sudden drop in fuel miles question

[mbozz282]

My truck usaully gets 5.2to 5.6 mpg.I went into colorado and the truck run great at 10,000 ft,but since i left colorado my mpg dropped to 2.75 and has slowly gone up to 4.5 over 3000 miles.i bought the truck to cat after changing all fuel and air filters.They ran it on the computer and found nothing wrong and dont know where to go with this short of taking the motor apart.Another mech. thinks that maybe the computer adjusted for the higher elevation and never recalabrated itself .The truck runs great the same way it always has except for the fuel miles.This is driving me nuts and costing money.Anyone have any ideas i dont know which way to go with this ?

[Orangetxguy]

Cat checked the ECM software? They should have done a quick EC dyno.

Another thing would be check the charge air for small air leak.

[solo379]

Check your "air to air" for leaks...
BTW You got a terrible fuel mileage to begin with...

[mbozz282]

Cat checked the ECM software? They should have done a quick EC dyno.

Another thing would be check the charge air for small air leak.

They checked the software but did not run it on the dyno.I will have to check for the charge air for air leaks.The truck is a w900 with a 550 i dont think it will ever get good fuel milage.Thanks for your help.

[solo379]

The truck is a w900 with a 550 i dont think it will ever get good fuel milage.

What kind of tranny, rears, and tire size? What you are pulling? Where? How fast? Idling %?

[mbozz282]

What kind of tranny, rears, and tire size? What you are pulling? Where? How fast? Idling %?

It has 18 speed with 335 rears and 22.5 tires .I pull refer. loads from 55000 to 80000 mostly east of miss. river .My idle time was around 31% and i drive alittle fast i know between 68 and 72 mph around 1400 +1475 rpms.I know my fuel miles arent great but to go down to2.9 and slowly rise to 4.5 really bites.I checked the air to air this morning and no leaks but notice autofan is coming on when engine is still cold.We had changed 2 sensers for fan a couple of months ago.

[Orangetxguy]

They checked the software but did not run it on the dyno.I will have to check for the charge air for air leaks.The truck is a w900 with a 550 i dont think it will ever get good fuel milage.Thanks for your help.

It's called a "Cut-out" test, and the mechanic, if he or she is properly trained up on the spec's' should quickly determine if there is an issue with the fuel system, from pump to piston. (I couldn't remember the name of the test yesterday...but they use a laptop computer with the proper software, at the shop where I have my work done.)

[solo379]

It has 18 speed with 335 rears and 22.5 tires .I pull refer. loads from 55000 to 80000 mostly east of miss. river .My idle time was around 31% and i drive alittle fast i know between 68 and 72 mph around 1400 +1475 rpms.

You got an almost perfect specs, for the job you do. Try not to go over 1400, preferably 1350, cut down on idling a little, and you might be very surprise. I mean after you'll figure out, what's wrong with it.

[YerDaddy]

Are you getting your mpg figures from the computer or from topping off your tanks and dividing gallons into miles since last top off? Computer mileage figures are notoriously inaccurate, last I heard.

[Papa Rick]

Not saying this is your problem, but I have a W900L Daycab, Cat C-15, my mileage started dropping off the charts and done everything I knew to do. The computer still showed getting 5.5 mpg but according to everything else it was showing 4.0 to 4.3 mpg. Changed all filters, had air to air checked, did overhead, ran dyno and checked for turbo boost, fuel pressure, drained my tanks thought might be getting water in them real bad, done more stuff to it and spent over 2,000.00 and still nothing.

Well one day it showed up when I took a trip and I knew how far the mileage was and found out the odemeter was stuck. It did not affect the speedomter part but the mileage it did. Took it out and blowed it out with air, (was gonna get a new one, but whew $500.00 for that, I figured I didn't need it that bad) but anyway after I blew it out with air real good it has worked great.

Not saying it is your problem, but after all I spent and then found it to be the problem was a real funny thing except for my billfold was alot lighter.

I would get the cat people to run the computer on it and see what the mpg is showing and any codes it may have (you can check for codes as well) On mine I turn the cruise off and hold down the resume button and then it will start lighting up, 5 blinks and pause 5 blinks more means nothing is wrong.

The computer might be able to show when the fuel mileage started to drop, and unless it has been cleared it will show what the fuel mileage was from way back.

