Keep an eye on your coolant usage; when I first noticed some minor coolant loss, I did not think too much of it. However, at about the same time my Avalanche Z71 5.3L engine began to have a little valve tappet noise. My dealer added some Mystery oil, with an oil change and my valve tappet noise cleared up. Unfortunately, within a short time my trucks valves found their voice again, and I noticed that the coolant was again low, I than checked my oil cap and noted some emulsion �further checking by the dealer revealed coolant in the oil along with a cracked head.