Sudden drop in MPG 2004 Cummins

[TKarnas]

I usually get about 18-19 MPG with my truck. Last weekend I needed fuel and did the hand calculation and it came out at 14. 7GPM. I drive the same roads and had the TST Box on the same setting, No load No high speeds.



Fuel pump was replaced with a FASS pump about 1 year ago. transmission recently had the gov solenoid replaced but no other mods.





Can this just be some bad fuel or is there something going wrong with the truck?

 

[GaryOwen]

Check your oil level to make sure it hasn't risen. A cracked injector body will dump diesel into your oil. Oil will also smell like diesel.

 

[dan.poitras]

I don't know where you live. But up here in Canada when they start distributing winter fuel, power and fuel economy drops

 

[firemansdiesel]

I usually get about 18-19 MPG with my truck. Last weekend I needed fuel and did the hand calculation and it came out at 14. 7GPM. I drive the same roads and had the TST Box on the same setting, No load No high speeds.



Fuel pump was replaced with a FASS pump about 1 year ago. transmission recently had the gov solenoid replaced but no other mods.





Can this just be some bad fuel or is there something going wrong with the truck?

I have the same exact problem as your truck w/ my '05. On average mixed driving I get 16 mpg and highway 18/19 mpg. Since August my mix driving has been in the 14 range and highway 15/16 mpg.



I have an Air Dog and filters are good, air filter is good, used some additives (no help), no oil rising, injector cut off test shows all six operate good, cleaned the IAT/MAP (no help).



Still have good power (stock) and no smoke other than sometimes a puff of black smoke on cold start up (usually when warm out). So I'm baffled w/ the drop in mileage.



I did use a Smarty and Smarty Jr in the past around the time I noticed the drop when I put it back to stock programming. I don't know if the stock flash did something w/ the new updates it provides when you flash it back to stock. I also changed all fluids out within the last three months on everything.

 

[JHardwick]

did the hand calculation and it came out at 14. 7GPM.

Is that Gallons Per Mile ... ... ... ... :-laf



Probably winter fuel, would be my guess.

 

[EArp]

I also notice on my truck that driving into a bad headwind makes a huge difference on my mileage.

 

[TKarnas]

Sorry all... I work in a industry that has Gallons per minute and did not realize the error. I meant Miles Per Gallon... .



Update... I live in South Central PA. I put some Diesel power w cetane in and the grey bottle that Walmart carries... . I dont remember what that was called but its a common additive... . Waiting results for next fill up/

 

[JDiepstra]

Winter fuel, cold starts, longer warm up time, denser air... takes a big toll on mileage. Or gallonage, as some people prefer



Also, take it from Obama and make sure your tires are aired up. My tires went down 10 psi as it got colder. Just aired up today!

 

[TKarnas]

Update... I added some Cetane boost and the numbers went back to around 16 mpg. I will say the fuel changed.

 

[carson6]

I just used cetane boost from wallmart and am waiting to see what happens...

 

[steved]

I changed from my summer tires to my studs and mileage dropped... rolling resistance between tires is vastly different.

My guess is winter fuel... never had a problem before this year... and I actually saw a drop this year. I'm down to around 18mpg on the highway between fuel and tires.