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Fuel economy suffering, don't know why.

I have an 06 ISX 15 450HP. Over the last two months Ive noticed my mpg go down when I thought it would go up. First summer with this truck and new o/o. Just had an overhead last wk and one injector was replaced. Tests for egr, injectors good, no loss in power, fuel rail pressure good relaced both fuel filters. Im losing about .5mpg to .75mpg. I do have an apu and use approx 4-6hr a day (didnt think the apu would use enough fuel to have such an mpg drop). The only thing I can notice different is it feels like I need more pedal pressure to get up to speed even with light loads. None of my brakes are sticking. Im lost and two mechanics Ive talked to are lost. Any ideas?

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Did they check the charge air cooler (CAC) by chance to make sure it's not leaking. That's about what you would lose in fuel mileage with a CAC leaking. With a cummins their going to say anything up to 7 LBS in 15 seconds lost is acceptably. I run a Duralite and it has no lost even at 4 hours.

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I got my CAC tested. They put 35psi in it, went down to 10psi within two minutes. Guessing its a bad leak? The mechanic was quite green so he didn't know how bad a leak it was. Ordered a dura-lite, should get it in a wk or so. Hopefully will gain my mpg back, seems truck is running worse and worse.

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Try closing off the EGR. Wait till the engine is cold turn the key on and allow the computer to cycle through. It should close the EGR. Unplug the wires on the cooler actuator and then turn off the key. There may be a better explanation in the ISX Group of how to do this but run it for a week or so with it in the closed position and check your fuel mileage again.

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test CAC

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best thing would be to put it on a dyno and see what power it's making and where the problems may be. your fuel pump could be getting weak, the turbo fins could be warped, could be restriction inside the fuel lines (from deterioration), bad ambient air pressure, intake air temp, boost sensor or another one of many potential culprits.

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