Download Subject: Information on Gradual Coolant Loss Over Time With No Evidence of Leak Found Models: 2004-2006 Buick Rainier 2001-2006 Cadillac Escalade Models 2001-2006 Chevrolet Avalanche, Blazer, Silverado, Suburban, Tahoe, TrailBlazer Models 2001-2006 GMC Envoy, Jimmy, Sierra, Yukon Models 2001-2004 Oldsmobile Bravada 2005-2006 Saab 9-7X with 4.8L or 5.3L VORTEC(R) GEN III, GEN IV V8 Engine (VINs V, T, M, B, Z – RPOs LR4, LM7, LH6, L33, L59) Supercede: This bulletin is being revised to include engine RPO L59. Please discard Corporate Bulletin Number 06-06-01-019A (Section 06 – Engine/Propulsion System). Some vehicles may experience a gradual coolant loss over time. A very low percentage of cylinder head(s) manufactured with an embossed Castech logo may develop a porosity crack in a very specific area. Inspect the cylinder head assembly to determine if the casting was manufactured by Castech. This can be accomplished by inspecting for their casting logo located on top of the intake port, under the rocker arm support rail and in the spring deck cavity portion of the cylinder head. If the cylinder head(s) are Not a Castech casting, follow normal diagnostic procedures in SI to determine the cause of the coolant loss.