Declining fuel economy

[fourtenposi]

I have a 2005 GMC 2500HD 4x4 with the LLY engine.Truck has approximately 50000 miles on it. Truck is stock. When truck was new, had no issues getting 20+ mpg going down the highway, and 17-18+ for my daily driving routine. All mpg numbers are hand calculated. Pulling my 10,000 lbs enclosed trailer has dropped from 12-13 mpg at 65-70 to struggling to get 10 mpg. Truck has been regularly maintain. Changing the fuel filter made no differance. I have a egt and boost gauge. EGTs are higher then usual. Pulling the trailer on a flat grade at 65 mph, it was around 1000 degrees, now I struggle to keep it below 1200. I also have the edge EZ which yeilded improved fuel economy and more power. I took off the edge box, but no improvement in fuel economy. Any ideas on what I should be looking for? Truck also has a catilytic converter, could this be clogging up?

[Mark Rinker]

Clogged cat definately a possibility
Clogged air filter / restriction in intake
Any changes in tire diameter / pressure
Dragging brake caliper
Injectors not spraying/atomizing well - do you run cleaner?
State mandated biodiesel? This will drop fuel mileage, as well as winter blend fuels if you were testing in winter......or a combination of all of the above... :)

[fourtenposi]

I am going to replace the air filter just to be on the safe side, but it looks ok. Any way I could verify a clogged cat? Tires are the same size that were originally on the truck. I was running that total power stuff for a while but it didn't really seem to make a differance. Put I will put some in my next tank. No biodiesel mandate. Unless they have change something with fuel, in the past, I never really noticed a difference in fuel economy Maybe 1 mpg during the summer. I always keep the air pressure up 80 in the rear and 60 up front. Same as always. I will take a look at the brakes.

Thanks

[a5150nut]

To test back pressure, drill a small hole in the exhaust pipe in front of the Cat. It can be plugged with a sheet metal screw after your done if you don't have a mig or torch.. Use a low reading pressure gage to check back pressure. 0 to 15lbs range will do. To confirm it's the Cat you can check between the Cat and muffler too. ;)

[fourtenposi]

What kind of pressure is considered to much?

[rat4go]

I might be imagining things, but I recently replaced my batteries (one was iffy and the other had a dead cell) and I'm pretty sure my MPG got better. Makes me wonder if the alternator was always busting a$$ to try to get that bat with the dead cell up to a decent voltage and thus was extra engine load all the time. Maybe it doesn't work like that, but....

Another thought....You mention that tires are same size, but your wording suggests they are not the tires you got the better MPG with. Tire tread design and compound can make a pretty big difference, too.

[fourtenposi]

Well I put a air filter in the truck and it seems to have helped. I will continue to drive and see what it shows at the gas pump. Big test will be once i get the trailer behind it again. EGTs seem to be lower now. Air filter didn't really look that dirty. How is that little filter gauge suppose to work on the side of the air filter box. Mind was on the very bottom of the gauge showing full flow.

Rat4go, I think what you are saying about the battery is very possible. I have a dragcar, with an alternator, and when I have had battery issues in the past, It slowed the car and reaction time significantly. Based on what is slowed my car down it was eating about 5-6 hp.

I was told by the bridgestone dealer that the tires that were originally on my truck were no longer made, but I got the equivilent. I don't recall it changing the fuel economy then, because I would have immediately associated the fuel economy drop with that.

Thanks for the help.