sudden drop in fuel miles question
 

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 ?

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.

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

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.

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.

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.)

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.

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.

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.