While idling and cruising around at low speeds my blms and ints are within +- 4 of 128 and my o2 fluctuates back and forth. Normal.
As soon as I start to hold speed around 55 or 75. The blms go down to 120 the ints are in the 128 range and my o2 goes way way low, down to 40-70mv. I thought low o2 readings meant lean? But my blm says rich and my int says normal. No knocks, runs fine.
And then when I let off the gas and decel the o2 fluctuates and the blms and ints go to around 126ish. Normal again.

Is my o2 not functioning while cruising or is my ecm lying to me about my fuel trim?My plugs look decent, a light brownish color.
Running a 357 mild build with a 165ecm and a modified ARAP bin.

 

When you say your O2 goes down, do you mean it goes down and stays there? If so, maybe your Highway Mode is invoking or your O2 sensor may be "going".

 

It reads about 40-70mv and fluctuates back and forth between those two, but it won't go past 70mv, it doesn't just stick there like it's not reading, but then when I let off or give it throttle it'll jump back into +-450mv.

It only happens when a constant speed is applied above 45-50mph.

It's a brand new ngk o2 sensor.

Is there a way to tell check if it's highway mode? what other values will drop out?

-Thanks!

 

yeah that is your highway lean mode. it helps with gas mileage. thats what my guess would be.

 

I can't say for sure on the $6E (it's been a while since I've done a scan on one on the Highway), but when Highway Mode is invoked on the SD TPI, your AF Ratio goes leaner from 14.7:1.

If you see that, then you are definitely going into Highway Mode. If your eprom is properly tuned, your mileage should increase. If your eprom requires some tuning, your mileage may actually decrease.

 

yeah on my 350 i get 27 highway with lean cruise, not sure what computer i have, it is from a 91 rs