Ok this is hard to understand, I'm still tuning every time I get in my car, when driving around or trips, so while taking a 90 mile trip saturday, on turnpike at speed 75 to 80, the WB is operating normal 14-15 area, then it goes to 12.5 steady and stays there for about 10 sec or so, then goes normal again, it did this on the whole trip. So when I got there I changed PCMs just to check and see if the PCM was screwed up, it did the same on the way back. can the map sensor cause this or what. I have a DST tuner and use scanxl elmscan on Lt1 OBDII 96Z, it also does this at 55-60.

 

Is it staying in closed loop when it does that?

 

that I'll have to check on, but I think it is, someone else said to check and see if cats overtemp was off, cats programed out, don't think overtemp be a factor.

 

It is probably Catalytic Converter Overtemp Protection. When the oxygen sensor reports that its getting hot it lowers the AFR to decrease combustion temps. You can turn it off by setting the threshold temperature to a unattainable level with tuning software if you no longer have cats.

 

thanks alot, I'll change that and try again over the weeken..

 

I have my cat overtemp disabled by setting the min temp to 151 degs (max it will accept) but my car still does this.
When going WOT it will hold 13:1 A/F ratio in 1st, 2nd and 3rd gear, but will switch to 12.2 A/F ratio in 4th.
Any suggestions?

 

I changed the target A/F ratio to 13:1 for the cat overtemp feature and that did it. It will hold 13:1 all the time.