove my fuel rails to create enough room to get my hand behind the motor! Hope this helps!