Passat Active Regens - How Often [Fans run after shutdown]

Santa2ya

Well-known member
Joined
Oct 14, 2011
Location
Ohio
TDI
None
I have interupted several regens after parking in the garage. The first couple of times there was a hot plastic/metal smell. I really wish VW would have put a light or something to let you know it was in a regen. They could have made it ding like the one for 39°. ;)
 

Jason4

Member
Joined
Aug 12, 2007
Location
Bellingham, WA
TDI
2012 Golf
I had an interrupted regen once after I parked and turned off the engine, fan under the hood running like crazy with loud noise, after a few minutes, didn't know what happened, I restarted the engine and the loud sound was gone, hopefully it didn't do much damage..
"Interrupting a regen" does NO damage of any kind. When the car is moving, during a regen, the flow of air through the radiator is usually sufficient to cool the engine. If the vehicle stops moving, the heat rises, triggering the high-speed fans.

As soon as the engine is turned off, any regen in progress is stopped. However, the residual heat might require the fans to continue to run. Therefore, if the fans are still running after the engine is turned off, it is ONLY to dissipate residual heat...the regen has stopped.

An interrupted regen will re-start, once the engine is re-started, and comes up to a temp hot enough to trigger the regen.

In traffic, when I hear a regen in progress (fans running, raised coolant temps, RPMs near 1k) I will put the car in neutral, and keep the engine at 2k rpm, to provide sufficient exhaust flow to assist the regen. This may not be necessary, but I like to think the added exhaust flow keeps things cleaner.

FYI, this scenario (active regen while not moving) has only occurred a dozen or so times in 50k miles. All the other regens must have occurred while at speed (over 2k rpm).

The newer, possibly cleaner, 2012 TDI might require fewer regens, but not because of the DEF. The addition of DEF has nothing to do with the DPF, and the number of regens required.
Stop second guessing VW engineers. Raising the idle based on the accelerator pedal will not "assist the regen" and "increasing the exhaust mass flow" is what we do at work when we get a runaway regen that needs to be put out.

The addition of urea should reduce the frequency of active regens unless those crazy Germans forgot to take advantage of the additional complexity of the system. It should actually improve fuel economy too. Without SCR (urea system) the in cylinder temps have to be lower in order to reduce NOx, if you have an active system to reduce NOx in the exhaust then the engine out NOx can be much higher (higher combustion temps). Higher combustion temps reduce soot production in the first place and higher exhaust temp increases the passive regen effect on the DPF which further reduces the need for active regens.

Higher combustion temps also improve the BSFC on a diesel engine up to a certain point.

Feel free to tell me why I'm wrong, if you understand this stuff better than I do and need a job let me know.
 

tdiatlast

Top Post Dawg
Joined
Jan 21, 2009
Location
Fort Worth, Texas
TDI
2009 Sportwagen (boughtback); 2014 Passat TDI SEL (boughtback)
I assume your admonition to "stop second guessing VW engineers" was directed at me. Thanks for the explanation about exhaust gasses, etc, as they relate to soot production, and that raising the idle above what VW has designed into the system is unnecessary, and perhaps even counterproductive.

My (old-fashioned) concern was with underhood heat, while sitting still. I thought that by increasing the rpms it would help the coolant dissipate more heat. I'll need to change the habit (only did it a few times, as the regens are so infrequent!) so that the exhaust gasses can stay as hot as VW designed.

Thanks for the clarification. I WILL, though, continue to park the car outside my garage when a regen is taking place!:D
 

darrelld

Veteran Member
Joined
Jun 6, 2007
Location
North Texas
TDI
2014 Tesla Model S85, 2017 Chevy Bolt
I assume your admonition to "stop second guessing VW engineers" was directed at me. Thanks for the explanation about exhaust gasses, etc, as they relate to soot production, and that raising the idle above what VW has designed into the system is unnecessary, and perhaps even counterproductive.

My (old-fashioned) concern was with underhood heat, while sitting still. I thought that by increasing the rpms it would help the coolant dissipate more heat. I'll need to change the habit (only did it a few times, as the regens are so infrequent!) so that the exhaust gasses can stay as hot as VW designed.

