Weird O2 ALDL readings (Page 1/1)
ZiZ097 DEC 11, 10:22 AM
Hello, I'm having difficulties on diagnosing what's wrong with my car. The car runs fine, but I have some issues with correct AFR. (all original 2.8 V6)
I'll share most detail I can possibly, as I've been looking around different bits and pieces to figure this out.

First of all, I machined the heads and when I reinstalled the engine in the car, I also put a new O2 sensor from Fiero Store as the old one used to throw SES light.
After a couple of months I noticed my spark plugs went pink, so I was scared of a lean condition. I got my hands on a ALDL adapter and software and found the engine was in fact running lean and the O2 reading was very sluggish.
The engine would struggle to get out of open loop for some reason, and when it did the O2 readings were slow.
I changed O2 sensor and all seemed fine.

One day I took the car on the highway and drive it for longer than usual, the car started to loose power and throws a rich O2 code. Luckily I had my ALDL adapter plugged in and recording, so I can share what happened.
Here is the graph of that run, I've been recording for 26 minutes so it's a bit squished.



After this happened I restarted the car and crawled to the closest rest area. The car apparently figured out it was very lean, so it gave the O2 lean code and runs badly. This is the graph of that second, shorter run.



Lastly that day, I resetted the ECM, unplugged the ALDL adapter just in case, and I got home nice and safe. I drove gently but the car got home and felt solid.

I'm really struggling to figure out why the ECM is acting up like that. With this episode I feel it's getting worse and worse.
Any input or ideas on that would be very appriciated as it's over 6 months now that I'm in the dark.

Additional info:
  • I now have new spark plugs but they haven't caught any coloration yet.
  • The fuel pump primes at 40 PSI, the regulator seems to hold it, when I start the engine the fuel pressure swings from 32 PSI at idle to 40 PSI at higher throttle. I'm guessing swinging due the vacuum controlled regulator.

[This message has been edited by ZiZ097 (edited 12-11-2024).]

Dennis LaGrua DEC 11, 01:46 PM
Once the engine is warmed the ECM will go into closed loop. The Oxygen sensor output will now vary quickly from lean to rich and back except at WOT. This is normal operating as it is seeing the fueling adjustments as you drive. Some but not all scan tools will provide an average O2 sensor reading and that can provide the true operating parameters, Most OBD2 scanners provide this but many even top end OBD1 scanners do not.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

ZiZ097 DEC 11, 03:08 PM

quote
Originally posted by Dennis LaGrua:

Once the engine is warmed the ECM will go into closed loop. The Oxygen sensor output will now vary quickly from lean to rich and back except at WOT. This is normal operating as it is seeing the fueling adjustments as you drive. Some but not all scan tools will provide an average O2 sensor reading and that can provide the true operating parameters, Most OBD2 scanners provide this but many even top end OBD1 scanners do not.




Hello Dennis, thank you for your input. The GUI I'm using has such feature, it's very complete actually.



As you said, the ECM seems to operate correctly, for most of the time.
The O2 seems to average slightly on the lean side and you can see the ECM trying to compensate for it with the BLM (long term) and INT (short term) values.
At one point tho, the car lost all power and I looked to my laptop to see that the O2 is not going up and down anymore. That can be seen at the very end of the first graph I shared. For some reason, out of the blue, the engine started running rich or better, the O2 sensor started measuring rich.

I'm trying to figure out why it's running lean in the average, and also why at some point it goes nuts.
From the graph I shared above you'd say it's not averaging lean, but that average includes the last bit of very rich reading that is misleading.
82-T/A [At Work] DEC 11, 04:06 PM
Hi, can I ask... which software is this? It does not look like Tuner Pro or WinALDL... it looks pretty awesome. Can you share the name?
Dennis LaGrua DEC 11, 04:12 PM
Just a comment on my experience. I have not had any luck with almost all aftermarket O2 sensors. We found that they all read inaccurately. Only the AC Delco which was made by Denso worked. Just a suggestion but you may want to try the Denso O2 sensor. Can't guarantee that it will solve your issues but we won't use any other brand here in my shop.

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

ZiZ097 DEC 11, 04:44 PM

quote
Originally posted by 82-T/A [At Work]:

Hi, can I ask... which software is this? It does not look like Tuner Pro or WinALDL... it looks pretty awesome. Can you share the name?



It's an adapter and GUI made by Paul Romsky. It's a relatively new tool he developed specifically for Fiero.
Currently he is selling individually on his website here.


quote
Originally posted by Dennis LaGrua:

Just a comment on my experience. I have not had any luck with almost all aftermarket O2 sensors. We found that they all read inaccurately. Only the AC Delco which was made by Denso worked. Just a suggestion but you may want to try the Denso O2 sensor. Can't guarantee that it will solve your issues but we won't use any other brand here in my shop.




I've been trying to find an original AC Delco O2 sensor that would ship to Switzerland but I only found the Fiero Store one, which was defective straight away.
I'm currently waiting for a 2nd Bosch sensor, maybe I'll be more lucky. But I feel there is some major electrical issue that throws off multiple sensors at once.
82-T/A [At Work] DEC 11, 06:46 PM

quote
Originally posted by ZiZ097:

I've been trying to find an original AC Delco O2 sensor that would ship to Switzerland but I only found the Fiero Store one, which was defective straight away.
I'm currently waiting for a 2nd Bosch sensor, maybe I'll be more lucky. But I feel there is some major electrical issue that throws off multiple sensors at once.




Thank you for the link! I'm totally going to buy that right now!!!

As for your O2 problem... have you thought about getting this?

https://www.casperselectron...oduct&product_id=101

It allows you to upgrade your single-wire O2 sensor for a 4-wire heated O2 sensor. It requires no modification, it just needs switched power and a ground. It use your old factory harness too. I have one that I'm going to install on my Corvette and my V6 Fiero.
jdv DEC 16, 05:09 PM
Good site to bookmark. I love there door sign.