You can test the coolant for exhaust gasses. Take the radiator cap off and use the same sensor they use for emissions testing. It will "sniff" the gasses coming out of the radiator and it will detect if there are any hydrocarbons. If so, them the head gasket is the logical answer.

I even heard the guys on Car Talk a couple of weeks ago say they use this method. They said there is another test for the coolant and it is positive if the coolant changes color. It was not very reliable because it is technique sensitive, and coolant can come in different colors, changing the end color giving a false positive or negative.

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2002 BMW M3 Convertible

2000 Harley FXDWG Dyna Wide Glide