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I have a 04 F350 that I just had the FCIM replaced and now I lost 3 miles per gallon and when I let of the gas it slows down like I am pushing the break. It feels like I am hauling a load behind it. Any ideas?
 

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Ok I have a question... If your a highway speed and you let your foot completely of the throttle does the motor go silent (injectors shut down)?
 

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It's not the ficm, how does it shift?
 

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Checked your calipers?
 

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How does your turbo sound?
 

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Does it FEEL or Sound like a jake break
 

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Where did you get the ficm?
 

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I think the vanes in the turbo are shutting down
 

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There is an actuator on the passenger side rear of the motor connected to the firewall it has a loomed vacuum line that runs down to the intake. It is what tells the vanes to open and shut. You may have a leak in that line it's common. I think you vanes are closing when you let off the brake acting like a jake brake it's called turbo braking some tow tune have this programmed in
 

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As far as mpg goes this really won't affect that. It is the tune on the new ficm. The late tune make the truck start better in cold weather but kinda kill mpg you can order a ficm tuner that can load the old programming to your new ficm
 

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Here is a picture of the vacuum line
 

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