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Old 04-30-2023, 02:44 PM   #1
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Sooty Exhaust, Excessive DEF usage?

First of all, I'm completely ignorant with the new diesel engines, DEF usage, regens, etc. I have a 2021 Phaeton with the L9? 380 HP engine. From the time I took delivery, until today, DEF usage was modest and the exhaust pipe was always clean; no soot, no nothing. Today, I noticed the DEF seemed to be being used at an unusually high rate. Filled up at Racetrac and noticed the exhaust pipe was smutty, except it wasn't a black smut; more like a grayish-black smut. I usually average around 8.2 - 8.3 mpg. On this trip I averaged 7.6 mpg (528 miles) and used approximately a quarter tank of DEF. I now have 13,400 miles on the coach.

Can anyone give me some direction here? I've searched the forum and the internet trying to figure which steps to take, came up empty. A literature source for these issues would also be appreciated...


Thanks!
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Old 04-30-2023, 04:06 PM   #2
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Assuming you have a 11 gal def tank, if you used 2.5 gal for 528 miles, that might be a little high but not excessive. What you don’t provide is the type of roads you drove, hilly, flat, etc, or the amount of stop and go traffic. Normally the latter eats up your mpg and def. That said sometimes my mpg fluctuates by 2 mpg. On open road with a toad, average 7.5. But have strong winds, numerous speed fluctuations, etc, drops to about 6.5. Really a lot of variables to consider with both fuel and def consumption, so wouldn’t panic yet.
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Old 04-30-2023, 09:22 PM   #3
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Your mileage and DEF usage sounds about right. With that mileage, you would have burned around 70 gallons of fuel and with 2-2.5% DEF usage, that would be 1.4 to 1.75 gallons. Our overall average for 50,000 miles has been 7.5mpg and 2.1% DEF usage. The soot could be the result of a DPF regen where extra fuel is pumped into the exhaust system to burn off soot and could account for extra fuel usage.
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Old 05-01-2023, 07:11 AM   #4
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After reading these replies to my query, feeling better about this situation. One more question, please. What causes a regen? I've heard this term used before and don't quite understand it. Is a regen an automatic maintenance action triggered by normal usage, or is it triggered by improper cumulative actions of the driver? IOWs, am I doing something that causes the regen or is it going to happen anyway, regardless of my driving habits?


Much appreciation for the replies to my concerns; I was close to heading out to the Cummins dealer...
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Old 05-01-2023, 11:40 AM   #5
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Try this link. Per my research most regen caused by the fuel. I suspect some place on your trip you took on some bad fuel. That would explain the soot on your tail pipe. I’ve over 71k miles on my coach and fortunately have not had to perform one yet. Keeping my fingers crossed.

https://www.cenex.com/about/cenex-in...-regen-hassles
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Old 05-01-2023, 11:46 AM   #6
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Quote:
Originally Posted by Warhorse6 View Post
Try this link. Per my research most regen caused by the fuel. I suspect some place on your trip you took on some bad fuel. That would explain the soot on your tail pipe. I’ve over 71k miles on my coach and fortunately have not had to perform one yet. Keeping my fingers crossed.

https://www.cenex.com/about/cenex-in...-regen-hassles
Are you talking about a forced regen? With 71k miles , your engine has done numerous regens.
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Old 05-01-2023, 12:15 PM   #7
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Originally Posted by SAWillie View Post
One more question, please. What causes a regen? I've heard this term used before and don't quite understand it. Is a regen an automatic maintenance action triggered by normal usage, or is it triggered by improper cumulative actions of the driver?
The Regen is a normal function of the exhaust system and will usually take place while you are driving. The only indication will be the HEST (Hight Exhaust System Temp) light will come on for about 20 minutes. It does this to burn off accumulated soot in the DPF. Ours will go through a cycle every couple of thousand miles depending on the terrain we've been on. Heavier pulling in hills causes a faster soot buildup.
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Old 05-02-2023, 07:34 AM   #8
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The Regen is a normal function of the exhaust system and will usually take place while you are driving. The only indication will be the HEST (Hight Exhaust System Temp) light will come on for about 20 minutes. It does this to burn off accumulated soot in the DPF. Ours will go through a cycle every couple of thousand miles depending on the terrain we've been on. Heavier pulling in hills causes a faster soot buildup.

This and the genex URL answered my questions about regens. Much appreciated!
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