Why is my fuel milage so bad?
 

I am only getting around 14-15 in my truck. I am going to change the engine oil and I want to change the differential and transmission oil but I need a new job first. Anyways, is there maybe some other reason that Im getting bad milage? I put a new CP3 which was what my check engine light was on for and my diesel mechanic said it should fix my problem if I got a new one but even after I replaced it the check engine light is still on. I took it to autozone and they ran the scan tool. It said the truck passed but the light is still on. Do I need to run a different kind of scan tool on it? What could be my problem? Thanks guys.

What was wrong with the CP3 that it took a crap? Whens the last time you changed the fuel filter or clean the air filter? Are you runnin any type of programer/chip? Auto or man? Are you hand calculating the milage or lookin at the overhead monitor? Also is the speedo recalibrated for the larger tires? Lotsa possiblities... :pca1:

Oh boy! The CP3 failed because the lift pump failed and the guy that owned it before never fixed it. I changed the fuel filter less than a month ago. Cleaned the air filter when I changed the fuel filter. No chip. Manual. I am hand calcualting and the overhead says im getting 3 miles to the gallon better than what im calculating. I don't think the spedo has been recalibrated. I also dont know what kind of gears I have. I would think they are 3.73s but maybe they are something else.

It should have 3:73 which is what mine came with and I know 35's threw off my speedo by 5 mph verified by gps. That will def. screw with you mpg readings! :pca1:

What should I do then? Are the 3.73 gears ok with 35s or do I need to put 4.10s or something?

The gears are fine, you just need to get your speedo recalibrated. Dealer might be able to help, mine wouldn't. I just reprogramed mine with my Smarty. Because with the speedo being 5mph off the trucks mileage is showing less than you actually traveled hence bad fuel mileage calculations.

Thats where im having a lot of problems. I don't have access to a smarty anywhere and the dealerships around here are only out to take my money. I will see if I can get a hold of one somewhere along the line. Thanks for all the help!:U:

I wish I were closer.. I have a Smarty!
I feel your pain... I have the exact same problems with mine! If one of us ever figures out what it is we will have to make sure to tell the other!
Look in your glove box for a label that has your gear ratio on it. Or you can go here and see where your rpm is on the chart for the equipment your truck has. You can even change tire size to a different value to re-figure the rpm!

klenger.net

To be honest the only thing I haven't checked on mine is the intake system for leaks (Under boost). I started buying the 'tools' to check it and never finished. Guess I better get that done.
Let me know if you find anything or if you have more questions.
HTH,

(Check around, you may find a dealer that will do adjust the speedo for you! There are some good ones still out there!)

some of our more "advanced" genral garages in our area are able to refigure for larger tires. i put a little bigger tires then stock on my new chevy, and the garage that did it, changed it right away, with out even askin me. and he was some backyard mechanic, with a really big barn for a shop.

Try cleaning the AIT (air intake temp) and MAP (manifold absolute pressure) sensors, located on the driver's side of the engine in the intake manifold above the fuel filter. These can get carboned-up and throw off the readings to the ECM, causing it to dump way more fuel than needed.

Somebody tried to recalculate for 35's on my 2nd Gen but he said if he did that then the Smarty wouldn't work on his truck anymore? Anybody know if this is true or were we just doin something wrong?

That is true since your changing the stock programing in the PCM then it's vin locked since it has to save your factory program. All you can do without vin locking it is checking trouble codes...

I forgot that was required with a smarty. Oh well. I found a fella down here that does independent diesel mechanic work and from what everyone says, he's damn good at it too. I was told he could do everything I need so I am going to go ahead and give him a call. Also, how do you clean the sensors? With a wire brush or something?

No. Pull them out with the applicable socket, clean them with a dry lint free cloth or some sensor cleaner - never use any harsh chemicals.

You can thank me later.

Cheers,

Lol well I wasn't contemplating dipping it in vinegar but I do appreciate the tip! Thanks Wyatt!

