Two years ago I thought we had this fixed. Did a full tune and replaced a few parts, drove 80 miles and the check engine light stayed off. Got My plates and all was good. Within the next 50-100 miles the $&( light came back on. I haven't done a darn thing about it except verify it's the same codes. 0171 & 0174. Banks 1 & 2 lean.
For referance here is the old thread when this adventure began https://www.ford-trucks.com/forums/7...nd-2-lean.html

To recap the truck in need :
99 Explorer, 4 door, xls, 4.0 (x code), automatic trans, 4wd, 85,xxx miles (maybe 2000 miles since I thought it was fixed 2 years ago). Drivability is good. Engine seems to run well, accelleration could be a little better, but is good over all.

I'm having a bit of trouble beleiving the forward cat cons can both go bad at once or both O2 sensors for that matter. I've read a bit here that many have found a bad elbow in the vaccum line for the pvc but my engine lay out is alot different that the threads I found. They all have elbows in inaccessable places, mine has the pcv on the drivers side valve cover and a short hose with formed double elbows right to the upper manifold I guess ( big black box with the throttle body at the forward end). This appears to be good.
I'll have to look the vaccum over extra good, but need some input as to where common causes may lie.

 

I had the same codes on mine. I found that the green seal around the EGR tube to plenum had blown out. I replace that seal and the plenum gaskets. I used this kit MS98006T.

 

Those codes can also be indicative of a bad Differential Pressure Feedback EGR (DPFE) sensor, also referring to as an "EGR Pressure Sensor" or "EGR Position Sensor" at some of the auto parts stores.

-Rod

 

can also be intake manifold gasket leak--- I had that on 2002.

 

Thanks for the ideas. The DPFE was one of the parts replaced last time. I priced the gasket kit and it comes with the "green seal". Bought 2 cans of Gumout and going to start sparaying and listening tomarrow. If I find nothing, I'll roll the dice on the gaskets.
Keep the suggestions coming.

 

Are you noticing any coolant loss?

 

Really not sure about the coolant loss. Seems it will always take a half gallon or so. Check it once a month or year, takes about the same to top it off. Never worried too much about it, heat is good and temp stays in the norm range even during the 2 months or so we had of 100+ temps.

 

Well... I think I got it.
Started out spraying every vacuum line I could find, every connection, every hard mounted part that might have a seal or gasket. Started at the drivers side and worked my way around to the passenger side. When I sprayed a vacuum line on the passenger side I inadvertantly sprayed the upper manifold as well. The motor dang near died. I moved the line aside and sprayed the manifold only. Nearly died again. Now I'm off to get the gasket set.
Nothing I read here or else where prepared me for what I was getting into. What a pain. I couldn't get the PCV tube out of the manifold and dismounted it from the EGR. That allowed me to remove it as an assembly, but not easily. Oh, by the way, Who's bright idea was it to mount the oil dip stick tube support to the same bolt that mounts the EGR tube ? Not funny !! Thought I was going to have to pull the dip stick tube to get the EGR tube off the mounting bolt. I got lucky and was able to flex/ bend the support tab enough to get it off the stud. Putting it back wasn't any better, Ford did us no favors on that part.
I've read somewhere that the felpro gaskets were paper and the motorcraft were rubber. Well I got felpro and the set included 3 rubber "o" rings, a thick paper gasket and the EGR "o" ring. I was really scratching my head on how all these gaskets are installed. (not having it apart yet). I expected a paper gasket under the upper manifold with rubber rings on the manifold. Nobody, nowhere said anything about changing a gasket under the fuel rail or even removing it for that matter.
Maybe I'm just calling stuff by the wrong names. As I understand it there is the upper manifold also called the plenum, then the fuel rail then the lower manifold.
Now that I knew where the paper gasket actually went, I knew better that leave the old one there once I got this far. Now I have to run all over town looking for a socket to fit the top of the studs. After 2 hours and 4 parts stores I ended up at Harbor Freight. The size is an E 7 for those who might need to know or want to save time and have the right tools when they start.
Back to the job at hand. This is where I really didn't know what I was getting into. I managed to pull the fuel rail. I had no idea how the fuel injector were mounted, but found out REAL quick. Lets just say it was brown trouser time. I had injectors going everywhere. Some stayed in the fuel rail, some stayed in the lower manifold, one just flopped on the wire. I had to pause for a moment of prayer.
I had to quickly get things back together before I forgot how it came apart. All I can say is prayer is a good thing. It didn't go back together easy, but I got it right the first time I think. NO left over wires hoses or connectors. Truck started on the first turn over and ran great. Once it warmed up and settled on it's warm idle, I sprayed everything again. I found nothing.
Now I just got to put about 80 miles on it and if the light stays out get it inspected.
Thank you one and all for the help and info posted here. Without you and Google, I'd have never gotten to this point.

 

The gasket between the fuel rail and lower manifold I left alone as mine wasn't leaking. If you do want to change it you will need inverted torx to remove the studs holding it in. Does the paper gasket have 2 small holes and one large hole?

 

Well, 85 miles and no light yet. Truck ran fairly well. I did have to do a (forgive the term) Mexican tune-up. I got behind an old man who insisted on doing 40 in a 55 on a 2 lane state hwy with no passing zone. He tried to wave me past, but no dice. I got expired plates, a no passing zone and very limited sight distance. Good thing I stayed put, turned out to be a good call. There was traffic coming I could not see. Once I got to the one and I think only passing zone for several miles, I nailed it. Oh crap, it got scary fast. I went from 40 to 85 in the blink of an eye, passing zone was coming to an end and I got a 30 mph curve ahead. Truck ran even better after that little tune-up. Now I just got to find time to get it inspected for emissions.