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New Member 2500 6.0 fuel questions help!

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Hello!! Just joined. And before anyone asks YES I did a search inquiry for my issues but couldn’t narrow any info down. Recently purchased a 2006 2500 with a 6.0 and I’m a little concerned. Previously had a 2004 with a 5.3 but wanted something with a bit more susp and snort to pull my fish house which is 4500lbs. This thing feels like a DOG. It has 160k, absolutely flawless condition. Starts up a runs and idles smooth as butter.  Last weekend I pulled my fish house 118 miles and it took me 22 gallons of fuel!! 5.4mpg. That can NOT be normal. I didn’t expect teens but WOW I did expect 9-11 as it’s only 4500lbs. It feels as though I need to put the pedal to the floor board to get it to downshift a gear for more power on hills or it loses speed.  No codes. No CEL. just asking you guys for ideas as I’m not familiar with the 6.0 but from what I’ve been told this is NOT normal and maybe some issues with fuel trims, O2 sensors, MAF, throttle linkages etc. truck has brand new plugs and wires and brand new manifold gaskets. Any directions I can go? It seems to run smoothly with and without trailer just seems to blow thru fuel at a crazy ass rate. Thanks for any input. 

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Wow, 5mpg?


Could the brakes on your trailer be sticking? Or is the brake controller not working correctly?


Are you using Towing Mode? If so, you've probably noticed the increased rpm before shifts, and harder shifts. The higher rpm will reduce mileage. Does your 2500 have a 4L80E or the 4L60E?


BTW: my '03 K2500HD 6.0 4L80E 4.10 gets about 10 mpg empty.

Edited by David89GMC
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Is it all factory? Like stock size tires and everything?


Anything over 65-70mph while towing will tank fuel mileage but 5.4mpg is pretty darn low. Our old shop truck at my last job had 4.10's with stock tires and it got 10-11mpg empty, maybe 12 in the summer time. It was a crew cab truck though, so it was heavy.


If you had a scan tool you could watch things like fuel trims. Very possible trims would mean it's lean and adding in a lot of fuel all the time. Negative trims means it's rich and trying to pull fuel away. Most of the time fuel trims should be within ~5% of zero while at idle and driving. They will jump around a little but if they are like +20% or -15% all the time, then there is a issue.


Easy things to do are check tire pressure, clean the MAF sensor, run some seafoam through the fuel tank.


PS: All 2500/3500's have a 4L80 if it's an automatic in that body style with a gas 6.0 engine.

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Common things that can fail, and kill mpg but may or may not turn on the cel, are

-fuel pump failing (can't maintain full pressure)->test fuel pressure on the rail both at idle and when you speed the engine up to say 2 or 3k rpm.

-exhaust manifold bolts break off (causes exhaust leak, which screws up the O2 sensor readings so they read lean, and the computer starts dumping in fuel) Replace bolts (which can be a hassle) or get exhaust manifold clamps for the end bolts (particularly useful for the 2 rear bolts), and perhaps even replace the exhaust gasket as well.

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The tune for the 6.0 from the factory makes it quite undesirable. It is quite a difference from a 5.3/4L60e combo to a 6.0/4L80e though in a bigger truck. The 6.0 makes more power than a 5.3 but in the 2500HD it is harder to notice sometimes. 


You do have something major wrong though. I would check fuel pressure and make sure that is in spec.

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Somethings not right with 5.4 mpg.  Need to dig deeper than codes for the issue like Davester suggested.  I carry a 3,200lb camper in my 6.0 and still get 8.5-10.5 loaded.  Have larger tires been installed?  They can be heavy, have much more rolling resistance due to width, and will give improper distance on the odometer.

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