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Discussion Starter #1
So I went back to the dealer today to see about getting back into my trade- in (2008 Duramax) and they said it has already been sent off to auction. I tried explaining to them that I basically hate this truck and it was a horrible decision trading my old duramax for it. I have about 3-4 tanks through it with about 1100 on the truck total. I drive 60 miles round trip daily with 90% of it being highway with cruise set at 70, my best tank has been 22.8. I have tried telling the dealer that there has to be an issue with my truck since I am getting that mileage but of course no cel= no problem. Yes it is a lot easier to drive than my old truck, it's more like a smaller suv. But going from my old duramax to this canyon, I am dreading my decision for the sole purpose of the trade off was to get the better mileage, but I had a whole lot bigger, better truck. Oh well I guess lesson learned.
 

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I purchased a Colorado the end of May. My typical driving is similar, about 45 miles on interstate at 70mph and a few miles in stop and go. My truck is getting around 31 mpg on the highway.
 

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Discussion Starter #3
I purchased a Colorado the end of May. My typical driving is similar, about 45 miles on interstate at 70mph and a few miles in stop and go. My truck is getting around 31 mpg on the highway.
I have pics that I have given to my dealer, on the sand stretch of highway same direction just different days, Monday my average for my highway drive was 33 mpg, Tuesday it was 21.3 mpg, Wednesday back at 33mpg. I am definitely having an issue, supposedly waiting from gmc about my issues for a fix. Today I drove it about 400 miles pretty much all highway and averaged 28.5 mpg which I was happy with for it was pretty hilly and for the first about 30 miles it was doing the less than 20 mpg on the highway thing. I have lost the ability to get back to my old truck and I'm not going to get rid of it to put me in the hole money wise, so either just keep hounding my dealership for a fix, or I will just delete the emission systems.
 

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4am travel on I95 with no traffic. CC set and didn't have to speed up/slow down for 40 miles, pretty much a standard morning commute. Afternoon headed home, not so much.
 

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Discussion Starter #7
4am travel on I95 with no traffic. CC set and didn't have to speed up/slow down for 40 miles, pretty much a standard morning commute. Afternoon headed home, not so much.

what MPH have you noticed for the best mpg? I've been keeping mine at 70
 

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I typically stay between 65-70 on the highway. It looks to me that the engine seems to like to run around 1950 rpm's.
 

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Discussion Starter #9
I typically stay between 65-70 on the highway. It looks to me that the engine seems to like to run around 1950 rpm's.

I typically stay between 65-70 on the highway. It looks to me that the engine seems to like to run around 1950 rpm's.
I've been doing better but still getting like one day a week just horrible mileage and it is killing my average
 

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It looks like the system is staying too long in regenerate mode wasting diesel. The pressure differential sensor might be in need of checking. Perhaps the diesel injection system is not attaining temperature fast enough so the catalytic element can "burn" off the carbon and rapidly complete the regeneration cycle.
 

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Discussion Starter #11
That is what I am leaning towards. I tried another dealership and they hooked it up and checked everything they could think of and found nothing. Since the dealership have said they have done all that they can, so I have submitted a case with GM with my issue. I am supposed to hear back from them again by Monday. If I do not get anywhere with GM/dealership , my plan is to just delete the emissions and be done with it.
 

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One other problem could be the diesel itself. Low cetane diesel will create more soot clogging the unit faster than normal. When plugged up the pressure differential triggers a regeneration cycle. Too frequent a regeneration cycle will spoil the average. Please keep us informed as I am seriously thinking about buying a Colorado diesel next year.
 

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Discussion Starter #13
I'm not saying this is a flaw with all the 2.8, out of everyone i have talked to on this forum only one other person has experienced this issue, while everyone else loves it. i always fill up at the same station near my house that has been recommended to me by local diesel mechanics, that it had high tier diesel and I've never had a problem with it for my past diesels I've owned. besides my mileage issue, i think my problem lies with that i traded in my slightly modded lmm which was a "true" duramax compared to the 2.8, so i just find it lacking.
 

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A different diesel source is a very easy try though. I read regeneration occurs about once with each tank. It would be nice to know how many miles between cycles.
 

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Discussion Starter #15
so far i have about 132 miles on this tank and it has already gone through one cycle at about 50 miles on this tank. I did add a cetane booster and injector cleaner to this tank to see if it would help any.
 

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Hmmm. That is the stuff I use in my tractor to prevent gelling in the winter to clear the driveway. Does the job. No regens 1985 in a tractor though. Max speed ~ 14 MPH so I cant say much else other than I have no engine problems.
 

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Discussion Starter #20
yea I've used it and 2 stroke in every diesel one owned and never had a problem. I've tried varying my driving style, driving like i stole it, to help burn it out or babying it, either way, it still goes through it's cycle.
 
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