Fuel mileage still sucks
10-28-2021, (Subject: Fuel mileage still sucks ) 
Post: #1
Fuel mileage still sucks
So, I’m seeking suggestions now

I’ve done a tune up, brand new CAC, and new rubber CAC silicone hoses.

My engine runs strong and fine. I’ve tried changing driving habits. Also considered trailer against the wind. I pull a chassis trailer that has removable containers. When I have an empty box, I weigh 36-37k. When full, I weigh 78-80k. Not sure of the weight without a box. Between all 3 options, my fuel mileage barely changes. Below are the numbers. What can you guys suggest me checking?

80k - truck, trailer, full box - 4.3 to 4.5 mpg
37k - truck, trailer, empty box - 4.75 mpg
? - truck, trailer, no box - 5 mpg

I keep looking for fuel on the ground, but none. No idling either. I have a generator with portable inside the cab.

All hand calculated. But even the onboard computer only shows between 4.8 and 5.2 on its readout.
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10-28-2021, (Subject: Fuel mileage still sucks ) 
Post: #2
RE: Fuel mileage still sucks
Doser injector seeping fuel into the exhaust?

could be how its driven, how fast you go, or somethng actually wrong somewhere that you have not found.

I know at 4 mpg, I would have gone out of business.

You might also want to mention what make/model truck, engine, some other history, etc.

Is it de-mandated?.. if so, who's programming?... what hardware mods have been done?.. etc.etc. also helps to know too.

There are 10,000+ things that can effect fuel mileage. The driver being the biggest influence of them all. Could also be engine or truck related too, however your question is also very vague with what might be going on.
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10-28-2021, (Subject: Fuel mileage still sucks ) 
Post: #3
RE: Fuel mileage still sucks
I’ll try to give as much info as I can.

2009 Kenworth T660 Midroof
CM871
11R22.5 tires, virgen rubber on drives
Rear gears around 3.70’s
Overhauled by Kenworth dealership approx 400k ago
Everything in tact, nothing de-mandated. DPF replaced approx 70k ago
Drive down in south Texas. Mostly flat terrain and small hills

I’ve mixed driving habits to test. Drove aggressive and very conservative with similar results. I’ve read the suggestions of driving by horsepower vs boost. Higher rpms vs lower rpms.

When I disconnected the exhaust pipe to put the rubber plug to pressure test, I didn’t see anything abnormal that would indicate the doser leaking.

I plan on de-mandating the dpf filter and associated components within the next few weeks with the mm programming.

When driving on the interstate, I drive at 67 mph / 1450 rpm. I shift around 1800 rpm.

Everything else on the truck is in great condition. In fact, I got stopped recently for a level 1 inspection. I’ve got one of those pretty orange stickers on my windshield.

Everything is all original. Only thing that my uncle mentioned to me before selling the truck to me was that he brought it to the dealership and had it turned up to 525hp.
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10-28-2021, (Subject: Fuel mileage still sucks ) 
Post: #4
RE: Fuel mileage still sucks
Although, while the boost system was leaking, I would notice the little regen light while driving on more often. But I associated this with a boost leak creating more black smoke and filling the dpf filter more often. Just fight it was doing it’s job cleaning itself. But now there’s no boost leak.
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10-28-2021, (Subject: Fuel mileage still sucks ) 
Post: #5
RE: Fuel mileage still sucks
Even if I’m running 67 mpg on flat ground and reset the instant trip fuel economy, it registers at either 5.2 -5.3 mpg on the computer. It seems that a sensor is reading something odd.

Could a dirty or defective fuel or air filter contribute to a problem like this?
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10-28-2021, (Subject: Fuel mileage still sucks ) 
Post: #6
RE: Fuel mileage still sucks
Don't you think 67 is a little fast? It takes more fuel to go 67 than it does to go 57.


User's Signature: Warranty??? Yeah right, I am the warranty!!!
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10-28-2021, (Subject: Fuel mileage still sucks ) 
Post: #7
RE: Fuel mileage still sucks
67mph is slow here in Texas. You drive 57mph here and someone will run into your back bumper. Speed limits here range from 75-80 on the interstate. Where I run is pretty much 75.

I’d be content with 6.5mpg. Not looking for 7+, but I know it capable. I just don’t know where the problem is
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10-28-2021, (Subject: Fuel mileage still sucks ) 
Post: #8
RE: Fuel mileage still sucks
(10-28-2021 )Lead_sled Wrote:  67mph is slow here in Texas. You drive 57mph here and someone will run into your back bumper. Speed limits here range from 75-80 on the interstate. Where I run is pretty much 75.

I’d be content with 6.5mpg. Not looking for 7+, but I know it capable. I just don’t know where the problem is

no one will actually run into your back bumper at 57 i worked for prime inc for years and ran new york to riverside and back in my governed 58 mph truck you are never gonna get 6.5 at 70+. i use to be on a dedicated route that ran from decatur alabama to waco texas that had to be ran in a 11 hour shift to deliver on time at 70 + in AL, MS, LA and 75+ in texas and the truck would do it and did do it from the time it was new until it hit around 350Kmiles but only did it at 4.6 mpg and I paid for it later with long term wear problems from pushing it that hard
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10-28-2021, (Subject: Fuel mileage still sucks ) 
Post: #9
RE: Fuel mileage still sucks
Sand boxes?
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