really poor fuel economy please help!

Ruckie, i have no Diesel experience, but try to find out what is hissing --I have no idea if the truck operates anything with vacuum --but if it does that could be your problem. Maybe someone with Diesel experience will chime in soon.
FYI: if it is the problem of your poor fuel economy i personally would be pissed that the mechanic blew off being informed of the hissing.

 

you need to have charge/air/cooler CK. The 4 hoses going to it might have a small hole making that hiss.

 

I'd agree with Baha, Check the charge air cooler. That being said, this time of year, fuel economy is all over the place. Cold air is denser (negatively effects aero). Cold trans and dif fluids are thicker and harder to turn. Snow and ice hugly effects MPG. Is not uncommon to loose 1-2 mpg in the worse oft he winter. What was your mpg a month ago? If a trans goes, In my experience, they get difficult to shift and wont stay in gears. I havnt heard of one providing more resistance although im sure its possible. Are the fluid levels in trans and difs correct? What kind of oil pressure are you getting under cruise and what are your temps.

 

Ruckie I'm not a mechanic either but my experience is the biggest factor in fuel mileage is your right foot. That being said you should not have a hiss going done the road is this a new sound or something thats been there all along. There is no real test for the trans other than if your having trouble shifting(you did not mention that). Also winter fuel can greatly affect fuel mileage I know from personal experience that I lose almost 1mpg during the winter and that does not include what I burn idling or running my apu.

 

Baha has got it. Also check amd make sure you don't have a boot alittle loose on your air to air. I would loose power and hear a hiss pulled the hood and one of my boots was loose.

 

You can check for boost leaks your self . Set the engine to high idle , or set a stick on it ,etc. then spray all the intake system down with soapy water. Air filter all way around to the intake where it bolts to the head. Any bubbles start you have a boost leak. That is most likely your hiss and big hit to your fuel mileage . May not be all of it but a fair chunk. Any leak you find is too much! Best of luck .

 

loss of power along with m.p.g. could be turbo going out, but with the air noise, a hole in the air to air. or a air to air hose. those hoses are like all hoses, they can & do vibrate & work loose over time. usually not a lot but they do. if you get behind dump trucks you can get a rock thrown at your rad, or air cooler & not even know it. most all truck shops have a air to air sys tester.

 

I think I'd worry about that.

The only thing I'm mechanically competent to do is throw tools under a truck, but could it be your air compressor is always under load? I'd check out that loud, continuous hissing, first, myself.