Thanks for the clarification. I WILL, though, continue to park the car outside my garage when a regen is taking place!:D
When getting close to my destination and notice RPM's hovering around 900 if possible I take an extra block or so to allow either the regen or the cool down cycle to complete.

I usually park in the garage but leave the door open if the fans are still running.
 

Jason4

Member
Joined
Aug 12, 2007
Location
Bellingham, WA
TDI
2012 Golf
Tdiatlast-
Didn't mean to come across as being rude but I wanted to be clear that the OEM should have done their homework with this. I deal with operators misunderstanding how things are meant to work on almost a daily basis.

I think you're probably doing the right thing by parking the car outside while it finishes a stationary regen just be sure you aren't parking over dry grass or on top of a pile of oily rags or something equally combustable. I don't know what the exhaust out temps are on a VW but in the applications that I'm familiar with it is high enough to use caution and could potentially cause some damage.

It's good to hear that regens aren't causing too much grief with these cars, I wish I could afford a TDI Touareg but that's more than I'm comfortable with and I need a little more clearance than I can get in a Passat or JSW.
 

GeekDrew

Veteran Member
Joined
Jan 23, 2008
Location
Columbus, OH, USA
TDI
None
I've had my Passat for less than a month, and have around 1500 miles on it -- it seems that I've interrupted regen (or for some other reason, the fans were running at high speed after engine shutdown) 4 times.
 

Paulsfin

Active member
Joined
Mar 13, 2012
Location
Kansas
TDI
2012 vw passat tdi
I read a lot of posts about interrupting regens so I was a bit concerned about it till posts started popping up that stopping a regen doesn't hurt the car. I listen for the fans before stopping the engine and if they're on I let the engine idle till they turn off. I've noticed that it only takes a couple minutes at most and I figure that letting the cooling system circulate can't be bad. Also, if I notice the fans on during a drive, the engine has a little more power. Pretty nice actually. I don't try to rev up or increase the rpm as IMHO, If I needed to VW would've programmed it into the system or had a warning label somewhere.

Is it correct to say the the DPF regen wants the exhaust side to stay hot for a while but keep the engine side at standard operating temp?
 

Jason4

Member
Joined
Aug 12, 2007
Location
Bellingham, WA
TDI
2012 Golf
Is it correct to say the the DPF regen wants the exhaust side to stay hot for a while but keep the engine side at standard operating temp?
I'm not sure that I understand your question, I think you are mixing different aspects of the engine. For an active regen to occur the engine-out exhaust temperature needs to be high enough to feed energy into the DPF for a red-ox reaction to occur and for the particulates to go from soot to ash. If VW is taking advantage of good multi pulse injection strategy then they probably aren't dumping much extra waste heat into the cooling system and could possibly be reducing the load on the cooling system if they are reducing EGR during regens.

In short, the exhaust gas temp is very high and the engine coolant temp should be within the normal operating range (~200-220F?).
 

TomB

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May 1, 2003
Location
Cle Elum, Washington/Las Vegas, Nevada
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2015 Audi TDI Prestige Sport
I'm not sure that I understand your question, I think you are mixing different aspects of the engine. For an active regen to occur the engine-out exhaust temperature needs to be high enough to feed energy into the DPF for a red-ox reaction to occur and for the particulates to go from soot to ash. If VW is taking advantage of good multi pulse injection strategy then they probably aren't dumping much extra waste heat into the cooling system and could possibly be reducing the load on the cooling system if they are reducing EGR during regens.

In short, the exhaust gas temp is very high and the engine coolant temp should be within the normal operating range (~200-220F?).
Correct.

So when are you getting a TDI? Where are you going to buy?
 

Niner

duplicate account, banned
Joined
Jun 3, 2011
I can think of 3 being done on mine. Something like electrical wiring or clutch smelled like it was burning the first 2 times. The smell seems to lessen as the car gets more broken in.
 

TomB

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Location
Cle Elum, Washington/Las Vegas, Nevada
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I thought of something as I pulled into the parking place and heard the fans and saw the increased RPM's, sitting at idle is not probably the best way to complete the regen.

