LLY Duramax / catback-low mileage

frmotorsports · July 6, 2005

I have a 2005 HD and recently put on a new cat back banks monster exhaust. Before I did this I avg 18 mpg and was fine with that. After the exhaust my mileage went down almost instantly. Now I get like 13 mpg avg. I call banks and they say they don't think it was the exhaust. I call chevy and they say that there is 3 recalls for a reflash of the computer for low mileage and black soot and smoke. Three trips to the dealer and no change. I told them about the exhaust and they just shrugged it off. Then when I get upset and start raiseing hell they tell me that they can't do anything else the truck is normal. Normal opperation does not leave soot on the fender and bumper of your truck. Normal opperation does'nt have 3 recalls to fix it. If I drove like an idoit then mileage would suffer but not the way I drive. Has anyone had this problem or heard anything about exhaust causeing this on the lly?

dannyukonxl · July 9, 2005

is the banks kit a turbo back kit?.. in that case im thinking that the turbo downpipe has a great effect on the driveability of hte truck and the boost range that the turbo runs on. I believe a larger down tube means slower velocity gases which equates into turbo lag but less heat. My 2 cents is that your runing the truck in the turbo lag which is causing drag in the exhaust and dropping your gas mileage, kind of hte the opposite of what the exhast system is supposed to do, id check your egt's theyre probably nice and cool creating a rich condition which will explain the soot on the fender, to fix this, take off the down pipe and put hte stock one back in or get a tune or intake to free up the turbo and make it use the larger down pipe..

frmotorsports · July 13, 2005

is the banks kit a turbo back kit?.. in that case im thinking that the turbo downpipe has a great effect on the driveability of hte truck and the boost range that the turbo runs on. I believe a larger down tube means slower velocity gases which equates into turbo lag but less heat. My 2 cents is that your runing the truck in the turbo lag which is causing drag in the exhaust and dropping your gas mileage, kind of hte the opposite of what the exhast system is supposed to do, id check your egt's theyre probably nice and cool creating a rich condition which will explain the soot on the fender, to fix this, take off the down pipe and put hte stock one back in or get a tune or intake to free up the turbo and make it use the larger down pipe..

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No its just to cat back system. I am going to try to help it by turning it up a bit with a diablo. If this works I will let you know. Thanks for the input.