[mbozz282]

Not saying this is your problem, but I have a W900L Daycab, Cat C-15, my mileage started dropping off the charts and done everything I knew to do. The computer still showed getting 5.5 mpg but according to everything else it was showing 4.0 to 4.3 mpg. Changed all filters, had air to air checked, did overhead, ran dyno and checked for turbo boost, fuel pressure, drained my tanks thought might be getting water in them real bad, done more stuff to it and spent over 2,000.00 and still nothing.

Well one day it showed up when I took a trip and I knew how far the mileage was and found out the odemeter was stuck. It did not affect the speedomter part but the mileage it did. Took it out and blowed it out with air, (was gonna get a new one, but whew $500.00 for that, I figured I didn't need it that bad) but anyway after I blew it out with air real good it has worked great.

Not saying it is your problem, but after all I spent and then found it to be the problem was a real funny thing except for my billfold was alot lighter.

I would get the cat people to run the computer on it and see what the mpg is showing and any codes it may have (you can check for codes as well) On mine I turn the cruise off and hold down the resume button and then it will start lighting up, 5 blinks and pause 5 blinks more means nothing is wrong.

The computer might be able to show when the fuel mileage started to drop, and unless it has been cleared it will show what the fuel mileage was from way back.

Thanks Papa Rick i just got it back from cat on mon.The transfer pump was going bad no fuel pressure .They did an overhead even though i had one done about 90,000 miles ago.I was told all the valves were to tight on the motor. Only put about 1000 miles on it since getting it back and i got around 5.5 mpg 1st tank and its running stronger so will see what happens.

[tracer]

Thanks Papa Rick i just got it back from cat on mon.The transfer pump was going bad no fuel pressure .They did an overhead even though i had one done about 90,000 miles ago.I was told all the valves were to tight on the motor. Only put about 1000 miles on it since getting it back and i got around 5.5 mpg 1st tank and its running stronger so will see what happens.

It's funny but I seem to go through the same weird fuel mileage drop! Cat C15 Acert, dyno-tuned by CAT for 550 HP at 1,425 RPM. Pulled like crazy with 3.73 ratio and low pro tires and then in December I:

- installed an extrude honed manifold for 130 CFM extra air flow (must provide 0.2 MPG gain)
- changed from 3.73 to 3.42 axle rears
- installed a rolling tarp on my dropdeck trailer.

My Wilson is pretty low and now with the tarp I didn't think it should give me worse fuel mileage than a dry van. I got 6 MPG consistently at 59 MPH 1370 with 3.73 ratio pulling a dry van with 45,000 lbs. Now, I pulls a similar looking rolling tarp dropdeck and at 62 MPH 1330 RPM with 3.42 (same load) I did ... 5.5!

This was going from Ontario Canada to Missouri. On the way back, I had 4,200 lb load (!) and driving the same 62 MPH I got 5.9 MPG. Something is amiss.... The truck does feel sluggish but I'm governed at 65 so I can't even go past 1380 RPM. Also, I checked it while bobtailing this morning and at 1300 RPM the speed changes very little if I push on the throttle. In other gears and other RPMs it's okay (quick response) but for some reason in top gear (13 speed), the fuel pedal feels like it's not connected to the engine

I should probably go to CAT and have them put a body with a laptop in and ride around. I suspect a fuel pump or something like that. The truck is a 2004, C15 ACERT, with 440,000 miles, 3.42 axles, 295 low pro tires, 13 speed tranny, and as I mentioned it does 65 MPH at 1,385 RPM.

Any comments would be welcome.

[tracer]

It's funny but...

My problem with poor fuel mileage was completely solved after I re-installed my full-roof deflector on top of the truck's sleeper. Turns out the Nose Cone was a waste of money Who knew ...

[sammy]

I have a 2005 Volvo 780 with the 535 ISX Cummins engine. The truck does pull good but my fuel economy is bad, its between 4.8 to 5.2 mpg. Anybody have any idea that will help me out? Thank you.

[terrylamar]

I have a 2005 Volvo 780 with the 535 ISX Cummins engine. The truck does pull good but my fuel economy is bad, its between 4.8 to 5.2 mpg. Anybody have any idea that will help me out? Thank you.

Slow down.

[heavyhaulerss]

disconnect the egr valve

[Aufgeblassen]

My truck usaully gets 5.2to 5.6 mpg.

Holy cow! Your "normal" MPG is low. My '01 Volvo 660 w/ 465 HP Volvo engine gets between 6.1 MPG (@ 80,000 lbs.) and 9.0 MPG (MT trailer) to 10.3+ MPG bobtailing.