Detail: I suspect a head gasket lak but car runs great would expect O2 sensor problems, can you suggest something to confirm??
Solution: A common problem on the 4.9 , 3.8 , and 3.4 is intake manifold gasket failure GM has not recalled this problem , look for oil and anti freeze marks on side of the block . Or the oil dipstick for milky residue . Sometimes an engine light will be triggered also . You can also have the emmissions test to detect High levels of Hydro Carbons . From the coolant cap location this will tell you if its a internal problem like head gasket or intake . , Hi, a 6ya Mechanic can help you resolve that issue over the phone in a minute or two. Best thing about this new service is that you are never placed on hold and get to talk to real repair professionals here in the US. click here to Talk to a Mechanic (only for users in the US for now) and get all the help you need. Goodluck!