OT, OBD-II O2 problems

Some of you fuel injection gurus may be able to help me here. I have a '97 Land Rover Discovery that is having some O2 sensor problems. The problem is a slow lean/rich transition time. Previously I had thought this problem was related to a bad fuel pressure regulator. I fixed the regulator, and now the light comes on every time the car finishes it's warm up cycle. I would think it was bad sensors, but both the right and left upstream sensors are showing the same issue. I was told by numerous people that the low fuel pressure would cause this problem. I no longer have low fuel pressure and the problem seems more consistent now than before. What other things can contribute to this slow transition time, on the O2 sensors? I really don't want to replace O2's, they are $100 each, but my wife is getting really aggravated with the CE light coming on all the time. Any help at all is appreciated, not to mention that mileage goes right into the toilet when the CE light is on (not a good thing with $2+ gas prices).

[LEVE]

1. The first thing to check is,
2. What sensors are enabled,
3. When the engine goes into Closed Loop?
4. A flaky MAF will cause the engine to run rich.
5. Have any voltages been checked on the sensors?
6. I'm assuming the 02 sensors are good...
7. Have you checked the OBDII codes?
8. Do you have an OBDII code reader?
9. You suggest the Fuel Pressure is OK...
10. Is it at the required 37 psi mark,
12. During operation, or only at zero flow?
13. If it's only 37 at zero flow...
14. The fuel pump is bad.
15. Back to the MAF...
16. OEM part number is ERR5198.
17. Atlantic British has it of $659.95.
18. The lowest aftermarket replacement was at
19. Speedycarparts.com,
20. For a mere who $624.
21. If that MAF can be cleaned...
22. Clean it.
23. Has the Idle Air Bypass valve been[*]Pulled[*]Cleaned[*]Mating surface cleaned?
24. If ya need a new one you may be able to use a[*]GM part #25527077, or[*]NAPA part number is 2-1738, or[*]AC-Delco part number 217-437 or, 17085051, 17086051 and 1717111288[*]Echlin part number CRB21738
25. O2 sensors are somewhat generic,
26. As example try a Nissan part number 22690-88G01,
27. Just swap output connector and it should work for about $65 each.
28. Or Bosch part number 13021 from NAPA for about $70.
29. Then there's automedicsupply.com,
30. They stock both "universal" senors,
31. That you rewire with a supplied splicing kit.
32 For about $75.
33. Ok.. that'll get you started. [img]images/graemlins/40BEER.gif[/img]

Leve,
Thanks for the response. I do have OBD-II software, I have done real time monitoring on the O2's. I am mostly getting the slow transition on the upstream sensor on the driver's side. I have a new pump as well as the regulator. I will look at the MAF, but I have seen no codes related to the MAF. I am told that the rover computer is really picky and it doesn't take too much to make it throw a code. I ordered a new O2 from automedicsupply.com so I will see if that helps. The service manager at the dealer was actually pretty helpful, he thinks the symptoms point to a bad O2. Only one sensor has consistently thrown the code, and I am going to replace that first. I am just getting familiar with the OBD-II system, so this has been a learning process (unfortunately a very expensive learning process). I thought the fuel pressure was the problem, I only had about 25psi under load. I now can easily get the 34-37 that rover recommends. I will look at the idle air bypass valve, I assume I can clean that with carb cleaner? Thanks for the help. Again and again I am constantly reminded that this is the best BBS on the net.