OBE1 95Z28

I have a scanmaster and while monitoring my O2 sensors on my 95 Z28 both left and right readings are 920's at idle and low load cruising. As soon as it's under modest load the O2 sensors bounce nicely between 100 - 900 (could be due to the exhaust properly heating the O2 sensor). The O2 sensors are low mileage AC Delco's. I verified that there's 12V supplied to the sensor heater element. I measured the sensor heater element at 22 ohm at 70F; the service manual states the resistance should be 3.5 ohm at 68F. Can someone please measure an O2 sensor heater element and let me know what resistance they measure? I would think that if the sensor heater wasn't working properly that the readings would be closer to 450 mA and not 920's. Thoughts?

mrmint69

You want to wait until the car is warmed up to see what the readings are on your scanmaster. You should not have to give it gas to make the readings bounce around and you want them to be all over the place except when going wot. Is the car going into closed loop when it should?

OBE1 95Z28

The readings are after its in closed loop; the O2's start cold at 450, then start bouncing back and forth while warming up. Once the car goes into closed loop is when the rich condition occurs and the O2's stop bouncing. The car has a custom tune and the tuner swears there's nothing in the tune to cause this condition; says it's a hardware issue.

Kevin Blown 95 TA

A couple possibilities to check out - if you had large injectors and the injector offsets aren't right or the minimum pulse width is too large.

OBE1 95Z28

I'm running 75 # low impedence injectors through an Acceleronics convertor. I'll investigate the injector offset/min pulse width. Thanks!

mrmint69

900's should happen when the car is in wot fuel enrichment. Assuming your O2's are working then you car is doing the samething it would do at wot. Sounds like a tuning or setting error. Just guessing though.

bobdec

W/O heaters working, the O2's will stay at 450 Mv longer. I did some testing with my heater fuse pulled and my pacesetter LT's and they stay at 450 Mv almost all the time at idle. Giving it throttle will get them moving, but they are slow switching w/o the heaters. I run Denso O2's, they seem a little slower heating up when compared to my original old Delco's when I did the testing. When cold the PCM puts 450 Mv bias voltage on the O2 sensor wire and as soon as the O2 begins switching (gets hot) it overrides that voltage and starts swinging, the PCM then signals BLM ready (assuming CL timer is hit & CL coolant temp is OK). As far as 900 Mv at/or slightly above idle AND in closed loop... Stock minimum injector pulse width is 1.404 Ms (constants setting in TC) wondering if that's to much for your 75# setup. Not familiar w/Scanmaster, does it show injector pulse width or ST BLM's (or AFR/fuel trims) at idle, does it show coolant temp at PCM.. at 900 Mv your AFR s/b pretty rich with those O2 voltage readings. In closed loop LT BLM's will tell how much the PCM is trying to correct mix, ST's will tell the actual AFR result of the PCM correction.

OBE1 95Z28

I recently purchased TunerCat to look at my minimum injector pulse width. It is set at 1.404 msec which according to the post above is the stock setting. As I'm running 75 lb/hr injectors so I followed the suggestion to reduce my minimum injector pulse width. I reduced the min to 1.144 msec and it appears that the O2 sensors in closed loop are now bouncing back and forth between .1 and .9 but now I have a surging/bucking under constant low level throttle. Increasing the min pulse width up to 1.251 msec got me back to the rich condition with no surging. I plan to try 1.205 msec but would like to get input as to how to manage the surging condition if it comes back.