I was driving tonight while watching fuel trims trying to troubleshoot a rich condition. After driving for awhile, I went WOT a couple times, and then noticed that my STFTs dropped to 0.00 under all conditions, while the O2s were sitting around .9V. It's almost like it dropped out of closed loop mode for some reason and just stayed rich. I have noticed this happen before, but not every time I drive it. Has anyone experienced this, or have any ideas what could be going on?

 

That is normal wot operation.

wot is a form of open loop because fuel trims/O2s are not used. O2s stay around .9 because the wot mixture is richer and stays relatively constant.

 

yep he is exactly correct sounds perfectly normal to me.

 

Sorry, I didn't make it clear... The STFTs stay at 0.00 well after I return to normal driving. I drove for several miles afterwards at different speeds, light throttle, and even at idle they were always 0.00. I know that it is normal to be 0 at WOT. My scan tool still showed it in closed loop mode even though it obviously wasn't. Any more thoughts?

 

Does anyone have any thoughts?

 

Sorry to dredge up this old thread, but I finally figured out my problem and thought someone else may find it useful someday. My cat overtemp protection was kicking in and reaching the 3rd level (>98 kpa MAP and >846 degrees C). Apparently once this happens, the PCM doesn't go back to closed loop mode until the car is re-started. I disabled COT by raising the temperature thresholds and it stays in closed loop mode now. Now the question is whether the cat will disintegrate on me when it gets too hot.