About 2 weeks ago, after my Expedition had been running for about 30 mins, I came to a stop. The engine didn't want to idle quite right, and or acted like it wanted to cut off. About a week later, the check engine light came on. It seems to run fine when cold, but after it warms up, the engine wants to idle at around 500rpm's, almost knocking off.

I pulled codes 171 and 174 out of the computer, which is system lean bank 1 and bank 2. A friend said this was the O2 sensor, and that there were two upstream and two downstream.

My question is how do you know which one to replace, if that's the actual problem. It's a 5.4 with 97,000 miles. Should I just replace them all and not have to worry about it again?

Thanks!!

Buddy

 

Try cleaning your MAF sensor element first.

 

How do you get trouble codes? My engine needs service light just came on.

 

Sounds like the infamous PCV elbow problem.. Around 100K miles ,5.4's are bad to rot out the pcv elbow on the back of the trottle body, this will cause bad idling problems and will throw lean codes when it gets really bad.

find the PCV valve in the passengers side valve cover and trace the tube around to the back of the throttle body.. With the engine running feel of the rubber elbow and see if you have a vacuum leak..

This is a simple check for a common problem. and a 17 dollar part from Ford.
Good luck.

 

Silly question, did you reciently change out yoru airfilter ?

Those codes can mean a multitude of things, but starting out with un-mettered air getting into the system would be the #1 place to look ( as 864Lightning noted ). PCV rubber boot on the back of the intake, vac lines rotting, etc. There are a few places that un-mettered air can get into the system, but would be something large such as the rubber boot, to set the CEL.

Did it just start running rough, or did it slowly get worse ?
The reason I aks is if the Intake elbow was on part way, and finally worked its way to a larger gap, the engine would have been kind of running rough, then slowly got worse.

Cleaning the MAFS is a good thing. Carefully clean the wire in the pillar. I use Windex and a Q-tip, JMC gave me that idea, and it works well for me. Some prefer to use T/B safe T/B cleaner ( which brand that is I don't recall, look for a note on safe to use, usage directions ).

The next list of items would be the DPFE ( did I weite that correctly ? ), which has to do with the EGR and EVR.
If you try a search on P017*, you will find a few posts on the topic, and the steps that memebers took in problem determination, as I can recall 3 soultions to similar problems ( code the same area P017* ).

Good luck, let us know if you have any other questions or get stuck on the topic again.

 

The OBDII error codes you received could be very serious:

P0171 System too Lean (Bank 1)
P0174 System too Lean (Bank 2)

It could be any number of things to consider:

1. Replace the front two O2 sensor (every 30K miles!)
2. Clean MAF sensor (especially if you've installed a K/N filter - could coat the wires with oil that collects dust.
3. Change the fuel filter (under drivers side frame)
4. Check fuel pressure
5. Don't put this off - could fry the motor!