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Originally Posted by Larry Derouin
With Mileage in the 6 to 8 mpg range I'd look at a bad fuel pressure regulator 1st. It's inside the Intake Manifold. Quick way to check if it's bad is to take an inspection mirror on a stick (sound like Jeff Dunham now......) open the throttle WIDE OPEN, and look in the intake. If the passenger side is "washed clean" and the drivers side is dirty, you need to replace it. Takes about an hour or two, depending if you've opened one of these intakes before.
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A leaky fuel pressure regulator will cause a drop in fuel mileage however, opening the throttle will not allow you to see if anything is leaking on this engine. The older Vortec engines have a tuning port on the upper intake plenum. You can remove the cover and look into the port and check for leaks. The only way to "see" a leak on the newer engines is to remove the upper intake. A much easier way to check for a leak on
any 4.3L W or X engine, is to run a fuel pressure leakdown test. 100% conclussive results and takes about 5 minutes to do.