2008 f150 rough idle?

Question

I have a 2008 f150 with a 5.4 3v (newer version with brown coil boots) for a while now it’s been running rough at idle with high fuel pressure, high rpms, and positive fuel trims on both banks. Mostly happens when I’ve been idling in park for a while but when I put it in gear it smooths out but as soon as I put it back in park it runs rough again. If i put it in gear and drive it around a bit and put in park again it’s smooth until 10 or so minutes later it’ll start running rough again. Sometimes when I’m cruising down the road and it’s in overdrive, I’ll lightly press the gas just enough to raise rpms without it downshifting it will feel like it misfires almost as if it has a bad coil but all 8 are pretty new. The fuel pressure should be around 35psi but it goes up to 70psi and the fuel trims go to around 10 on each bank. Rpms at idle when smooth is around 600 but when it’s rough it’s about 800. Plugs, coils, fuel filter, air filter are all new, just cleaned injectors and replaced the o-rings, clean the MAF sensor several times and it seems to be working right, no vacuum leaks, no codes or check engine light. Any idea what it could be? Trying to avoid taking it to a mechanic as I don’t have the money to be up sold anything. The only other things I can think of is vvt, fuel pump and fuel pressure control module. It hasn’t been rough enough to feel like it wants to stall or has a dead cylinder or anything but it does have a very noticeable vibration.

falconry2 · Answer

I mentioned on a prior question that the V.V.T. solenoids are a common issue, more so than cam phasers or other things. The 3 valve engine is pretty notorious for it in fact. I think I even linked to the technician video to introduce the issue/solution. The solenoids clog easily and produce the symptoms described, with not just rough idle but a near dieseling "huff" sometimes. You can disconnect the leads to put them into the default non-advanced mode to see if the idle settles out after having it run at speed. Again, it will run decently at speed but not idle well from what you describe.
With it doing it on tranny changes there is a possible issue there, so you might have to purge the computer and have it relearn, again there are videos for that process. 
Hopefully, you don't need to tear into the front cover yet, eventually, the nylon timing chain guides fail or the oil pressure driven tensioners fail; that means so major work but the blocks seem to hold up pretty well so you shouldn't need a rebuild the first time you have to change chains.

Anonymous · Answer

Check vacuum leak with a smoke machine.  Could be a bad intake manifold gasket.  Get a dealer level scanner and check misfire counts on every cylinder.  If you are unlucky, cam phasers might need replacement.  Good luck.

2008 f150 rough idle?

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