I'd wait and do the test with the Napa kit. Check your oil at the dipstick carefully. As a head gasket failure progresses, you notice your oil start to get a milky look due to water and coolant making it's way into oil. Let us know how the exaust gas test goes. Hopefully it's something simple. I had one customer on an early eighties Honda Accord, that ignored his overheating so long, that when I drained the oil before tearing it apart, I filled up 3 oil drain pans, 5-6 quarts each. Only a Honda or Toyota would keep running like that.
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