On the 16 yo car , I ' ve changed the AT tranny fluid with Honda ATF Z 1 fluid . Did the manufacturer recommended flush at the right interval . I also did the spark plugs 6 years ago , although not the wires . That ' s around the time I noticed the fuel economy was going bad . Fuel filter is super old . I ' ve never changed that . But , I don ' t think you can change that on my car . There ' s no specified Honda maintenance schedule that recommends changing the fuel filter . Air filter is super clean and also changed not that long ago . I do get gas at the same place . I ' ll look at getting some fuel system cleaner , though . That might help . Never looked at the vacuum lines , but I think that would eventually throw a CEL code , no ?