Help - 2003 Chevrolet S10 Crew 4.3l 152K code P0442?

[Indiana Jones]

Has following symptoms:
* Occasional P0442
* Poor MPG, down to about 16
* Slightly rough idle
* Pinging while pulling steep hills at highway speeds
* Rattling sound when accelerating

So far I have replaced the plugs, dist cap, charcoal filter assy and gas cap, that helped, reduced the code from P0455 to P0442, but no cure.

[Ronnie Wrench]

Your two codes are both related to vaccum leaks