I've always had what I think is poor fuel mileage while pulling my 8000-lb travel trailer (8.5 - 9.5 MPG). It doesn't matter if the OATs are 60* or 110*, and it doesn't seem to matter if there is a lot of fan action or very little. Speed also doesn't affect my mileage; I will travel anywhere between 55 and 65 MPH, and see no change. Empty, I can get 19 - 20 MPG on the freeway, and 12.5 MPG in town (lots of stop-and-go; I got 14.5 with stock tires, but dropped down when I went to 265/70R17 Bridgestones).

While chasing down something else on The Diesel Garage, I ran across a similar question posted there. Someone suggested that there might be a problem with the MAP (Manifold Air Pressure) sensor. One post suggested that they get damaged under high heat conditions. This brought up a bunch of questions:

1) Obviously, this measures the air pressure in the manifold (intake? exhaust?). How does the system use this information?

2) Where is it physically located, and what does it look like?

3) How could I tell if it is damaged? Would it throw codes? Can I self-diagnose it, or do I need to take it in?

4) Is it a pricey item to replace? I'm just beyond the standard three-year warranty. In general, would a typical dealer cover it under some other warranty (required for emissions control, etc), or am I at the mercy of my local dealer?

Thanks for any info/pointers,

Joseph

Well, I think I found an answer for #4. I found a MAP sensor online for about $175. I'm sure the dealer will charge me more for it. But I also found on the forum that the engine should be covered for 5 years or 100K miles, with a $100 deductible.

I also put a code reader on, and verified that there are no codes being thrown (MAP sensor or otherwise).

Still looking for answers for #1 - #3, though, or any other reason why my towing mileage is so low.

Thanks,

Joseph

#2.....It's located right after the stock air box filter indicater. Mounts on induction tube leading to your turbo mouthpiece. Round, flat with a wire harness attached.

I think that's the MAF, not the MAP. Interesting enough, I couldn't find any posts on the forum talking about the MAP sensor. Hopefully, that means that it's fairly bomb-proof, and I don't have anything wrong with it. Which means I have something else wrong, if I'm only getting 9 MPG pulling 8000 lbs at 55 - 60 MPH.

Thanks for the input, though!

Joseph