sorry if im posting this in the wrong place,but my teacher has a 05 duramax pretty much stock.only mod is a bully dog chip...he says here lately it just doesnt have the power it used to...any ideas where to look?

he just changed oil and air filter.said it has been a while since he changed fuel filter,maybe a year,but not too long...any ideas on what else could be checked?

sorry about lack of info,i dont know much about truck.thanks for any and all info

 

change the fuel filter if not it could be a weak pump and then id just add a lift pump such as airdog

 

also check/clean the maf sensor