Ok so when I bought my truck I was averaging 17.5 MPG. We took the truck to see my parents in South Dakota. I got 18 MPG going up, 17 MPG while we were there and 19 MPG coming back. When we got back I hauled a load that weighed about 700lbs. After hauling that load my fuel milage has dropped to 15.4 MPG.

I removed the cat and changed the fuel filter and my mileage went up to 16 MPG. I haven't changed the way I've been driving so why the drop in mileage?

The terrain here is hilly and I do some city driving but most of my driving is on state road running 55 to 65 MPH and a little highway driving going 65 to 70 MPH.

 

Get a smarty! and some guages!

 

Air filter, change the oil, maybe check tire pressure. I don't know maybe there's a catastrophic failure going on inside your Cummins but I'd start with the easy stuff.

 

Bad/poor quality fuel...I see it all the time.

 

Tire pressure is good. I'm changing the oil and air filter on Thursday. Don't know about the fuel quality I buy from the same place and they sell quite a bit of diesel. Is there a way to check the quality?

I filled up the tank last night and checked the milage it was 16.5 MPG so I will check it again after I change the oil and filter.

 

Changed the fuel filter? You may have a turbo on it's way out but I wouldn't get to crazy thinking of that just yet. Check your aftercooler piping as well.

 

You could try buying fuel from a different station...

 

If you had a gauges (specifically rail pressure and boost), that would go a long way to diagnosing an issue.

If its not a simple fix like fuel, air filter, or fuel filter, and it just happened...I would check the rail cascade valve to make sure it hasn't started "leaking". Low rail pressure can cause this and not trip codes.

 

Ok well I changed the oil and air filter. Tuthfully I think I got a bad tank of fuel because my milage has been slowly increasing since I filled up the tank last week. I'm back up to about 17 to 17.5 MPG which is where it was when I bought the truck (actually it's a little higher).

I haven't gotten the chance to see what it is on the highway yet though. I think I'm going to have to find a new place to get fuel though.

I thank all of you guys for your help I'm just glad that it wasn't a big issue. Oh and dangerous as soon as I have the money I'm going to get a Smarty and gauges.

Thanks again guys.

 

Glad to hear it! You'll pick up about 2-3 mpg and alot of power!