I have an very rough running F150. It's ok if I have it punched or not pressing the gas at all, but if I push the pedal down just a bit the truck shakes and bucks. The last time it did this, I changed the plugs and COP's and it ran good.

This time I changed the plugs, COP's, and fuel filter. It doesn't feel like it got any better. It still runs very rough. Does anybody have any ideas what else could be causing this. I was certain this would fix the problem but apparently not. It also has relatively new injectors so I don't think they should be dirty.

Thanks.
1999 F150 XLT, 5.4 V8, 215k

 

Suck test the EGR valve. Good? Compression test next. Unless you have the wrong the plugs in there. 99's can be picky.

 

I haven't checked the EGR valve. The plugs I had in before were actually the wrong ones (they were for the lightning). So I thought for sure putting the new ones in would do the trick. I used the OEM Motorcraft plugs. It actually threw an engine code today, so i'll try to get that checked out ASAP. Thanks.

 

You get it scanned yet?

 

Without hearing it i'm not positive but it sounds to me like a miss. Check compression of each cylinder at tdc. Another good thing to do is a cylinder balance test. Balance test is with the engine running ground one plug wire at a time when the plug wire is grounded the engine should make a noise change then when you stop grounding it out it should return to what it was at. If there is no noise change when the wire is grounded then you know there is a dead miss in that cylinder. Another thing is while then engine is running using an automotive stethoscope listen to the fuel injectors you should hear a pop of them opening and closing if one is not making that noise either the injector is not get voltage or the injector has failed. THats about all I can think of right now. Is the enigne throwing a code by chance, do you have good gas?

 

Dont forget the old PCV Valve vacuum leak, from the rubber grommet at the back of the intake manifold. Just had mine done.

Vince

 

also check the injectors

 

did you fix the problem. i am in the same boat right now. first time my 04 was running rough i changed the cops and plugs and it ran great. this time i did the same thing and it still runs rough. any suggestions?