RomRaider

Hey all, I've been running a MS41.1 - mildly turboed M52B28 stuffed into a Mazda RX8 for a few years, and I have had a persistent problem with closed loop while cruising on the highway. I have a separate wideband O2 sensor I can watch while driving, and when I'm cruising at 100-120 km/h it will lose closed loop control, add a whole bunch of fuel, and stay that way until I get off the highway or slow way down.

I did a bunch of logging with the wideband, narrowband through the ECU, load, STFT, etc and the only think I could imagine was the O2 sensor was old and would eventually stop responding fast enough while I was cruising. I should note I'm running through the Mazda RX8 transmission, so it revs higher cruising than stock, sitting at a little over 3000 RPM at 100 km/h.

Anyways, I replaced the O2 sensor and it didn't help, if anything it was worse. I started to wonder if the heater driver circuit in the ECU was dead, so I did a logging run with the heater % logged too, and was very surprised by the result! (See attached) The heater output drops a lot while driving, from nearly 100% shortly after startup to 10-20% while cruising, with dips right down to 0 heater output. In fact, heater output seems to be a mirror of engine RPM, so with my higher cruising RPM the O2 sensor is seeing very little heater power.

Since I'm running a turbo and I had very little space for the exhaust in the RX8, I had to mount the O2 sensor at the bottom of the turbo downpipe where it sees a lot of airflow. My hunch at this point is
-The exhaust gas has cooled a bit before reaching the sensor.
-The sensor is in the wind under the car and cools off even more.
-I'm running higher RPM at low loads while cruising, which cuts heater power too low.
-The O2 sensor gets too cold and stops responding.

So. Has anyone experienced anything like this?

Is there any access to O2 heater cutback tables or algorithms? I didn't see it in the definitions file, but maybe someone has figured out how to control it.

If you made it this far, thanks for reading!

Have you tried heat insulation wrap? I had problem to pass emissions test with racing catalytic converter and proper heat insulation helped a lot

I'll give that a shot. I already tried moving the sensor location as close to the turbo output as I could for maximum heat, but it wasn't enough.

Ideally we could find the O2 heater control table, but insulation might do in the meantime.