what all can cause a rough idle? i've changed all coils and plugs with motorcrafts and cleaned the maf and throttle body. it still has a slight rough idle seems to be smooth if you push it a little like make it shift at around 2,000 rpms its a 03 with the 5.4 with 249,500 miles on it. the idle rpms are 1,100-1,200 at start up and around 650 in gear while its up to temp its a steady idle just shakes a little. No codes either the fuel filter was changed maybe 6,000 miles ago and i've put lucus injector cleaner in the tank about 2,000 miles ago it has a K&N drop in filter ( it has the oil on it from the factory i have not oiled it or cleaned it yet ) the PCV elbow was changed the same time as the plugs along with the EGR feedback hoses (they we're burn't/Melted) one day i'll look for cracks in vacuum lines i'm pretty sure they are factory ones. Also I use 6.5 quarts of Castrol high mileage and a purlator pure one filter and looking at the history it has has the oil pressure light come on so it may have been very low on oil at some point. Could that cause the idle because something is messed up?

 

have you replaced the IAC valve? Its located behind the throttle body held on with two bolts, the needle starts to stick and that will definitely cause a rough idle.

 

Nope haven't touched it haha. I'll check it and clean it when it's not raining. Or just replace it. Probably clean it. It'll be cheaper.

 

i would go ahead and just replace it. You'll be able to tell when you pull it off if its got build up on it. Chances are thats your problem if its never been changed, if not i would start looking for a cracked vacuum line like you said. This is what your looking for. Its easy enough to get too

 

well the throttle body was very dirty so it probably is also.

 

I'm pretty sure it will be. :thumb up:

 

You test idle/vacuum at operating temp. Thes engines should idle smooth and shouldn't flinch..very steady @ 725/730 RPM's. That's considered perfect. Ford has it between 700-750 RPM's for normal idle. BUT, when these engines are without any sort of leak, warm idle is steady at the RPM's I've posted above. My 98 is 729 RPM's. Has been since new. If it were any different, I know I have a vacuum leak or something else isn't right.

 

Intake leaking vacuum??

 

My idle is around 640-660 when warmed up and I have a vibration. I haven't been able to figure it out either. I can watch water in a water bottle shake, the seat belt and passenger seat shake.

FordFan, what you can do is unplug your IAC when it's running. If your truck dies you have an issue. I doubt a steady vibration is caused by IAC.

The only time I've seen failed IAC valves, the rpms are surging and really "bouncy" at idle. Sometimes the truck will die. Or they sound like a moose call.

 

When it's on the low side like that, -something is either plugged or IAC, TPS, MAF, perhaps a low grade miss. Does the trans shift alright ? TPS, MAF problems also relate to your trans tables (shift) which in-turn hinders idle. All work together and something to consider with low idle problems. Air filter and IAC piping. Clogged air filter and pipe obstruction to the IAC will knock the idle down. Something isn't right, surprised you don't have a DTC.

Those numbers I gave were at operating temp in park.