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Duramax Fourth Generation: 2007.5-2010 (LMM) Discuss the fourth generation (2007.5-2010) of the 6.6L Duramax diesel engine & associated components. Engine related discussion ONLY.

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Old 04-18-2016, 09:52 AM   #1 (permalink)
tbeers11
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Fuel mileage decrease

I recently bought an 07.5 ccsb from my dad. He had the truck for 3 years and wanted to get a new one. While he had it he had the egr deleted, 4" exhaust with a muffler, and efi live put on it. After doing these mods he said that he lost fuel mileage. He mainly towed with the truck and said that fuel mileage (11-13 mpgs) pretty well stayed the same while he was towing. Unloaded he lost 3-4 mpg though, he would consistently get 14-15 on the hwy after the mods.
Fast forward to me buying the truck. Over the weekend I hooked up my camper roughly 8000lbs (weighs pretty much the same as my dads also) and headed out for the weekend. On the way down I noticed that it was just drinking the fuel. It was about a 100 mile trip one way after hand calculating my results I got 8.5 mpgs going down and 9.3 coming home. I was telling my dad about this and he said he never got less then 11.5 while towing his camper. We camp in the same place so we drive the same roads, our campers weigh pretty much the same give or take a couple hundred pounds, we drive pretty well the same 65-70mph.
My question is what could be causing this drop in fuel mileage? Like I said he noticed it after doing the deletes and going efi live but it wasnt that big of a deal since his towing stayed roughly the same. I just sold an LBZ that had 20's, 34inch tires, leveled, and only a straight pipe and would get 13 mpgs pulling my camper, I know that the LBZ's get better gas mileage, but something has got to be up for me to be getting 8-9 mpgs. Any advice is greatly appreciated. Thanks

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Old 04-19-2016, 01:05 AM   #2 (permalink)
Ron Nielson
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Does the truck seem to have good power? Has the DPF been removed?

My gut reaction is that it is likely in the tuning. I would set tuning back to stock, get a baseline, then go from there. Just a guess of course.

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Old 04-19-2016, 09:27 AM   #3 (permalink)
tbeers11
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The truck has great power and pulls very well. The dpf is deleted as well. I'm going to contact the tuner and see what he says about it also. Just trying to see what other options it could be though.
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Old 04-19-2016, 01:15 PM   #4 (permalink)
Ron Nielson
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There are two other things that could be robbing your mileage that are not commonly noticed because the happen quite gradually over time.

1) Batteries - when your batteries are weak, are not fully charged and your engine tries to charge them and this takes more fuel. Ever notice that your truck is a little zippier after you get new batteries? And then you recalled that it seemed a little sluggish before the new batteries?

2) Brakes can drag just a little for lots of reasons, mostly related to lack of maintenance/attention. The pads do not glide along the guides as they should and apply a little extra pressure to the rotor or they might just stick.

My fifth wheel is a very high profile and I get about 10 mpg when towing. I have been situations driving directly into a 30 mph wind and it has gone as low as 6 mpg. Speed of your vehicle and the local wind also has substantial effect of mileage.

You probably already knew about these but I thought they were worth mentioning.
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Old 04-20-2016, 01:41 PM   #5 (permalink)
tbeers11
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The brakes could be something that is an issue. I know I am due for a brake job just havent been able to find time yet. The batteries where just replaced in January so I would hope those are good. Thanks for the advice.
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Old 04-24-2016, 06:36 AM   #6 (permalink)
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My guess is the tuning. The more power you make the more fuel you will use.
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Old 04-26-2016, 07:30 PM   #7 (permalink)
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I've spent well over $3k in mods and tuning, didn't gain a ounce on mileage solo, towing 13.5k about 1.0 tops.
I'm old and don't have a lead foot.
More power and cooler EGT's heck yes.
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Old 04-26-2016, 11:15 PM   #8 (permalink)
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possibly the trailer? axle/trailer brake resistance, I knew of a guy who had an issue with his electronic brake system on his trailer, although unlikely, his didn't fully dis-engage, his trailer brakes were somewhat grabbing, not enough to make it noticeable while towing, he noticed when backing up on a rough muddy road 1 tire wasn't rolling as much as the others when that particular side did;t have the weight/traction.
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Old 04-26-2016, 11:41 PM   #9 (permalink)
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Wind.
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Old 04-27-2016, 09:44 AM   #10 (permalink)
tbeers11
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I would not care if it did not gain any mileage after the mods where done, but it actually decreased when unloaded after it was installed, pulling a trailer it stayed about the same which was between 11-12.

It could have been the trailer, I fought the electronic brake controller trying to get it dialed in correctly.

There was very little wind that weekend so I do not think that was it. I've pulled in much windier conditions and still got better than what I got on this trip.

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