1993 Geo Metro Repair Question
Head gasket tests are done with the engine idling. You don't see smoke at idle either, so the tests are being done when the problem doesn't exist. It isn't terribly practical to perform the head gasket test at the radiator while driving 60 mph and the mechanic running alongside the car, so I would suspect the head gasket based on your observations. Combustion pressures go up at higher speeds and higher engine loads and can push gases into the cooling system. The cooling system can't get much higher than 15 psi, but the increased turbulence from the water pump can force coolant into the combustion chamber at higher speeds.
Higher head temperatures at higher speeds can cause just enough additional head warping to allow coolant leakage. You could also possibly have very slow coolant leakage that you can't see at lower speeds. The coolant can collect in the muffler, then be blown out at higher speeds. Don't consider no coolant in the oil as a sign the head gasket is good. Coolant can leak three places, the oil, the combustion chamber, and outside the engine. You're only concerned with the combustion chamber.
A crack in the cylinder head can show up at specific times too. Magna-fluxing will detect small cracks that you can't normally see.
A better test in this case is to add dye to the coolant. Drive at high enough speeds to see the white smoke, then check the tail pipe with a black light. If the dark purple dye shows up as bright yellow, you know you have a bad head or head gasket. Of course, you kind of know that already because where else is antifreeze going to come from?
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