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Old 06-30-2012, 09:28 PM   #1 (permalink)
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Default Catalytic Converter Purges

I've been told by one source I think knowledgable that FAS'ing and downshifting both cause catalytic purges, in addition of course to the routine purges. I seem to have noticed the penomenon myself, particlularly with downshifting. I do seem to hang on to lean burn in fifth at the tops of hills by simply lightening the load a bit, but if I downshift lean burn is gone and very difficult to recover.

I realize of course that load is involved and I may simply be using too much load after the downshift, but still I've heard the account of purges.

Could some of the folks with vast years of experience and knowledge comment on this.
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Old 06-30-2012, 10:58 PM   #2 (permalink)
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Jim,

I spend a lot of time driving close to 34 mph, which seems to keep NOx purges to a minimum. I hardly have any at all.

However, when driving on a hillier portion of the drive home, and approaching speeds up to 40 mph, NOx purges seem to happen almost over every hill. I'm exaggerating a little but purges really pickup dramatically.

FASing does not seem to change the amount of purges, but I rarely downshift the motor either. My high preference for slowing down is coasting with the engine off.

So to answer your question, I have not noticed anymore NOx purges from FASing, but I probably drive slower than almost anyone else on this forum, except for maybe DiamondLarry.

Jim.
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Old 07-01-2012, 02:40 PM   #3 (permalink)
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What I really want to know is whether the car would purge with the cats removed, whether it uses pre-programmed mapped coordinates for air and fuel in the computer to determine purge intervals, or if it relies on sensors including the 3 (or 2) oxygen sensors to provide actual feedback of catcon NOx accumulation somehow.
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Old 07-01-2012, 04:47 PM   #4 (permalink)
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Quote:
Originally Posted by RedJellyBean View Post
What I really want to know is whether the car would purge with the cats removed, whether it uses pre-programmed mapped coordinates for air and fuel in the computer to determine purge intervals, or if it relies on sensors including the 3 (or 2) oxygen sensors to provide actual feedback of catcon NOx accumulation somehow.
The subject of catalytic purges was discussed a few months back on this thread:

https://www.insightcentral.net/forums...ge-cycles.html

I don't recall all the details there, but the emphasis was on finding a way to avoid purges, perhaps with direct fuel injection into the cats. I too need to reread that thread. I don't think it spoke to either of our interests.
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Old 07-01-2012, 04:50 PM   #5 (permalink)
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Quote:
Originally Posted by RedJellyBean View Post
What I really want to know is whether the car would purge with the cats removed, whether it uses pre-programmed mapped coordinates for air and fuel in the computer to determine purge intervals, or if it relies on sensors including the 3 (or 2) oxygen sensors to provide actual feedback of catcon NOx accumulation somehow.
I have no way to back this up but, I suspect the car uses a pre-programmed table to determine purges as I've never heard of our cars having a NOx sensor. I'm guessing it would use some formula having to do with level of load over time while in lean burn. NOx is produced at higher levels when the load is higher which would explain more purges on the highway than when just tooling around the countryside.
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Old 07-01-2012, 10:11 PM   #6 (permalink)
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There's no sensors past the NOx cat so there purges are calculated. Seems that on the highway at 60mph or higher that mine will purge around 1.5 miles plus or minus about .1 mile, most of the time. Less load less purges. Mine will still purge though, even if I'm going 30mph and the load is minimal but at 30mph I'm usually doing P&G in 4th gear without lean-burn because I get better gas mileage when the engine is off.
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Old 07-01-2012, 10:27 PM   #7 (permalink)
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There's no sensors past the NOx cat so there purges are calculated. Seems that on the highway at 60mph or higher that mine will purge around 1.5 miles plus or minus about .1 mile, most of the time. Less load less purges. Mine will still purge though, even if I'm going 30mph and the load is minimal but at 30mph I'm usually doing P&G in 4th gear without lean-burn because I get better gas mileage when the engine is off.
OK, I think most of us believe that this general relationship prevails, and that there is no activation by measurement. So, does downshifting cause an immediate purge? I ask because it seems on both my cars that downshifting drops the LB and it takes considerable time to nurse it back.
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Old 07-01-2012, 10:47 PM   #8 (permalink)
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It's not the downshifting that drops you out of leanburn but the clutch signal when you depress the clutch, if you have installed the Calpod switch just flip the switch without shifting and you will notice it will take you out of leanburn.
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Old 07-02-2012, 07:48 AM   #9 (permalink)
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It's not the downshifting that drops you out of leanburn but the clutch signal when you depress the clutch, if you have installed the Calpod switch just flip the switch without shifting and you will notice it will take you out of leanburn.
Of course, why didn't I think of that

Kinda makes one want to experiment with an "Inverse Calpod" switch to see if LB can be maintained through a low load shift.
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Old 07-02-2012, 10:37 AM   #10 (permalink)
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The movement of the throttle and load change needed with upshifting and downshifting will always pull you out of lean-burn, it's not the purge cycle. It's the same as if you release the throttle completely or push too much throttle while in lean-burn, it drops out of lean-burn. Lean-burn requires a fairly specific range of throttle(lower than around 32ish, less throttle at high RPM as you approach 78mph), load(below high 80's), gear(3rd and above), and temperature(over 155f or so and above about 20f ambient) to operate, if you leave those factors you aren't going to go into lean-burn.

If you drop out of lean-burn due to throttle or gear change, you need to wait a little bit but if it's being picky just hold the throttle at around 60mpg for about 5 seconds, then drop the throttle to 100mpg and it will drop right back in, if it doesn't add throttle to 50-75mpg again for a little while and retry. It gets tough when its freezing out and the first few times it goes into lean-burn during a warm up cycle. Both of my cars are consistent with their lean-burn behavior.
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