Hi folks,

I just joined this forum. I picked up a '94 Admiral Blue six-speed with 102K miles on it last October. I've been working through a variety of issues and could use some help on this latest one.

I was troubleshooting an afterfire on deceleration which led me to examine the O2 sensor readings to try an understand if the engine was running rich, and to see where that would lead. I have been using EEHack to data log my system. My O2 sensor readings happen to look terrible. I found that they would oscillate at about 2.6 Hz for a while, but then for periods lasting anywhere from 10 seconds to 2 full minutes, both upstream O2 sensors would read close to 0.9v and stay there until the event ended. I then looked at the post CAT O2 sensor and found that it was always correlated to the upstream sensors. This gave me a clue that my CATs were not working. But it didn't explain the shape of the curve in the O2 sensor data. I eventually found that the CAT Over Temp signal is asserted with 100% correlation each time my upstream O2 sensors stop oscillating and go to 0.9v. Now I had two data points that my CATs were bad.

I decided to have an exhaust shop examine the CATs. They removed the O2 sensors and looked at each with a boroscope. They found that the CATs were hollowed out. It confirmed why the data was showing that they were not working. But, that in and of itself didn't explain the CAT Over Temp signal that I was getting, especially when the CATs weren't actually doing anything. I had them install Magnaflow CATs and new upstream O2 sensors. I could not locate a replacement for the downstream sensor, so I had them reinstall the original.

I reset the computer and data logged a couple of hours of running with the new CATs. The car generated one tiny afterfire the entire time, which was a huge improvement over what it did before. I thought I had this all figured out, but when I reviewed the data, it showed me that 1) now the downstream O2 sensor no longer tracks the upstream sensors, so it seems to corroborate that the CATs are working, but 2) the CAT over temp signal is still being asserted, and with it, the upstream O2 sensors rail to 0.9v during this period just like before. The duration is about the same as it was before, anywhere from 10 seconds to 2 full minutes. It will do this periodically throughout a 1.5 hour trip.

Here are my questions. Does the ECM determine CAT temperature from a temp sensor built into the post CAT O2 sensor (or infer it from the O2 reading, or ??) If so, is it fair to assume that the downstream sensor has failed? Or does it determine CAT temperature from some other sensor? If so, what sensor? If I could locate a replacement post CAT O2 sensor, I'd install it and see what happens, but I cannot locate a single vendor who has one labelled as a downstream sensor.

Is it worth just disabling CAT over Temp in the ECM by uploading a tune that turns that off?

Any other thoughts on what to do?

Thank you,
Joel

 

First off, be thankful someone hollowed those POS out. Many people have had them melt down and cause blockages. Esp if it runs rich.

next. The popping on deceleration was normal on my stock 93 6spd. Its music to the ears. I loved it

there are big engine mgt differences between my 93 and your 94, so i cant speak if thats normal on a 94.

my speed density 93 was highly sensitive to the type of o2 sensors used. The auto parts store bosch o2 sensors were not the same caliber as NOS ac delco made in usa heated sensors that i installed. (I went heated bc I run headers).

also, the o2 sensor wiring is very sensitive. I had soldered on extension wires which is a no-no.
so make sure your o2 sensor wiring is good

lastly, could a bad optispark cause it to run rich by not firing (and dumping raw fuel out the exhaust?) ? I bet thats possible.

the fact the PO hollowed the cats tells you something was going on....

 

The post CAT O2 sensor (aka the third one) provides data that shows how the CATs are performing. It allows the computer to detect a failure. It doesn’t look like the program in the ‘94 looks at that to determine CAT performance, but it uses that sensor somehow to determine temperature. I’m trying to understand if it computes temperature from the O2 readings from that sensor, or if there is a separate thermistor in that sensor.

 

my 93 doesnt have the post CAT o2 sensor, but maybe you can see if the p/n of the o2 sensors (both pre and post cat are the same ?)

sorry i cant be of much more help to your specific question.

 

https://www.camaroz28.com/forums/com...5/#post4909815


I interpret that as:

gutted cat --> rear o2s tracks front o2s --> assume cat overheated --> dump extra fuel to cool off cat --> o2s pegged rich and afterfire

FWIW, search camaroz28 for stuff like this. It's a dead site these days but the archives are infinitely deeper than this site for things related to tuning, diagnosis and PCMs.

 

I had new CATs and new upstream O2 sensors installed. I can see that the downstream O2 sensor is responding properly to the CATs, so that’s good. The problem is that even with the new CATs and sensors installed, the computer thinks it sees an over temp. I was attributing that to a bad downstream sensor, since I am using the original.

As an experiment, I just disabled cat over temp and burned the flash with that tonight. I read it back twice to confirm my changes took. They did. However, after driving for 30 minutes with these changes, the data log still shows that CAT over temp is being asserted, and I get the same O2 rich sensor behavior as before. This was completely unexpected. Basically the change had no impact because CAT over temp mode is still being enabled.

I am at a loss for how that can be. The temp threshold was set to the max, 151.3C.

 

I thought about this some more and managed to have some success. I noticed that there are three other parameters that are used to determine when to enable "CAT Protect Enrich Mode". Besides "Min Coolant Temp", there is "Min Temp at Low Pressure", "Min Temp at Mid Pressure" and "Min Temp at High Pressure". Based on the comments in the XDF file that describe what "Min Coolant Temp" means, it looked like the entire mode could be disabled by maxing out the value of "Min Coolant Temp". The max value is 151.3C, which is significantly higher than my coolant temperatures ever get. Mine stay below 90C on every run I have logged. It's reasonable to think that setting the temperature to a value that is not actually reached would effectively disable the mode. However, that didn't actually work.

After setting the new Min Coolant Temp and still getting the CAT over temp, I decided to have another look. What if the ECM is looking at the other three values and will trigger CAT Protect Enrich Mode regardless of the Min Coolant Temp if the CAT temp meets the threshold? Having nothing to lose but a little time, I adjusted the three Min Temp at Pressure settings, and maxed them out. Now my Min Temp at Low/Mid/High Pressure are set to 1065C, up from the defaults which were in the 800's C.

After a test drive to check it out, the system did not enter CAT Protect Enrich Mode as it had before. It was successfully shut down. The upstream O2 sensors reported normal values the entire time, and the downstream O2 sensor showed what looks like good data, too. So far, so good.

Joel