Thanks, I don't believe the head gasket is the problem...there is a possibility, but various tests need to be done to verify it. This will include a Block Test (which is a chemical test that checks for combustion gases in the cooling system), or a HC (Hydrocarbon) test. At this milage it would be unusual for the head gasket to fail with no other symptoms other than coolant loss, and no history of overheating the engine. There are common area of the engine where coolant can leak out so little, it takes weeks for it to go low, and in some cases there are little visual signs of leakage. One of the more common at this milage is the water pump and the water pump housing cover. You should look around it for any signs of orange or pink colored corrosion. In fact the system should be pressure tested and determined for pressure loss as well. Do not add any types of sealants to the system anymore, it can me detrimental to the Northstar engine. Also, if this was a head gasket problem, not only is it most likely NOT covered under warranty any longer, but rarely can the Head Gasket be replaced successfully, but since yours hasn't overheated - this may be a different situation. The head and block are alluminum and rarely is this engine repaired, but rather replaced. I recommend you take the vehicle down to a quality facility experinenced in Northstar engine's and have the appropiate testing done to issolate where the coolant is beling lost. The missfire your experiencing may not even be related to the coolant loss.
~Rip
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