I have a 2000 Dodge Cummins three-quarter ton long box has an air dog fuel system brand new RV 275 Bosch injectors new cold air intake valve lash recently set all new fuel filters cleaned map sensor  missing muffler and tailpipe from exhaust running 265 7516 No matter if I’m pulling a load or completely empty I average between 10 and 12 miles per gallon  90% highway miles  .  I have not had it into the shop 

what speed are you driving on the highway?

I will add when I change my injectors afterwords I could not get it to prime without hitting it with ether repeatedly

I drive between 65 and 70

Edited April 4, 20196 yr by JAG1

Have checked for any trouble codes? What rear gear are running? I am just over 20 mpg with a similar set up as you with 3:55 gears.

When you tried to prime it did you open any injector lines up?

Lots of things that can cause poor mileage.  Tire pressure, tire size, air filter, dragging brakes.  Do you have a EGT gauge?  If so, are your EGTs high while driving? 

you should reinstall your injectors, if it doesn't start without ether then you have issues with your fuel lines at some place.  

 

Could be the return, it could be the cross overs, or connections to the cross over.  

EGTs have to be below 600°F for good MPG numbers. Like with my current tune set the cruise for 60 MPH and it hovers right at 550°F EGT's and twisted out 20.3 MPG. Can't complain one bit. 98% all highway miles. Just in 3 days done 566 miles already.

 

Cold air hurts MPG's. Optimal IAT is 100-140°F. Below 80°F the MPG's fall off.

 

Drag shows up as excessive high engine loads, EGT's above 600°F, etc

 

 

 Yes I did bleed the lines  five injector lines were open to just get a little bit of fuel to spit out  I do not have an EGT gauge my truck ran the same before I got the cold air intake I drive like an old grandma 

My truck runs about 1700 rpm’s at 70 miles an hour

I had the fuel pressure checked in at idle it sitting At 15 psi at the filter

 I feel like there’s definitely no issue with the cold air affecting my miles per gallon I’m driving it from 6 o’clock in the morning till 5 o’clock in the evening and same mileage the entire time it might affect it for the first 30 minutes of my drive 

 Just spit bawling here but could it be an issue with  The computer assuming everything is all right and not mixing the fuel and air correctly 

37 minutes ago, Jesniccad said:

My truck runs about 1700 rpm’s at 70 miles an hour

Should be closer to 1900, 

Do you blow out black smoke, have you I have you checked for boost leaks

Umm.. Should be closer to 2200 at 70 MPH. At 82 MPH I'm a perfect 2,500 RPM.

I Cruz right at maybe 1850 1900 rpms and no black smoke

3 hours ago, Jesniccad said:

Just spit bawling here but could it be an issue with  The computer assuming everything is all right and not mixing the fuel and air correctly 

 

The computer doesn't control air/fuel mixture like it would on a gas engine. Just amount of fuel. Adds more fuel when more power is demanded.

 

Have you ever compared your speedo to a GPS speed signal to check calibration? If the speedo is off, so would the odometer, and that would skew your calculations. However, it seems yours may be off the other way... So I don't know. Just thinking out loud. 

Yes i am baffled aside from the hard purge of the fuel rail it shows no signs of issues plenty of power no funny sounds smells or leaks

I just started driving it everyday these last 3 months I’ve owned it for 12 years only put about 15 thousand miles in 12 years now I put 800 miles a week on it 

Ok. Another stab. 

 

RV275 injectors - How many miles on them? If they are over 100k miles they are done. 

 

As for the automatic transmission how many miles on that transmission? Has it been upgraded with better cutches, bands etc.? What is the fluid like (color and smell)? 

 

Do you have a live data tool like a OBDLink LX or similar? I would suggest checking the ECT and IAT sensor at start up. Both sensors should match the same temperature at key on. Then you might watch the engine load value the lower the value the better the MPG. Engine load is the commanded amount of fuel on a 0% to 100% scale so if your seeing high numbers this is how much fuel you commanding.  

Nothing funny about fluid color or smell 

My truck with the same size tires as you, 265 75 16, turns 2000 rpm at 70 mph. Several others here have quoted the same rpm on there 265's. Are sure about your tire size? Big tires eat fuel. 1700 at 70 is screaming bigger tires.

 

3 hours ago, Mopar1973Man said:

Umm.. Should be closer to 2200 at 70 MPH. At 82 MPH I'm a perfect 2,500 RPM.

Your 245's would do that but 265's wont be that high. 

Had a code reader on it a week ago there was nothing at all have not tested the iat  sensor or ect sensor is that something that wouldn’t show up on a scan

I am sure that my 265/75/16 tires at 70 mph run at 1800 to maybe 2000 rpm what should that matter 

It’s not over 2000 rpm for sure

MAP sensor giving false reading causing the ECM to command more fuel at the VP? 

4 minutes ago, IBMobile said:

MAP sensor giving false reading causing the ECM to command more fuel at the VP? 

 

I think that could affect acceleration, but at cruise state that situation you described sounds like a potential runaway situation. At cruise state it should be all good. I wish I had a better idea to contribute.

Edited April 5, 20196 yr by kzimmer
Grammar

25 minutes ago, IBMobile said:

MAP sensor giving false reading causing the ECM to command more fuel at the VP? 

 

Possibly command more fuel. Again this would be seen in the Engine Load reading. It will show the percentage of fuel commanded. The MAP sensor only limits how much fuel per boost pressure. So if the boost pressure is low then the commanded fuel limit is low. This is part of the stock ECM smoke control. 

 

1 hour ago, dripley said:

Big tires eat fuel. 1700 at 70 is screaming bigger tires.

 

Sure sounds like bigger tires to me... Even my 235's or 265's where about 1,900 for 65 MPH. The 245's now are 2,000 RPM at 65 MPH.

Had a code reader on it a week ago there was nothing at all have not tested the iat  sensor or ect sensor is that something that wouldn’t show up on a scan

So is there no tips as to what it could be because it’s definitely not my 265/75/16 tires that have proper inflation and rotation and balance and no brake drag