At idle the engine has LESS opportunity to do the post injection to feed the regen. So I rev'd it to the 2500 rpm parked limit. It ran for about 5 more minutes and I watched the #1 MFI MPG go from 38 MPG to 35 MPG in those 5 minutes.

One thing was the exhaust/engine noise change when the regen ended. You could distinctly hear it quiet down and change. The fans were still going and making their noise.
 

VeeDubTDI

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That's just silly. You could have turned it off and let it resume its operation during your next drive.
 

tdiatlast

Top Post Dawg
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Location
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TomB, I used to do the same thing, until someone with much more knowledge than I suggested that revving the engine only lowers the EGT, which might hinder the efficiency of the regen.
VeeDubTDI, when you hear the fans blowing, and feel the engine revving, and smell the odor, you might be tempted to do the same thing. There's an amazing amount of heat being generated, and I guess old-school training triggered the "raise the rpm" signal...which of course harkens back to the days of rpm-speed mechanical fans...of course, before you were born...
 

VeeDubTDI

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I hear you on that, and I remember mechanical fans well - see Mercedes money pit in my signature. ;)
 

tdiatlast

Top Post Dawg
Joined
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Location
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So...could you please delete the "silly" reference? You probably hurt TomB's feelings, and you know how he gets :mad: when he's been hurt...
 

TomB

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TomB, I used to do the same thing, until someone with much more knowledge than I suggested that revving the engine only lowers the EGT, which might hinder the efficiency of the regen.
VeeDubTDI, when you hear the fans blowing, and feel the engine revving, and smell the odor, you might be tempted to do the same thing. There's an amazing amount of heat being generated, and I guess old-school training triggered the "raise the rpm" signal...which of course harkens back to the days of rpm-speed mechanical fans...of course, before you were born...
I was saying I revved it to allow more opportunity for injection of additional fuel. at 900 RPM less cycles than at 2500 RPM. Remember it injects in the off cycle of the four stroke. The CRD can only inject so much per cycle.
 

darrelld

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I was saying I revved it to allow more opportunity for injection of additional fuel. at 900 RPM less cycles than at 2500 RPM. Remember it injects in the off cycle of the four stroke. The CRD can only inject so much per cycle.
We have the same short trip issues with our Golf. 5 miles one way to work and I keep a check on regen requests with VCDS. If we see the requested regent hit 4 or above it's time for short run up the nearby freeway.

Passat regens are less frequent, VCDS values are here.

 

VeeDubTDI

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I had interrupted it TWICE already. My drive is too short, so I wanted it to complete.
Fair enough. Good decision on doing that, as it will eventually get cranky if it can't complete a regen.
 

TomB

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Fair enough. Good decision on doing that, as it will eventually get cranky if it can't complete a regen.
Scary part is that I am now noticing them when they start, which is good. Before I would just turn off the engine and then hear the fans and slap my head (lightly, TDIATLAST, so that does not explain anything).
 

Niner

duplicate account, banned
Joined
Jun 3, 2011
Scary part is that I am now noticing them when they start, which is good. Before I would just turn off the engine and then hear the fans and slap my head (lightly, TDIATLAST, so that does not explain anything).
Do you get on the freeway or highway or interstate much, where a 70 mph blast for 10 or 15 minutes would allow a passive regen to occur? Is there much biodiesel in your fuel there in WA?
 

Jason4

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Aug 12, 2007
Location
Bellingham, WA
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2012 Golf
Correct.

So when are you getting a TDI? Where are you going to buy?
I've decided that I'm going to replace my current B5 A4 in August, it'll be my birthday and I'll hit the 200,000 mile target around then. I'm not sure what I'll get to replace it, I've been doing a lot of hiking and climbing lately and there are a few trail heads that have pretty rough logging roads on the approach. Like I said a few posts ago a TDI Touareg would be perfect but my price range is more like $30K.

As for raising the idle when you notice it's in the middle of an active regen go back and read my earlier post (#32). I work with this stuff for a living and trust me when I tell you that you aren't helping things.
 
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