Two problems related to coolant leaking have been reported for the 2005 GMC Sierra. The most recently reported issues are listed below.
Took vehicle in for new brakes and coolant leak. No complaint about brakes, but complaint about coolant leak. Coolant leak issue is connected to gm TSB 06-06-01-019b and affects castech 706 cylinder heads. The coolant is leaking into the heads because of a known porosity issue. My engine will have to be replaced if the problem is not addressed because of glycol contamination. Gm dealer said to just "keep an eye" on it. A second, independent mechanic, did confirm the coolant leak is not external and my engine will fail if the heads are not replaced. This appears to be a known issue with gm for manufacture years 2001-2006 on the vehicles with the 5. 3l engine.
Faulty castech heads are leaking coolant into engine oil. Gm is aware that the 5. 3l v8 engines have faulty heads, and they refuse to place a recall to fix the problem. It is not right to not stand behind your product. The repair costs are in the $5000 range.