Dwindling fuel milage

[Showgood1]

My uncle's 04 lb7 fuel milage is steadily getting worse the best it ever got was 14 in town and about 15 or 16 on the highway. All of this unloaded. The truck is a crew cab short bed 2 wheel drive. He has had it in the dealership twice for this and they said everything is ok. (With the morons at our local dealership that and .50 will get you a cup of coffee). Still the truck runs great pulls great with no drivability issues no codes. The truck is now down to 12mpg unloaded in town. Jeez my '96 454 gets 11. Oh and he doesn't drive like I do either. Beyond the obvious filters, tire size, fuel quality where else would you look for a problem? I wonder if since the older the truck gets the worse it gets if we are looking for an injector replacement soon? But on a '04? I think he presently has about 30k on the clock. Where would you start looking from here?

[markrinker]

1) What oil does he run? If he's not burning a synthetic, now is the time to switch. Add +.5mpg.

1) Check for any obstructions or restrictions in the air filter. If the filter has been wet - replace it. How about a foam aftermarket filter?Add as much as +1mpg.

2) Check for any obstructions or restrictions in the exhaust system. You may have a plugged cat. Add as much as +2mpg.

3) Check for any dragging brakes or wheel bearings by putting the truck up on a hoist and spinning the wheels. Improper alignment? Check for proper tire inflation. Add as much as +.5mpg

4) Where is he buying fuel? Any chance that he fuels up at the same station with problems in their tank? Fuel filter ever been changed? Add +.5mpg

5) Does he drive it like a gasser? Ask him (after performing all of the above) to drive one FULL tankful, including a diesel additive/injector cleaner, NEVER exceeding 2000 RPM. Add +1mpg

I'll bet its some combination of two or more of the above, and not one thing will fix the problem. Do all the above and drive it right, he should see 16-18 at least.

[Showgood1]

Thanks the one thing I didn't think of was the cat. Just what should the back pressure be? This isn't his first diesel and he does take good care of his equipment.

[markrinker]

Don't know...anyone else help with a good (legal) test of a plugged cat?

[DmaxMaverick]

Every time my wife's cat got plugged up, she used this stuff called "Cat-a-lax". Worked every time in about an hour.

[Showgood1]

I thought that if you had a clogged cat you could just stomp on it and blow the ......soot out of it. ;)

[markrinker]

Not sure on diesels, but I have had gasser cats come apart internally, partially blocking exhaust flow. The truck ran hotter and slower until it was corrected. The inside 'honeycomb' was broken, rotated, and fused together from the heat.

[madmatt]

there is no backpressure spec for the duramax. at least not given from the factory.

[markrinker]

Have you eliminated the other possible physical causes - i.e. brakes, bearings, fuel, air filter?

[Showgood1]

I talked to him last night he had the edge put in and was very happy said that he got about 1.5 mpg better but I didn't get any more details. I know that he doesn't change his own oil. You know how those oil change places are about air filters.