Reasons why O2's are dropping during a WOT run in 3rd gear

Dr. Jeckel

New Member
Jul 29, 2003
I get no KR, except occasional transitional knock(.7*-1*). I'm running 15psi of boost and 91 octane(highest here). What are some reasons for my O2's to drop while doing a 3rd gear WOT run? If I get the car to about 65mph and hammer it then O2's slowly decreases from about 840 down to 770 before I let off. It drops about 10mv's per frame. The O2 sensor is good and is an AC delco. I went through two denso sensors that did the same thing before going back to the ac delco. I don't believe it to be the senor but perhaps something I added to the car. I did everything pretty much all at once so it's not easy to say when it started. Can anyone see anything or think of a reason for this?
I have a fuel pressure gauge on the fuel rail and static pressure is set to 43psi. Maybe someone has had this heppen to them and can give me some ideas. The car runs great and pulls hard on the top end. Any kinda help is greatly appreciated!!!
Thank you, James.
Not sure what you're asking...
Of course the O2's will drop during a run. In general, there is an increasing load on the motor the faster you go (drag). If your O2's are at 770 at the top of third, you're good.
More importantly, you're not seeing KR.
Well when I had my stock chip and not alot done I would do a 3rd gear punch and the O2's stayed steady at 820 for as long as I kept my foot on the gas. I though that once you got to 3rd gear your O2's should level out and stay steady. Is this not true?
Nope. Never, ever, IME. O2's always drop, just like Snorman reflected.

BTW, O2's are a poor indicator of tune, because they are sensitive to EGT, which increases through the run. Don't hang your hat on O2's...

Thanks for the clarification. I'm not using them to tune by just wanting to make sure nothing was wrong. I had just remembered posts where people had talked about O2's dropping and they had said it was a bad O2 sensor. I thought since i had ruled out the O2 being bad it might be something else. Maybe I was mistaken.
Like I hinted earlier, and strikeeagle confirmed, O2's are not the very best way to determine tune, but they are okay up to a certain level.
If you don't have knock, you should be good as long as everything else is good.
Also, usually your CC's will drop dramatically with a bad O2. When people are talking about O2's dropping with bad sensors...they are really dropping. I've gone through 2 sensors in the last year or so. When the first one went bad, the O2's would drop to zero under WOT with a few seconds. The last one was showing O2's in the 300's, even though an hour earlier I was in the mid-700's under WOT at the top of 3rd. Your Densos may have still been good...keep 'em for spares (or in case you ever get the urge to run race gas...;) ).
At a certain point, and EGT is the way to go. I'll definitely be running one by spring.