06 Silverado type x 4.3

[oomag12]

From an earlier thread on this model concerning the fuel filter, they "are" in tank. Bummer, but thats the deal. I still have mpg issues. air filter clean and new. Map sensor is stock, not sure manifold pressure could be off, but should get a code i would think. I am not getting codes on O sensors. I'm 'told' if they are below their operating threshold, they throw codes. The plugs that were installed during my cat converter mess ARE the AC delco correct plugs, BUT, they are soot black and dry. The old plug chart says the plug is to cold and hotter plug is needed. AC and GM says it the right plug, but mpg went from 18 before the entire cat. con. mess to 14. Curious if anyone has had similar exp. with this motor.