i have a 2001 expedition 4.6 lt. in the last week it has started idling very rough, and when im maintaining a certain speed is also starts shaking like its about to die. but only while im maintaining a certain speed, if i accelerate it doesn't do it. and if i let off the gas completely it doesn't do it until im at a very low speed.

thank you for any imput

 

How many miles on it?
How many miles on the spark plugs?
Have you inspected the COPs for signs of arcing to ground?
Any chance you got a bad batch of fuel?

 

Check the MAF meter and the throttle body to see if they are dirty. If they are, "take them off" and clean they with the "appropriate" spray cleaner. EGR valve and vacuum leaks can cause a bad idle as well. The previous post listed other items as well to address.

First thing is to get a code reader on it. It may speed up the fix, as opposed to just throwing parts at it.

Hope this helps...