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Old 07-21-2014, 04:12 PM
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MPGs and regens

After several weeks of CRD ownership, I feel like I'm getting used to how the car drives. One thing I've noticed is how the MPGs drop at various points throughout the regen cycle. I have identified several flat spots on my daily commute* and when I can reliably drive at a constant speed I have tried to observe how the car behaves. Here are my observations:

A) When a regen is actively occurring**, instantaneous fuel economy drops to 25mpg @ 65mph on flat terrain.
B) After regen has completed, MPGs rise to ~32 @ 65mph on flat terrain.
C) After a few days of my normal commute, fuel economy does not seem to get much above 28mpg @ 65mph.

This has led me to wonder if there is a "third state" beyond regen / non-regen, where the system tries over and over again to passively regen but never really ends up clearing itself out. I can get back to state B by doing a decent drive (~30 mins) on a clear freeway which seems to trigger an active regen. To date I have never received the "DPF nearly full" message or whatever. Knock on wood.

During my daily commute I am not really above 45mph for more than 10 minutes. During that time I don't see my MPGs drop to 25 so I doubt that the car attempts an active regen, but who knows.

Does anybody have any thoughts on this, or any similar experiences?


* "Daily commute" consists of 12 miles each way, 5 of which is usually in moderate to severe traffic, plus 2 miles of city streets, with the balance being on a clear/free-moving freeway.

** I have no mechanism to know for sure that a regen is occurring. I am going off of knowledge gleaned from the diesel supplement and the reports of others on this forum that fuel consumption increases during a regen. I also check my DEF gauge periodically and it will often report a lower level immediately after this dip in MPGs.
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Old 07-23-2014, 12:44 AM
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Re: MPGs and regens

My experience of DPF function is the same as yours. The only hint it is happening is mileage drop to about 24 from the usual 30-32 on the freeway at 60-70 mph. I don't notice any change in power or pickup.


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Old 07-23-2014, 11:33 AM
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Re: MPGs and regens

A few weeks ago I noticed my MPG really dropped off, I thought I had gotten a poor batch of fuel or something. I then got the "Please drive at highway speeds" notice for the first time in ~3500miles. After that went away my MPG jumped back up.
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Old 07-24-2014, 12:44 PM
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Re: MPGs and regens

Rumline, here's a round trip average from Pasadena area to Palm Desert and back last weekend. 34mpg.



I kept a pretty close eye on things, and noticed a couple drops here and there. Assuming now, that they're regen related. But overall, it did really good cruising. My wife putts around town during the week with the JGC, and it's our weekend family hauler, and gets lots of leg stretching then.

In 1800 miles, we've not come across issues requiring the regen notice in the EVIC. This includes an entire two days in a row worth of about 200+ miles driving low speeds around Orange County for family party related stuff.
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Old 07-24-2014, 11:23 PM
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Re: MPGs and regens

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Originally Posted by Rumline View Post
I also check my DEF gauge periodically and it will often report a lower level immediately after this dip in MPGs.
That is probably coincidence. DEF fluid is used continuously and is injected after the DPF into the Catalytic Converter to reduce NOx.

The best way to monitor regen activity is with an ODB2 interface. The Torque app for Android and a Bluetooth adapter can be had for 30 bucks. CAT temp will rise to 1200F+ during regen.
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Old 07-25-2014, 06:42 AM
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Re: MPGs and regens

I wave just over 3000 miles on my diesel and have not noticed anything that let's me know the vehicle has entered a regen cycle.

How are you guys noticing it?
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Old 07-25-2014, 08:20 AM
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Re: MPGs and regens

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Originally Posted by smljeep View Post
I wave just over 3000 miles on my diesel and have not noticed anything that let's me know the vehicle has entered a regen cycle.

How are you guys noticing it?
I think most of us won't notice if if we do a lot of highway miles where the DPF doesn't clog up as quickly (city and lot's of stop and go are what plug them up quickly). I had mine do it only once at 1,500 miles but we were driving it a lot from point a / b and not doing a lot of sustained highway or long trips.
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Old 07-25-2014, 03:09 PM
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Re: MPGs and regens

OBD tool is really the only way to know apart from the hot exhaust and odd smell. Mine was at the end of a regen yesterday when I pulled into the garage. It smelled hot and there was visible heat distortion rising from the exhaust tips. Torque showed the cat temp fluctuating between about 1050 and 1225F and fuel rate of greater than zero while coasting. EGT stayed in the normal range for the driving conditions.

I noticed some odd behavior on some cold starts. After about a mile of driving, the cat temp will shoot up to 1000F+ for a brief time like it's starting a regen and then return to the typical 400-600F range and stay there for the rest of the drive. Does the DPF or cat need to be preheated to be effective?
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