The vac line was false hope, I was tired overly optimistic that I had found something but underestimated the amount of air neaded to move the vacuum motor for the warm air draw for the air filter. still getting the check engine light at an idle.
I looked at the exhaust the best I could without actually getting under the truck, it snowed today and is a sloppy mess under there. From the top, front fender wells and with a mirror i can't see or hear any sign of a leak. I'm still stumped.

 

OK good feedback on the last fuel pressure check, the numbers look ok for this year/model.
Do you have any other trouble codes, if so, post All code Numbers you have, set, or pending, as they can offer up good trouble shooting clues.
If you have, or can come by a scantool that can hook up to your OBD-1 under hood DLC & display live sensor PID's to the computer, like the Actron CP9145/CP9150, ect, & have it take a look at O2 sensor switching range & speed, long & short term fuel trim, MAF sensor output, intake air temp, engine coolant temp & post up the numbers.
An acting out sticking open, or not seating well, worn out PCV valve can mess with idle fuel trim as its a calculated for vacuum leak thats down stream of & not monitored by the MAF sensor, as mentioned earlier.
So make sure the PCV valve is new, its rubber fittings, hoses & their connections are tight fitting & not dryrotted/cracked/soft mushy, loose fitting, such that they may be leaking vacuum.
Test the engine for blow-by & that the EGR valve has No vacuum applied to its diaphragm at idle & the EGR valve is Closed at idle, no carbon deposits holding it open, as all that if acting out some can mess with idle fuel trim, as any leaks from them are a much larger % portion of the idle air/fuel trim ratio.
More thoughts for consideration, keep us posted on your trouble shoot.

 

Checked the codes again for the ? Time to reassure myself that I was counting the blink/beep's correctly, 11-11—1—41-41, "System OK" and "Oxygen sensor signal fault, switching not detected" for continuous memory or for "System running lean" Key on engine running. I haven't done a Engine running test yet so I think I've been looking for problems I don't have and haven't looked enough for o2sensor problems. I replaced the o2sensor with a new NGK after putting a small scrape in the wiring on the old one installing the transmission. The two possibilities that I can see think of is the new o2sensor is bad or the Engine wiring harness from the BII isn't compatible with my Ranger only for the o2sensor? The wiring harness is in great shape, still had kinda dry grease in all the plugs? I think my next step is check the o2sensor plug again and after that????

 

You may be on to something with noticing the gunk in the O2 sensor electrical connector. O2 sensors need to be able to "breathe" through the wiring harness wire run, to generate a reference voltage against which it measures O2 content in the exhaust gas passing over it. If that breathing path is blocked by crud, soldered leads, flux resin, rtv, ect, used in a harness, or O2 sensor wire run repair/splice, it can un-calibrate the O2 sensor reference & corrupt its PID output to the computer.

 

I have been slowly ironing out the little problems with this truck (I haven't posted the majority of them) and now I only have this problem to resolve.

The things I have done so far, verified the wiring harness I used was wired the same other than the old one had a ground wire that was not used (dead ended at a plug), cleaned every wire connector associated with the O2 sensor and replaced the O2 sensor again just because....

I'm really not sure where to go from here... The only thing I can think of now that could cause this is a broken wire in the wiring harness? I wold appreciate your opinions.