Coolant leak

Well, it could be as harmless as a coolant leak in the small coolant lines that go in and out of your TBI unit. If it is leaking and draining on the top of your motor, it will just evaporate. Also, don't forget to check your transmission fluid for "Milkiness" incase the transmission cooling lines are evacuating coolant from the radiator. One other place, check the coolant lines that enter the firewall that supply your heater core. If you have a 2003, there should be a access panel under the dash on the passenger side. You have to remove the kick panel (three 9/32 hex hed screws). There is a small "L" shaped access panel that holds the "Cabin air filters". Remove that, pull the filters out and see if the heater core is not leaking fluid (may be absorded into the filter material).

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.

Refer to the illustrations on how to identify Castech casting and/or the very specific areas of the cylinder head(s) for a coolant leak from porosity.

Cylinder head(s) location of the Castech manufacturing casting logo (1).

Close up view of the cylinder head(s) showing the Castech manufacturing casting logo (1).

If the cylinder head(s) is a Castech casting (1), inspect the area around the five oil drain holes for witness marks indicating coolant seepage over time (2).

If No evidence of coolant loss is found on inspection of Castech casting cylinder head(s), follow normal diagnostic procedures in SI to determine the cause of the coolant loss.

The crack location can be found in any of the five cylinder head(s) oil drains. This can be seen as a clean or shiny area, on an otherwise stained surface (1). Pressurizing the cooling system at this time may reveal coolant, air, or a combination, weeping in the described area. If inspection reveals evidence of coolant witness marks (1), replace the entire cylinder head(s) assembly.
follow this info,very common.