Our platform isn't known for the good gas mileage, 200 isn't horrible for these, but cleaning
the throttle body will help the most. Remove throttle body with fuses #10 and #28
removed from fuse box while cleaning. Lots of video's on how to do it. 4.2L straight 6
likes AC Delco 41-103 sparkplugs only. Also, a new upstream AC Delco or Denso 02
sensor will help. I believe 04 still has the fuel filter external from fuel pump, new one
would be good. Clean your MAF sensor IF it has one, don't recall what year they started
using them. OR clean your MAP sensor if MAF is missing near the airbox. If you really
want to get the maximum out of the engine, send your PCM to PCM of North Carolina and
have it tuned to your liking.