DTC P0171 and P0174 are set only during closed loop feedback operation when the PCM is monitoring the oxygen sensors. WOT is never closed loop.
The 171 and 174 codes mean that the Long Term Fuel trim has been enriched to the calibrated limit at one or more cells in the correction table. Basically, the PCM is saying that there must be something wrong to require this much enrichment correction.
When both banks are lean, the first thing to look for is an unmetered air leak before the throttle body, then look for a vacuum leak after the throttle body. A notorious leaker is the PCV system and it can contribute to either depending on where the leak is.
If that's OK, you could have a dirty MAF, clogged fuel filter or low fuel pressure for some other reason. Lower manifold gaskets, clogged injectors or bad O2 sensors are also possibilities but often affect one bank before the other.
EDIT- John beat me to it! Again! While I was composing, he was posting.
I guess I had already fixed the problem and didn't realize the computer had to be reset before the check engine light would go away. I had previously found the pcv line disconnected allowing unmetered air to the manifold. After resetting the computer to remove the codes from memory, I've driven over 150 miles today and the light did not come back on.
Thanks to John and Bob
On fords the main problem they have is with the hose on the PCV, it turns to junk quick normally. Now if you have a 3.8 and have a 171/174 its the valve cover doing it. If its a 4.0 its usually the PCV hose / dirty MAF.
The local consensus is I should replace my O2 sensors. Does anyone know the values to check them or other ways to test them?
Hi Ryan, it's been a while!
I agree with the other comments that say to check things that affect both cylinder banks first - vacuum leaks, unmetered air leaks, dirty MAF, etc.
Checking the oxygen sensors is a bit tricky because the voltage values should be constantly changing when the fuel delivery system is in closed loop control. Try this link for some ideas on in-vehicle and bench testing of the O2's.
Your oxygen sensors have 4 wires each. The color codes for the signal wires are gray/light blue (PS) and red/black (DS). For 97, the signal return common to both sensors is solid orange.
I know this thread is old, but I just had the codes 171 and 174. I found a nice size rip in a 90 degree rubber tube leading to the PCV valve, which is just near the throttle body. I got a $5.99 piece of similar angle rubber tube from Advance Auto and installed it the same as stock, reset the ecu and no more light. Simple fix.
well i hab been getting these codes for almost a year and couldnt track it down i changed the maf iac and did the intake gaskets and after all that it was a bad iac.
if you have a low and/or erratic idle its most likely the iac