WARNING: This page is about a different car, the 2012 GMC Yukon, 2012 Chevrolet Tahoe, and 2012 Cadillac Escalade. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Identifying Valve Guide Cracks, Damage Or Excessive Wear
Courtesy of GENERAL MOTORS COMPANY
Inspect the valve lifter guides for the following conditions: