8.1 Chevy Workhorse

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New here and reaching out for help I have a 2004 w22 workhorse motorhome with a 8.1 chevy and I bought a aftermarket ecm and now I am fighting high fuel trims and looks like my tuner is not or cannot help he says +16 fuel trims are normal I have tried different maf and map sensors and still the same fuel pressure is good I do have long tube headers and Banks ram air. I am a gm master tech and can reflash the vin my self and the operating system is 12585950 and I am looking for a used ecm and needing a part number for the ecm found several for sale but don't know what ecm part number will work thanks

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Thanks for all the help found the info on another forum