Morning Ford People!
I've got a 2000 Expedition AWD Built by Regency (oddly enough) with the 5.4 and it's got an issue. Been reading through the forum for a while now and really haven't come across a clear answer. I'm at 218K currently and it's starting to run like crap.

I Changed out the MAF and the other sensor on the air filter tube, clamped down all the vacuum lines that go into the throttle body and it's still running like crap. It'll start fine, but once the idle drops below a grand it shakes like a *******. It's died a few times while driving it as well.

My theory is it's either a substantial vacuum leak (throws the Bank 1 and 2 running lean codes P0171 I believe) or it seriously needs a tune up...

My question is - where are the most common failure points for the vacuum lines to take a crap? Figure I'll start there since I hear a tune up is a bearcat on these motors!
Thanks for any advice in advance!
Nate

 

The PCV system accounts for about 90% of those occurrences. Pay special attention to the notorious elbow around back, facing the firewall. It's a left-handed, check by feel operation.

 

On the smaller line going into the firewall or is there another connection I'm not seeing/feeling?

 

From the passenger side valve cover around the back.