I recently delivered a 7900 lb travel trailer to North Dakota from Dallas. I have owned the truck since 2000. I have changed exhaust, changed to K & N air filter, used a Hypertech programmer and G-force chip. I now get 12.5 MPG highway with no load. With this last load I could run up to 65 mph at 2500 rpm and then let of the gas a little to downshift into the overdrive and immediately I would slow down below 60 a keep lossing momentum. Needless to say it was like dring 1400 miles in high gear at 2500 rpms and used about $850 in gas averaging about 5.5 MPG on the drive up and 12.5 MPG on the way back with no load. Cantor anyone tell me what may be the problem? Could it be a Catalytic problem or a transmission/torque converter problem?

 

Welcome to the forum. We'll be glad to help. Do you have the 5.4 V8 or 6.8 V10 motor?

 

6.8 liter V10

 

Here are some places to start:

Check your fuel filter. Better yet, just replace it. Cheap and easy and can make a huge difference.
Also, clean the MAF. They get dirty and need some attention now and then. Pick up a can of MAF cleaner at an auto parts store for about $5.

But right off the bat, I suspect fuel pressure (filter or going-bad pump) and the computer not reading the right air flow so it's running lean (MAF).

 

Have you checked your spark plugs and coil packs? Sounds like what mine was doing when I had a bad #7 plug.

 

Good point. I dont think about spark plugs much any more...

But does sound similar to what my 5.4 did with 2 bad plugs. But that truck also had bad fuel injectors and 2 bad cats. Lets hope its not that.

 

just had a tune up and oil change before the trip. Also running 32 inch tires at 65lbs. Exhaust sounds like a sick toy truck instead of a big block

 

Are you sure they got the fuel filter? Its one of the most over looked filters on vehicles.

Double check and clean that MAF. Those are common culprits. What gear ratio do you have?

 

I just had 8000 miles of towing from 5k to 29' 11000lbs cruser andworst i got was 7.3mpg up VERY steep mountain in Pennsylvania, but rest of time in average between 9-10. I have 6.8 with custom headers, ypipe, straight 4" pipe with Banks high flow mufler, Banks trainy computer, 285 75 16s, stock air filter, fresh coolant and differential flush, new fuel filter, spark plugs, and coil boots.

 

After hearing this, I might tend to look at the cat and see if it's blocked (partially or fully). Exhaust shouldn't sound that way, and it would explain a few things too.