No worries.:c:

So I drove about 50 miles today and I noticed that if I slowly came up to 2000rpm and then shifted and kept it close to 2000rpm I got better gas milage than when I only sat it at around 1300rpm just cruising. I know they need to operate at a higher rpm but could my rpm and speedo gauges both be off?

mine runs the absolute best around 2,000 rpm. i ran a 400 mile round trip holdin it at just around 2,000 the best i could, and i got around 20- 22 mpg, most all highway. thats with 35x12.5r17, AND an alignment that was WAY outa wack. without my juice.

Hmmm maybe my rpms are off pretty bad then. I just had it aligned. I will call today and get they speedo and rpm fixed.

Well I'm super e xcited to say I'm getting better gas milage now. I changed the oil and it made a huge difference. I filled up my truck and got 200 miles out of a half tank, that's 50 miles every 1/8th of a tank! Well needless to say I drove it after the oil change, with 200 miles on the odometer and 80 miles later I only used an 1/8th of a tank! That even included racing with a guy that quite literally whipped my ass. He had to have had a chip but he didn't have any visible gauges so I don't know what he was running but he was quick. Anyways just wanted to thank everyone for their help and advice!

i never change out the speedo gears. they're not even accurate from the factory, and it's off a few percent with just low tire pressure.

like an old mechanics instructor used to say, "it's an INDICATOR, not a gauge". i.e.: don't trust 'em.:ohno:

265/70/r17 (factory).....
...265mm width x 70% sidewall height x 2 sidewalls(above and below the rim)= 371mm
.......371mm / 25.4mm per inch = 14.6
.............14.6 + 17" rim = 31.6" factory tire (could be anywhere from 30" to 32 1/4")

now ya gotta calculate the tires that're on there now, and come up with a percentage of increase........35" / 31.6" = 1.1 (10% increase):yeah:

so, 60mph on the speedo is actually 66mph. ...and 15mpg is actually 16.5. (about what i'm gettin:booo:)

my "35 inch" bfg 315s actually calculate out to an increase of 8%. i just tell the wife to "DO THE SPEED LIMIT" according to the speedo, and ya should be alright.

an easy way to verify all this with little math is to go 60mph on a straight stretch for 5 miles. it "SHOULD" take exactly 5 minutes. if not, do the math. 4 minutes, 30 seconds = 300 seconds / 270 seconds = 1.11 (11%)

cheese, some of you nancy boys need to tuck your skirts in:girlsmile: and think a little.:humm: y'all "have a shop do it" waaaaay too much.:moon:

kevin

I hate math....

I think I see the root of your problem. If you are getting GAS mileage in your diesel-powered truck, you really screwed up back there at the filling station!:w2: Try a little diesel next time and I bet your FUEL mileage gets better!:U: lol

Seriously, I just cleaned the before-mentioned sensors (it's been a while) because my fuel mileage has been slipping pretty badly lately and instantly picked up nearly 4 MPG on my daily commute (428 miles on 19 gallons = 22.52 mpg). Been struggling to get 19 mpg for the last several tank-fulls. On your 2004, the MAP sensor is located in the intake manifold, close to #4-#5 injector lines. The IAT sensor is in the intake tract between the air filter and turbo. ON the 2004.5 and above Cummins "600", the MAP and IAT sensors are both in the intake manifold.

If you have an air filter that needs oil, that oil can really play hell with the IAT sensor. I just use carb and choke cleaner and hit em with a puff of compressed air.

You use carb cleaner on a sensor? Maybe you should try ether too?:ph:

Never use anything other than a clean rag and electrical sensor cleaner on these little things. They are not cheap and are critical to the operation of the truck.:c:

Any dealer should routinely make the Starscan input to reclalibrate tire size. The one I use around here charges $17 for such service. Call and ask if you need to make an appointment to get it done. I did not even need an appointment to get the high idle enabled (similar type service) and it took a total of 15 minutes from the time I got out of the truck in the service entrance to when they brought it back to the parking lot out front. Just tell them you got bigger tires and want the speedometer adjusted. Pretty simple.

