Idle RPM question - Page 5 - Fuelly Forums
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Old 10-27-2008, 08:44 AM   #41
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The cat may not hav a direct heat sensor but it works better when hot and if it is giving a reading that is not ideal it may assume that the temperature is low. Remember there is usually a before and after O2 sensor and the 4 wire O2 Sensors may have something in the heater connection to measure the temperature indirectly.

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Old 10-27-2008, 09:11 AM   #42
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Originally Posted by 1993CivicVX View Post
It didn't bounce or anything--stuck at 550rpm. So to me this indicates the heat generated going up the hill meant the catalytic converter was hot from going up the hill so the idle didn't need to warm it up.
Thanks for the data.

I was thinking about this a few days ago and realized that it can't be about the cat, but I no longer remember why.

Originally Posted by dkjones96 View Post
You are assuming that there is a temperature sensor in the cat so the ECU would know it didn't have to idle up to keep up the temperature.
My VW has one. I used to watch it when I had the VW-specific OBD cable and software.

Honestly, with a heat shield and everything I don't see the cat really cooling down THAT much.
It varies by HUGE amounts, and changes very fast.

Plus, emissions tests never test high speed neutral coasting emissions so I don't see them putting something like that in.
Very good point. They engineer to the test, not necessarily about real-world emissions, and especially not under weird driving behaviors like we do.

Most likely the fast idle at speed is to keep the hydraulic pump in the auto trans going and they just use the same engine management code for the manual trans engine management. It could also be that the same exact ECU is used for auto and manual versions of the car
That's the most plausible theory, IMO. They wouldn't necessarily design it with neutral coasting in mind (or maybe they would, in case it's being towed, engine on, drive wheels down). I'm not sure that it would manifest exactly the way it's been described, but it could.

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