New o2 sensor. Notable differences
 

Hello,

I changed out my o2 sensor as I suspected my old one had gone bad. The purpose of this thread is to note my differences and document my symptoms for use by the community. I am making this thread in conjunction to this one: https://www.rx7club.com/3rd-generation-specific-1993-2002-16/new-o2-sensor-%3D-difference-acceleration-553767/
Starting off:

My Rx7:
1993 VR Touring with 41k. Mods are a Pettit 3" downpipe and Azeknights solenoids.

Symptoms:
I had been experiencing the typical 3k RPM hesitation as well as a light surge at cruising RPM, poor fuel economy [14-15 MPG], and poor partial throttle low end response. My 3k RPM hesitation occurred with AC both on and off. It felt like an automatic transmission equipped vehicle up shifting under mild throttle [best description I can give] at ~3k - 3.3k Rpm. It occurred with AC on and off and was prominent while temps were both cold and warm. Whilst cruising at low speeds and steady RPM, I could feel a light surge of on and off power but steady speed and RPM. No CELs.

Attempted fixes:
To remedy the 3k RPM hesitiation issue, I added additional 8g grounds and cleaned the factory ground connection points. I added 1 ground wire from the negative battery terminal to the drivers side shock tower [bare metal] and 1 ground wire from the alternator body to the same point on the shock tower. This DID NOT remedy my 3k RPM issue. I had replaced the plugs and wires roughly 2k miles ago and the issue had still persisted. I got a new battery 500 miles ago.

What led me to the o2 sensor:
The supplied thread at the beginning of this post had me believe my o2 sensor was at fault. I believe the o2 sensor is used as a reference for the ECU to properly supply fuel at a set cruising RPM for maximum fuel economy and performance. It is also a reference during acceleration. The cause of the 3k RPM hesitation is believed to be the transition from primary injection to both primary and secondary injection. If the AFR read by the ECU is not proper, then the onset of additional injection may come on as a "surprise" to a lazy o2 sensor.
http://www.pettitracing.com/rx-7-recommendations/ This page also indicates that a new o2 sensor may quell hesitation issues. The FSM on page F-193 indicates the o2 sensor may be a cause of poor acceleration. Page F-195 indicates it may contribute to hesitation as well.

My o2 Sensor:
Attachment 718583

I believe this may have been partially my fault. I believe I in part caused this during the installation of my downpipe. There were 4 broken strands I saw after I removed it. I saw the exposed wire prior to removal of the sensor. I replaced this with a Denso direct fit sensor from oxygensensors.com [a link in the FAQs]. I disconnected my negative battery terminal and stepped on the brake briefly during installation to reset my ECU. I installed the new sensor and embarked on a 32 mile cruise.

Noted changes:
I immediately noted a smoother idle warm up. Before when it warmed up, it would miss here and there and generally be pretty rough. After the idle would drop to ~1100 RPM it would begin smoothing out. With the new sensor is idled smooth as butter. Like a fat kitty purring in your lap. :P I took her out for a cruise and the drive through my neighborhood was easy. Its 15MPH until I leave housing on base so I had time to play with low end response. My response was improved and she accelerated better under light throttle. She definitely felt stronger. I took her out through some nice roads and she accelerated smoother than she used to. I also tried to get her to hesitate at 3k with the AC on under light throttle. No matter how much I tried, she would not hesitate like she used to. Now, as I accelerate under light throttle through 3k RPM with AC and headlights on ["heavy electrical load] I feel a smooth but notable increase in power as I pass 3k RPM. I attribute this to the additional injection provided by the secondaries coming online. Almost like a smooth "vtec" from 2.9k-3k RPM. During steady cruising RPMs, she holds steadfast, smooth as can be. Accelerating into boost and up to redline, she feels notably stronger. My reference to the increase in power is as follows: My friend owns an 04 s2000, bone stock and well kept. Now and then we comparatively drive on the highway to see who can accelerate better to the speed limit, if you know what I mean :angel: The driving would usually go as follows: he would get the jump on me by half a car and we would hold steady at that until about mid 4th gear when he has to grab 6th and attempt to go any faster. At that point I would walk right past him. In our recent driving comparison :angel:, I had walked away from him like it was no big deal.

Conclusions:
FD3S.net has a survey that studies Rx7s with the 3k RPM issue and surveyed fixes and failures. I would like to add myself as the one and only who had their issue quelled by a new o2 sensor. I would like to advise everyone to be careful upon installing their next o2 sensor as to not twist the harness as they thread the sensor in as damage may result. I will note my possible increased fuel economy upon my next fill up. I hope this thread helps others and any input is appreciated.

Matt

Very well written and researched post. Glad you got your problem fixed! Thank you for sharing.

excellent, this is next on my to do list!

Just filled up my tank today. My previous fill up was just under 15mpg with my usual spirited cruises on the weekends. This fill up was 18mpg with my same spirited driving! 18mpg is saying much but it's within oem spec of 17-21 now :)

Glad to hear that this worked for you. I have had the 3k rpm hesitation myself and am looking into swapping this part out myself (should be done anyway regardless as my mpg has dropped below oem spec). Hope to have similar results myself. I'll be sure to update with my experience as well when I get around to it which will hopefully be sometime this month.