Have a 97 Expedition 5.4 and have the DTC PO171 & PO174 for lean on bank 1 and 2. Already checked and replaced the MAF and the performance has increased but still having same problem with low idle. Idles fine at 900 and then drops to 600 and sounds like it is going to die. Any ideas?

 

Lean codes on both banks means you need to start checking things that are common amongst both banks. A common failure item on Modular engines is for the PCV valve tube leading to the back of the throttle body to corrode and crack, allowing an unmetered air leak.

<edit> And where are my manners? Welcome to F150online.com

 

Thanks for the advice, I will check it out!!! Also thanks for the welcome, it is appreciated!!!

 

I also have a 97 5.4 expedition and had the same codes on it.. Like Quintin said,, it is very common for these engines to rot out the rubber elbow ,from the pcv valve,on the back of the throttle body, and the plastic lines from the pcv valve can crack..
Find the pcv valve in the passenger side valve cover and trace the tube back ,, with the engine running check for a vacuum leak at the rubber elbows..
If you wanted to it wouldnt hurt to just change the whole thing out anyways,, the are about 20 bucks at the ford dealer. it includes the pcv valve and tubes and elbows.

Another thing to watch out for on these rigs is corroded vacuum lines behind the battery , look for a 4 way vacuum plug and associated little tubes that crack and cause a leak .. This caused the dreaded p0401 on mine ,, and it can also cause evap codes.

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Thanks for the info. I have a 100,000+ miles and do not plan on getting rid of it any time soon. So all the advice given will be appreciated and absorbed!!

 

I found and pulled the PCV and it hoses and found that the elbow on the intake had rotted away. Tried to get one from Auto Zone and had to go the dealer. $18 for a rubber elbow. Replaced the PCV and elbow and Expy runs like a Champ!!!! Thanks to all!!