Very common symptoms we all have at one time or another with our 5.4L is a rough idle where we will have high idle for a few seconds when slowing or coming to a stop followed by a drop off to a very low rough idle after stopped for a few seconds sometimes even to the point of stalling out.
I had this problem for a few weeks, and then the weather turned cold, and it got even worse to the point that my ‘ check engine light’ was even coming on. I took it to the dealer, and they replaced my IAC. No change. Still idled rough. Still had the CEL. The only change was the weight of my wallet!! So I turned to the internet!!!
By doing some ( a lot) of research on the subject from other Ford sites and forums, this is a culmination of what I’ve found out. The fix is a little rubber ’boot’ or elbow with a crack or a hole in it causing a vacuum leak.
The Ford dealer will run a diagnostic and indicate that it returned an error code P0171 and P0174, which indicates the engine is running lean. They will most likely recommended the following service:
Replace EGR valve - Replace PCV valve - Replace differential feedback EGR sensor -Replace mass airflow sensor -clean fuel injectors
all at a cost of almost $800. Yikes!!!!
The first thing to check is your vacuum assembly. You most likely have crack or hole in the little rubber elbow that connects to the intake from the PCV valve on the right hand side of engine. Even the slightest crack will allow more air to be sucked into the engine causing a too lean mixture.
The PCV valve is in the right valve cover... which is on the left side when your facing the engine (passenger side). It's under a lot of other hoses so it is very hard to see.. The hose goes back from the valve, then toward the drivers side to attach to the back of the intake way back between the engine and the firewall very close to the top of the engine. The common faulty part is the rubber boot that connects the hose to the back of the intake.
The way I found it was to follow the hose back from the valve cover. I sat right on the engine shroud, and felt for the tube sticking out of the top rear of the valve cover on the passenger side of the engine. With the engine off, it pretty much just pulled out and went back in real easy. Then I just followed it with my hand the best I could ( with the motor now running so be careful!!) until I got back to the firewall, and it actually comes up from underneath the rear of the intake. . You can kinda reach your finger up under there to where it's connected, and feel and hear a definate shoosh sound when your finger goes over the hole or crack in the elbow.. You can't see it, but you can definatley trace it back by following that tube. You'll know it when you find it because your finger will plug the hole, and your truck will start running smooth again!!!
The part number for the entire vacuum assembly with the correct boot on it is - F85Z-6C324-AA
or (updated F75Z-6C324-JA or JB) probably around $30 -$40. Or you might be able to just replace the elbow itself. Either way, make sure if you do have the leak, to make sure you have the replacement part handy before you take off the old one, because it will crumble in your fingers!!
Hopefully you just saved a few $100, and a lot of frustration. I know I was pretty happy when I found this easy fix!! Good luck!!!
The other thing to check is your MAF.
It's the Mass Air Flow sensor. It's located in the air inlet tube that comes from where your air cleaner/filter is. Just disconnect your air inlet tube ahead of the sensor, and spray some MAF cleaner through the screen at the little elements in the center of the sensor. Let it sit for 5 minutes or so...,give it another shot, let it sit for another 5 minutes, then start your engine. Some people just use electronics cleaner, but I recommend actual MAF cleaner from the auto parts store. You can also take the sensor right out and clean it that way too, but it's easiest to just spray it where it is. You can hold a rag or something behind it. Here's a pic of what you are looking for. Mine looks a little different on my engine, but it's pretty much the same idea.
Hopefully, between the elbow or vacuum hose, and the MAF, there will be no more CEL and your Expedition will run like a charm again!!!!
Feel free to add your ideas and suggestions!! But these are two quick, cheap fixes.