really poor fuel economy please help!

No offense, but you're not giving enough info for anyone to really help you.

What engine, what year?

Do you have a pyrometer & boost gauge? If so, what are they reading? How does your current MPG compare to your 30 day & lifetime averages?

How old are your mufflers? When did you last change the air filter?

I agree that the CAC is a good place to start. Here's a link to a tester; http://www.amazon.com/OTC-5039-Charged-Cooler-Tester/dp/B00063XRGA

The picture should give you a good idea of what you need. I built my own with stuff from NAPA Auto Parts & Home Depot.

Other than that, without knowing what specific engine you have it's impossible to give you any constructive advice about where to start looking.

You should pull an oil sample, look for soot and/or fuel dilution. Find an honest shop with good diagnostic skills and preferably a dyno. Have them dyno it, do an ecm dump and set the overhead. Top-down diagnostic structure, start with the simple/cheap stuff first.

 

Since you didn't provide much information, I'm going to take a stab at it. Check the hose from the intake to the air compressor. It can will split on the back where you can't see or feel it, but will leak air under boost. Pull it off and check it. If that's not it, check the boots on the intake air cooler(IAC). Also, if you are running Lucas in your trans, rears, or engine, that will get really thick in cold weather and drop your mileage. Anything that thickens any fluids in cold weather will cost you fuel mileage, including the wheel hubs.

PS....Keep the air in your tires checked. Cold air can drop them quite a bit, costing you money.

 

The hiss you are hearing, appears new or you wouldn't mention it. I'm with most everybody else as far as you are loosing air on the pressure side of the turbo. When air escapes under pressure it will cause a hiss. Even if your boost gauge says you haven't lost any a good turbo can still make pressure with a leak. Check the clamps on your CAC boots they will wear loose, look for black streaks running out away from the boots its a sign of air leaks. Like stranger said check comp hose from intake.
Not telling you your business, but any mechanic in just out of tech school knows poor milage with a hiss check air to air. The computer wont really do it, he coulda been busy that day and not had time. If so he shoulda said so not don't worry about it. What else might he say ahh don't worry about that you don't know about I guess is my thinking.

 

I worked at a Freightliner dealership for a while around 1980. You wouldn't believe the amount of stupidity as far as mechanics went, even back then. I had taken a break as a driver at that time, and even as a parts guy, when the techs came asking for parts, and they described the problem, I knew they were way off, but they wouldn't listen. We had one that was a nice and fun guy, but he didn't know crap. Everything he touched was wrong, and came back, or broke down on the road. It made me sick.

Service writers are usually not mechanics, and are only concerned about how much they can make off a comission of your bill, and they are not going to rat out a tech because all other techs will hate then, so the truck owner is screwed. In reality, most service writers don't know a bad mechanic from a good one, as most parts guys are the same. All that matters to them is that they have a diploma from a school. That diploma means everything, no matter how stupid the diploma holder is, as it is in most businesses today.

 

I spent about all my life as a field mechanic, I know just what you are saying. I have only been a part time driver till lately I bought a truck and gonna be rich. lol

 

So you have the only truck in history that gets worse fuel mileage the slower you go? Wow! It defies physics, but wow.

Check the CAC. I had a Mack with power issues. It took a couple mechanics, but we finally pinned it to to air compressor intake hose. Mack has the compressor drawing air off the CAC system and the hose blew out, causing me to lose boost pressure.

 

Thanks for following up on your thread Ruckie, a lot of people don't -they just leave us hanging.
Let us know if that was the loss in fuel mileage.

 

"rubber thing that goes to turbo"sounds like the culprit to me.good luck.