Okay, so I have a 1993 F150 XLT 4x4 with the 5.0L EFI. It seems to have a rough idle. The RPM needle does not move much but it's noticeable in the feel of the truck. Once started the truck goes to high idle which is fine, then falls down quick, and it idles low for a second, and very irratic. it will go up, and down up and down up and down (almost like I have a highlift cam), but then the jumps get smaller and they level off. Then the truck warms up a bit, and you can feel the miss in the seat on your back. I also have a CB in the truck and you can notice the miss on the cb, because you can see it shaking a bit. The needle doesnt move much, but it stumbles. I'm wondering if there is a way to fix this rough idle. The plugs are new in it, also, the timing is good and the oil was just changed, as well as the plug wires and rotor cap for the distributor. Also, when this pulsating idle is doing it's thing, the Volt meter doesn't move much, but you can see the lights dimming, and brighting, dimming and brightning. not much, but you can see it. Then if I just tap the gas right quick, the volt meter will shoot up about a quater of a movment, stay there for a second, but once the truck idles back down into its rough idle, the volt meter goes back down that quater of a movement. I am also wondering if someone could help me with that. I hope i explained the problems well, and thanks so much ahead of time for the help.

 

Wow... have you checked for codes? That might give you some leads on the rough idle. Might also try pulling each plug wire (at the plug) while the engine idles. If one doesn't change the idle, you've probably got a plugged or dead injector on that cylinder.

The voltage thing, I think I'd check grounds first - starting with the negative battery cable. They tend to corrode inside the insulation, and cause all sorts of issues like you're having.

 

sounds like your voltage is directly affected by the idle, so i wouldn't worry to much about that. When you say your idle goes up and down, up and down, where are the rpms at there lowest point? When you checked the timing did you have the spout pulled or plugged in? How does the truck do crusing and accelerating? is the miss only at idle?

 

you could also try seafoam. i have a friend with a similar miss on idle and it cured his

 

The local mechanic checked the timing and said it was alright. Umm, driving and crusing speeds are just fine. But idle is just a little rough. The rpms arn't very low when they do go down, they're just enough to make the truck run a little rough, and you can feel it in the seat and steering wheel, etc. The miss is only at idle. The codes did get ran not too long ago, and the only one I could think of would be EGR valve which showed up. I don't know if that would have anything to do with the idle or not. I will try the fuelinjector test tomorrow to see if ones plugged or something. Thanks for the idea, and I'll get back to you with the results. If someone has any more ideas, let me know. Thanks guys for the help. It's not a big miss, but it's just big enough to annoy me.

 

Sounds a lot like the issues I've experienced with the IAC valve as far as the start-up idle speed. Cleaning it out usually restores the idle.

As far as the perceived misses - perhaps check the coil to see if the resistances are within spec, also maybe consider some fuel injector cleaner - at idle speeds, the pulse durations will be the shortest, so if there's any problems with sticking or the like - should be worse at idle speeds.

 

My truck started doing the same thing, originally there were no codes, just recently I got a egr sensor voltage low code. Mine is a 93 5.0 xlt 4x4 supercab, the voltage drops about the same time as the idle problems and I believe that it is related though my altenator has started to whine during cold weather startups. I should pull off the IAC and clean it one of these days when it warms up.

 

I second the IAC my wifes mustang did the little idle hunt no vac leaks
so turned to the IAC and sure enough it was that.

 

well with that information, i also would tackle the IAC motor first. all you need to do is pull it off, and take some good cleaner and clean all the gunk out of it, they usually get gummed up. Might be fuel related, i just think you'd have more problems past idle, the injector pulse width was a good point though.