02 sensors CEL but thats not problem

(3.8 engine) ok so for 2 yrs I had P0171/p0174 and come to find out there was a rod of some sort that was missing off the bottom of the van. mechanic made a rod to fit and cleared the codes. mechanic said van should not have been running at all with that missing. then after clearing those 2 codes, the 02 sensor code is popping up but mechanic put in new sensors (2) and after driving 50 miles the CEL came back on for the same thing. he basically said he didnt know what it could be and gave me my money back for the sensors. so far ive had a new battery and new wires/plugs as they were fused together it took 3 hours to get them apart and put in new ones. air filter was checked. its running very rough like it wants to die on me (it did die on me at mall and i had to get a jump) and there is a smell (not sure if its oil or not) and i was told (by brakemasters who replaced plugs/wires and gave me the cel code for sensors) to take to a separate mechanic to figure out what it is or it could get worse and cost more money down the line. the weird thing is that last year my van had not passed smog then when the smog guy reset the CEL and after driving 100 miles the car passed smog which is a miracle in itself. I could wait for the next dMv smog inspection coming up in jan 2011 and once it fails have the state use $500 to pay for repairs but not knowing if the van issues can wait until then. Im wanting suggestions as to what this could be (fuel related or what?) Id hate to pay another 90 bucks for diagnostic if i already know code and have them tell me they dont know the problem.

Your story covers quite a bit of territory - but I suspect most of your underlying problems will be addressed at the two links listed below.

- The infamous P0171/P0174 problem is related to intake manifold problems and isolator bolt seals (depending on which model year your Windstar is - which you didn't mention). You'll find many, many references to this issue - and how to fix it dependably - on this site. However, most of what you need to know is summarized here:

http://leckemby.net/windstar/windstar01.html

- There are a number of things that can cause Windstar idle problems - but the most common is a dirty IAC (Idle Air Control) valve. Thankfully, the valve is easy to get to, and easy to clean. Take a look here:

http://leckemby.net/windstar/windstar02.html

Sadly, many shops seem to misdiagnose these Windstar problems, which causes lots of frustration. I've heard that shops commonly replace oxygen sensors - which are rarely truly a problem on Windstars (Example: My '00 still has the original oxygen sensors at 125K+ miles; they work fine).

One follow-up I've noticed: It seems that another sensor, the DPFE (Differential Pressure Feedback EGR), will fail with code P0402 at about the same time as the P0171/P0174 occurs. If it were my van, I'd simply replace the sensor at the same time as performing the P0171/P0174 fix (presuming your Windstar model year is affected the same way my '00 is). I'll also note that I've had no luck with aftermarket DPFE sensors. In this case, the Ford OEM DPFE is worth the extra expense.