On a side note, this is one aspect of the programmers I hate: Protecting their profits. Maybe to set a full tune. Maybe. How many people do they really think will be running their tuner all over the neighborhood flashing everybody and their brother's ECM? Most if not all the few people who will do this aren't gong to spend the money to buy one anyway. It's like downloading bootleg MP3s or videos. The market impact on corporate profit is insignificant at best. But for the simple stuff like tire size? Why won't Smarty let you do that on any truck you want? That's just greedy, and why the prices are so high for a little plastic box with some software and a cable. The profit margins on those things are insane. Just one of the very few examples where the dealer wins on price by a wide margin.

And just in case any of the BD, Mads or other guys or their vendors wants to come on here and tell me all about how their coding skills are oh so intangibly valuable - don't bother. I work for one of the largest Aerospace corporations in the world, presently in a shop using our own custom software flying imaging satellites in low earth orbit. So I "get it."

Rant off.

I hear you.... I almost agree completely too. Along with the R&D costs I think a lot of these guys do take a higher profit than necessary sometimes. I have to wonder with our current 'sue' happy and everything generates a liability type mentality that if they aren't putting money away for a rainy day.... It all ads up to higher costs for us the consumer! That is the part I hate!! The great thing about our country is that if it is just greed then someone can come along and 'do it better' and then all us consumers win!

A suggestion if you are going in to have your speedo re-calibrated: Look up your tire diameter and Revolutions Per Mile from a reliable site (Like the manufactures site!) so you can give them accurate info. Just because a tire is a '35' or what ever doesn't mean it is exactly that size. Different manufactures actually make them different from one another even though they are the 'same size'. As accurate of a figure you can give them will just put your speedo that much closer to being correct!

Sorry I will refer to it as "diesel milage" from now on lol.

Well I did the 60 second mile test and my speedometer is dead on. It took me exactly one minute to travel one mile. I am getting 22+mpg with an oil change and sensor cleaning. Everything is running great. Now I need to pull the water pump to see why its leaking and fix the axle seal and everything will be in running order (finally :argh:).

Maybe I will go out and check my sensors . . . . Gently of course!

My '06 still has an IAT sensor in the intake tube just after the filter . . . . Just like the older ones do. You didn't mean that they had been combined, did you? I do believe the MAP in the intake has a temp function also though....

Ok Nancy:w2:

On my 06 the dealer couldn't recalabrate my tire size their only options was factory size tires. On the 05's and older they could change the tire size. I used a smarty.

I couldn't DISAGREE more!!!! There is no such thing as "to high a profit". If someone or everyone wants a product, they'll pay whatever for it. And said producer should charge what the market will bear. Supply and Demand...........simple, basic economics. Are you self-employed??? If not, then I understand your answer.:argh:

The problem I see with this particular market is it's too small, and the "demanding" customers for the most part have plenty of money. There's an old saying that goes something like... "a fool and his money are soon parted..." :w2:

Ok guys Im back in the same boat again. My truck sat for a while but after I changed the engine oil the milage jumped up a lot. Well then I just started driving it again and I only got 30 miles out of an 1/8 of a tank :dang:. But when I got back to town I was at a light so in 2nd gear I decided to floor it and a huge black cloud rolled out. I was shocked because it barely smokes as it is. Is that good or bad? I dont know what to do anymore. There are no codes. Im extremely pissed. Any ideas?

Bump

If you are always easy on it you will not get all the carbon out of the engine. I know if I am on the highway for a long trip I always try and remember to get on it pretty good and it will smoke like a train for a minute, but if you try and do it again right after it will hardly smoke(unless the programmers are turned up). I have always been told to "clear" the engine out on long trips. Maybe somebody with more info could explain why that is, but it seems to be completely normal!

Ok thanks because it did sit for a while then did exactly as you said. Rolled a lot of coal and then barely any as normal after. I would like to understand the carbon build up thing more and how it effects the truck. Thanks for the info! :tu: