Quote:
Originally Posted by SentraSE-R
When I made those cat temperature observations, I noted that my UG and SG cat temperature gauges behaved oddly. Indicated temperatures shot >500 degrees F within 3 blocks of driving, which seems impossible, or at least improbable.
They also indicated cat temperatures dropped on every FAS from 500, 900, even 1100 degrees F to <200 degrees F within the 15 seconds to 1-2 minutes of the engine-off glide. This is another impossibility.
I suspect the UG and SG cat temp algorithms must rely on a voltage signal that resets when the UG and SG detect the engine-off condition. Obviously, the cat cannot physically cool 1000 degrees in a few seconds.
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The temperature rise you observe may be normal; it could take as little as 2 minutes to get >500 F, or something like that...
It does sound like your cat temp signal is erroneous after shutoff for some reason. It would be interesting to find out where your cat temp sensor is located. Or- is it possible your ECU attempts to predict cat temp, rather than measuring it, and there is no real sensor at all?
About 10-15 min after shutoff, I inserted a thermocouple into the air-gap between the cat inner and outer jackets on my 97 Civic. The reading was 200°C (392°F), and it very slowly fell to 170°C (338°F) over the course of several minutes. Since that location isn't a good thermal well, I was probably reading air temperature mixed with cat temperature.
I can say the cat's internal temp must have been higher than 200°C at least 10 minutes after shutoff, although I don't know by how much higher. I can also say that the thermal decay time is on the scale of 10-30 minutes, and certainly not <2 minutes.
So preliminarily that seems to support the 25-minute half-life I cited earlier, and casts strong doubts on the validity of the OBDII sensor (at least the particular sensor in Sentra's Scion). When I get better data I'll probably do a write-up in a separate thread, since I'm interested in smog pollution, and I'm suspicious of the conventional wisdom.
When you say FAS, the transmission is in neutral, no?