I apologize, new user cross posted this previously in wrong Forum, Experienced advice needed Please!

I recently bought a used Expedition with the 5.4. It has intermittently run very roughly and will not maintain an idle uless you apply extra throttle. It goes into this bad running mode intermittently, runs fine for a few days or even three weeks, then bad idling again for a few short trips.
I'm afraid to let my wife drive it as when it fails you stall when cornering with your foot off the gas, the power steering becomes VERY heavy and a safety concern.
I had autozone check the codes after the last failure, it cleared up 2 miles before I arrived at Autozone, they found no stored codes. I bought a Code Scanner to have ready and waited 7 days, this morning it finally failed again and I was ready. Unfortunately it finds no codes! I drove 15 minutes to work, rough running and wanting to stall at every light all with no codes found.
It feels like one or even two cylinders not firing. When it is not in this failed mode, the truck runs perfectly and gets 18 MPG highway.
How should I proceed? Can a mechanic find the problem with no codes and this very intermitetnt behavior? Right now it is in failed mode but by history will clear up probably after a few more miles. Warmup?

Any help much appreciated, thank you.

 

Just a wild guess just maybe you're getting a bad tankfull of gas on occasion. Do you always fill up at the same place or do you frequent several gas stations?

~DM~

 

Thanks for the suggestion, I do use multiple stations, same as my Eplorer with no problems. After waiting seven days for it to fail (To check Codes but no luck), it was in the failing mode when I started it in the morning. Ran bad to work, 15 miles, went out at lunch, started it, and it's fine again. Same tank of gas since last week.

 

Take a long screwdriver and 'tap' the idle air control valve and see if that may be the problem. They do get 'stuck' and that can cause lots of interesting problems which do not set codes.

Don't know exactly where it is on an Expy, but on my F150, it is on the very back center of the engine right under the right hand windshield wiper arm connection. Has a hose that goes to the left to a block on the firewall, then from there to the throttle body.

I understand that they can be removed, cleaned up, lubricated and reinstalled. Ford wanted $165 to replace it.

Larry

 

Search the "engines" forum for IAC*. You should either clean or replace the IAC. It's really easy to do yourself.

 

Yep, probably the IAC valve. It is a $40-70 part (depending on where you get it) that is attached by two small 8mm bolts to the backside of the throttle body and has a wire lead out. Unlike most other engine problems, a faulty IAC valve won't throw any codes. It is a very simple replacement and should fix your problem.

 

Did you clean or replace the IAC yet?