WARNING: This page is about a different car, the 2011 GMC Yukon, 2011 Chevrolet Tahoe, and 2011 Cadillac Escalade. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Inspecting Valve Lifter Guides
Courtesy of GENERAL MOTORS CORP.
Inspect the valve lifter guides for the following conditions: