Poor fuel mileage

lciambotti

Well-Known Member
I have an 07 9400i. Isx cummins 475 turned up to 565 hp. I've noticed a drop in mileage (now down to 4.8 mpg). I am running local with 2-3 stops daily with some stop and go traffic. Any thoughts on what to check?
 

mndriver

curmudgeon extraordinare
Supporter
Fuel and air filter, is your CAC leaking. Did this just start since October? It just might be the winter fuel blend. Is it your first winter with this truck?
 

Tazz

Infidel
How big of a drop? Like mndriver mndriver said it could be the fuel if you live up north. But yeah check the basics first air boots, CAC, tire pressures, etc etc etc
 

lciambotti

Well-Known Member
Air filter has 58k miles on it. I just ordered another one. Fuel filters get changed every 13k miles with regular service. The cummins dealer said they heard air leaking from my CAC when the truck was on their dyno but that was back in June. I bought the truck the end of February of this year. I started noticing a drop in power a bit. Actually I didn't notice much difference after it was turned up.
 

8978

** Commie Express **
Supporter
My fuel mileage went down and down over a period of years. Couldn't find anything until my trailer brakes were not releasing all the way to a point where they were locking up. Turns out it was my relay valve on the trailer. My mileage at one point was 3.8. It jumped back up to 6 and above after having my trailer fixed.
 

lciambotti

Well-Known Member
I live in PA. Tire pressures were checked two weeks ago. I guess I need to block the turbo lines and blow air to the CAC and see if I can hear it leaking.
 

lciambotti

Well-Known Member
I am currently using company trailers. A different one every day. I'll dig into it. And my mileage dropped from 6ish to the 4.8
 

8978

** Commie Express **
Supporter
This was an interesting note on boost pressure. I had a cracked CAC and was still making 28psi. When I had a new CAC put in there was no difference in pressure. My mechanic said you can have lots of leaks and still create your max boost pressure. Your boost pressure is throttled to a certain limit.
 

8978

** Commie Express **
Supporter
I am currently using company trailers. A different one every day. I'll dig into it. And my mileage dropped from 6ish to the 4.8
Could be your steer or drive brakes dragging also. Your boost pressure is fine too!
 

Tazz

Infidel
Air filter has 58k miles on it. I just ordered another one. Fuel filters get changed every 13k miles with regular service. The cummins dealer said they heard air leaking from my CAC when the truck was on their dyno but that was back in June. I bought the truck the end of February of this year. I started noticing a drop in power a bit. Actually I didn't notice much difference after it was turned up.
If you have an audible leak this will be your culprit. You can rig a pressure test for yourself to check. But if you hear a leak I would change that out first.
 
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