Just wondering if anyone else's 8 does this. On hot starts only I get 0.0 stft for 10 seconds right after startup. It idles a little rough. Probably because of there being no fuel trim. After the 10 seconds it starts working and idle smoothes out. BTW ltft is steady at 8.6 normally.

 

Does it say you are running an open or closed loop? Can you post data charts or numbers from a 10 second idle? You can also try the reset using the stomp the brake pedal 20 times trick and take new readings. Just not enough information here for me to really help.

 

It would be open loop in the first 10 seconds. The O2 sensor isn't even hot yet. That might be exactly why you're getting 0 trim, I've never really monitored what it does so early in the start up sequence, but since STFT is based on O2 sensor input, I'd bet that's it. LTFT should stil have a number.

By the way, 0 is the ideal fuel trim under most conditions. It means everything is fine and no adjustments are needed. 8 LTFT points to either a small vacuum leak or some other minor issue that we can try to investigate.

 

I've tried the brake stomp reset multiple times. I noticed this a long time ago. It's always the same. Next time I drive it I'll log the hot start.

The ltft has been around 8.6 since I can remember. I've tried looking for vacuum leak. I haven't found anything. I really want to get a smoke machine and figure it out. The good old spray and see if the idle changes didn't give me any results.

My motor has 140000 miles as far as I can tell. I called Mazda and they have no history of the motor being replaced. I bought it at 90000. Only mod I have done is a catless midpipe.

 

Again, this sounds like normal function. It takes time for the O2 sensor to heat up to a temperature where it actually reads AFR. Until then no fuel trim adjustment makes sense.