ULTM8Z
Veteran Member
Something I just learned after running stainless headers since the mid-90's.
Looks like you should run a 4-wire sensor.
Long story short, I had been getting significant fuel-trim variability at low MAP values for a long time that I couldn't really pinpoint the cause of. Because it was low-mid MAP values, the performance of the car wasn't significantly impacted, but it always kinda bugged me in terms of knowing the variability was there.
I recently found out that stainless steel evidently can have relatively poor electrical properties in terms of getting a reliable ground for O2 sensors that depend on the exhaust system for it's grounding.
The 3-wire sensors have a ground wire, but its for the heater element only. Whereas the 4-wire has a ground wire dedicated to the sensor element itself. On my old TPI computer, the O2 sensor ground in the harness is actually just a reference ground that is connected to chassis ground (since the 80's-90's O2 sensors were only 1-wire). So if the O2 sensor ground isn't exactly the same as chassis ground, it'll cause fuel trimming issues.
I bought an AFS-75 sensor and took the sensor element ground and tied it directly to the ground stud for the ECM.
At that point the fuel trim variability was eliminated. I had been getting BLM readings varying from 126 to 140 at times between 20 and 40 kPa. Now, the variabilty is only 126 to 130. What I notice is a bit smoother and more linear transition between closed throttle to moderate throttle (it's subtle, but noticeable).
I know it was this ground as the culprit since I've tried different 3-wire NB O2 sensors over the years and always got the same result.
Looks like you should run a 4-wire sensor.
Long story short, I had been getting significant fuel-trim variability at low MAP values for a long time that I couldn't really pinpoint the cause of. Because it was low-mid MAP values, the performance of the car wasn't significantly impacted, but it always kinda bugged me in terms of knowing the variability was there.
I recently found out that stainless steel evidently can have relatively poor electrical properties in terms of getting a reliable ground for O2 sensors that depend on the exhaust system for it's grounding.
The 3-wire sensors have a ground wire, but its for the heater element only. Whereas the 4-wire has a ground wire dedicated to the sensor element itself. On my old TPI computer, the O2 sensor ground in the harness is actually just a reference ground that is connected to chassis ground (since the 80's-90's O2 sensors were only 1-wire). So if the O2 sensor ground isn't exactly the same as chassis ground, it'll cause fuel trimming issues.
I bought an AFS-75 sensor and took the sensor element ground and tied it directly to the ground stud for the ECM.
At that point the fuel trim variability was eliminated. I had been getting BLM readings varying from 126 to 140 at times between 20 and 40 kPa. Now, the variabilty is only 126 to 130. What I notice is a bit smoother and more linear transition between closed throttle to moderate throttle (it's subtle, but noticeable).
I know it was this ground as the culprit since I've tried different 3-wire NB O2 sensors over the years and always got the same result.
