I don't know much about diesel engines, so please bear with me. I'm one of those 11 mpg owners trying to get the mileage better on my 04.5.

While on vacation in the Sierras last week at about 7,000 ft elevation, my truck got around 14.5 mpg towing my boat to/from cabin and lake, up and down steep grades, etc. Unloaded under flat suburban driving conditions at sea level (home), I get between 10.5 and 11.5 mpg at best. My question is, does the CTD utilize air/fuel mixtures like a gas engine and could my engine be running too rich at sea level, hence the bad mpg at home? In a gas engine, the ECU on a fuel injected engine would automatically adjust the air/fuel mixture to some extent, but not sure if this is applicable to CTD's. Changing air/fuel mixture to improve mpg would be an easy solution to this mileage problem. Thanks for your input.

 

answer no as you probably know diesel engines don't utilize fuel air mixtures outside the comfy confines of the cylinders. If you were running to rich you would see black exhaust smoke(unburned fuel) and this would only get worse in the thin air because of slightly less air in the cylinder. the only things I could think that would help you mileage in the mountains are thinner air ie less friction to over come and that you drive well and let the mountains do the work through gravity and your not in a big hurry to get up the other side.

 

Thanks Spudman. I figured it wouldn't be such an easy fix.

 

I noticed I was also getting better mileage in the hills than I do in the praries. The way I look at it when in the mountains not only do you go up hill but you also spend time coasting down them. That's my theory.

 

This is waaay out there and probably not likely, but I'll throw it out there anyways. We all know these trucks should be capable of 17-20mpg, however some like yours are getting 11-12mpg on the highway. *If* this is due to something such as a sticking injector, or some other part of the fuel system, perhaps the problem only shows up at moderate load, such as when cruising flat ground? It could possibly be that with the boat hooked up and going up hill, you're forcing more fuel through the system, but in a more effecient manner, and that coupled with the going downhill has improved your average. The theory someone mentioned about the downhills giving you better MPG wouldn't pan out on a "normal" truck, because the amount of work to go UP the hill you're going down would always be more than the difference, unless you have some quirk in your fuel system that negates that. More often than not, I see better MPG in the mountains, mostly because the turns require me to slow down, and cruising at 60mph is always going to give you better MPG than cruising at 75.

 

I'll go with Spudman

Noticed it years ago riding my motorcycle in places like Colorado, Northern New Mexico and West TX,
I usually got about 10% better fuel mileage than around home which is about 400'.

Air resistance is the big power grabber above a certain speed , say maybe 40mph?

 

I always get better mileage on my mountain trips too. Better than on my regular open highway, high speed trips. I just chalk it up to lower average speed. My truck always gets much better mileage when I slow down and in the mountains I am usually just cruising and enjoying the scenery. Wind resistance seems like the second biggest mileage killer (after hot rodding around town). And 70 seems like the turning point. 75 is a lot worse than 65, and 80 is noticably worse yet.

It can't be a lean/rich thing because diesel engines are almost always running lean. Meaning there is almost always an abundance of air for the amount of fuel being burned. The only time they could really be considered rich is when they are at full power and pouring out black smoke, not just the normal brown haze. They don't know how to be rich when they're just cruising around unless they have a severely clogged air intake and no mass air sensing system.

Wetspirit

 

Wind resistance is a huge factor at speeds over 50mph. Below 50mph, tire rolling resistance is a bigger factor. Wind drag goes up with the square of speed, so doubling your speed nets FOUR times more resistance.

Mountains DO NOT cause better mileage. There's on free lunch. The increase in fuel usage when climbing hills is NOT offset by the decrease in fuel usage when going downhill.

Think about it. You spend X amount of energy to climb a hill. The only way to "reclaim" that energy energy on the downhill is to NEVER TOUCH THE BRAKES. We all know that isn't going to happen when descending steep hills.

So what's at work here to overcome that natural situation of lower mpg in the hills?

First, the pilot injection may be calibrated to turn off that third injection event (under load), which wastes fuel when it's on. This partly explains why you get better mileage under moderate loads.

There's also a pressure sensor in the airbox of the 3rd gen trucks (IIRC), and this is one of the inputs the ECM uses to determine fueling. Lower pressure means less air; less air means the ECM injects less fuel to keep emissions low.

That's my story, and I'm stickin' to it

jlh

 

yes the expenditure of energy will be greater than the energy created rolling down the hill however I was trying to say if your not pushing the pedal through the floor on the way up the hill and allow gravity to propel you down the hill the efficency may be greater the the energy expended to propel the truck across a flat plane leading to the better mileage that was observed it the mountains than on the flats I'm not trying to change any laws of physics thats my story and i'm stickin to it