After replacing the pvc valve and the plastic tube connecting it to the intake manifold ( the tube was completely oil soaked and the rubber fittings were very soft and "gooshy" ) the CEL has not come back on and my laptop has not recorded any more fault codes.

I did have a PO300 code come up soon after the pvc valve was replaced ( multi random misfires) and I pulled the plugs and found them to be in pretty bad shape ( 85,000 miles and a good oil soaking from the faulty pvc valve )
replaced the plugs and now everything is fine.

thanks to everyone who offered there suggestions!!!

 

Glad you fixed it! I'd be interested in knowing what equipment and software you're using to do that with your laptop. That's cooool!

 

The only tool I used was the obd2 tool sold at www.obd-2.com.

it allowed me to view in real time the O2, MAF, and IAT sensors.

I also bought the Haynes 2001 explorer book @ AutoZone, it doesn’t cover sport tracs specifically, ( I could not find a book on just sport tracs anywhere, anybody know where to get one? )
but it covered the 4.0 SOHC engine enough to explain the sensors and how they work and interact with the PCM for me to see that the engine was running WAY to lean, in fact at idle the O2 sensors were "flat line" and running only at only .030 volts!!!
After that I posted my results on this board, and after checking a few things suggested by Ranger88, I found that my pvc valve was not working right.

Now the dealership told me that ALL three O2 sensors, MAF, IAT, and the throttle body position sensor were bad because of a “power surge” and that this was common on explorers…..
I am not a mechanic, however I am a electronic engineer and I knew that ANYTHING powerful enough to take out all of these sensors would have killed everything else like the radio, the PCM, ect, ect, anyway the dealership quoted a price of $1800.00 to repair my sport trac, using the OBD2 software, a book, and a little advise from this forum my total cost has been about $125.00 for the software and cable, and $45.00 for the pvc valve and the new lines running from it to the engine………..

 

Had same problem in 97 OHV, repeated PO171, PO174 codes, replaced MAF, IAC-still getting lean codes, finally replaced (missing) EGR gasket and had injectors professionally cleaned. Replaced PCV tube for good measure. 1000 miles later, no codes, and slightly better mileage. Also replaced upper intake O-rings but I think this was unneccessary as was the MAF and IAC.