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08 f150 rough idle?

I’ve asked this a few times before but since it’s still not fixed I thought I’d give some updated info. 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. The fuel pressure should be around 35psi but it goes up to 70psi and the fuel trims go to around 10 on each bank. Gets horrible mileage (about 10mpg instead of the 15 or so I used to get), Rpms at idle when smooth is around 600 but when it’s rough it’s about 800. Plugs, coils, fuel filter, air filter all new, cleaned injectors and replaced the o-rings, MAF sensor is fine, no vacuum leaks, no codes or check engine light, vvt is fine. Any idea what it could be? I don’t have the money for a professional. only other things I can think of is the fuel pump, fuel pressure control module, or pcv but my truck doesn’t have a normal pcv like other vehicles, it’s built into the valve cover and is not a maintenance item and from what I heard is just a mesh screen. 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.

3 Answers

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  • Anonymous
    2 months ago

    Clean the idle air control valve (if it has one) and throttle body.

    No luck, check O2 sensor output with a scanner.  If O2 sensor output is flat lining, O2 sensor could be "lazy" without setting a code.  If O2 sensor output is normal, check fuel trims at idle and when revved. 

    If fuel trims become normal when revved, suspect a vacuum leak.  CONFIRM vacuum leak with a smoke machine.  Suspect a bad intake manifold gasket, common on your vintage.  If fuel trims are the same when revved, suspect a bad MAF sensor.

    If rough idle still persists, suspect a bad fuel pump.  Check output of fuel pump control module with a scope at fuel pump connector.  If output is good, fuel pump is bad.

    https://www.youtube.com/watch?v=ZhmcwSCsnT4

  • Anonymous
    2 months ago

    Quick check.  Get it idling.  Now pop hood.  If you got a cloth rag it helps, or a glove like Michael Jackson...now start pulling off spark plug wires one at a time and see what the engine does.  If it runs "rougher"(OR STALLS) That plug/coil is good. Put the wire back on the plug and if you must restart the engine.  Take off the next  spark plug wire and note if engine gets "rougher" or NOT.   Then put wire back on.  You are looking for the ones that are NOT firing...as the engine idle will not change.  That is faulty(either the plug or the coil) So now you know what to check and how.  If it is a V8, it will run on 5 cylinders(or 4)-(a PoS for power but it will run on half a motor.)> I know, I ran a 4cylinder on 2 for 500 miles @60MPH on a dead flat road.

    .   If you find that 2 cylinders are not working and they are side by side, there is a good chance you crossed wires, so swap them plug connections.  It may be that simple.

    . If it is not crossed wires then change: spark plugs are cheaper than coils so I would change them first(even new ones can be faulty which I would return to the store I bought them at with the receipt and exchange them for some other spark plugs  and properly gap the plugs &. try again.(if the old ones are still available and not damaged by chucking them into a bin you can use them.  This is just a test.

    .  That may clear up your problems.

    You are getting fuel. It is just a DOG.

    Electrical problems first. Fuel second.

    Just use LOGIC

  • ?
    Lv 7
    2 months ago

    Sounds like the fuel pressure regulator. It's not dropping the pressure in the fuel system at idle. Either the regulator is faulty or the ECM is not getting the message that the engine is at idle.

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