Losing coolant water

I have a 97 Taurus that is losing coolant water out of the radiator cap. It is not overheating (I have the normal temp gauge) and is not overfilled. It happens when I turn A/C on or in hot weather. Anyone know what's wrong with it? Thanks.

Is it the proper cap for that vehicle and is it in good working condition? Is the filler tube on the radiator itself or did they move it to the resevoir already? My '94 Taurus has the tube on the radiator and my '99 Windstar has it on the resevoir with no tube on the radiator. Just curious as to when they made the switch and if they did it on all vehicles. If the cap is good then you may be getting some pressure from the engine combustion chamber causing a higher pressure in the cooling system and forcing fluid out of the cap. That is not a good thing as it would indicate a probable bad head gasket. Hope that's not the case.

It is a new radiator cap, but did not help. I have seen more than 15 technicians including two Ford service, most of them point to the bad cap. The pressure tesing is normal. It is not possible for Themostat because the gauge show normal, it is not possible for the water pump because of no leak and no smell of coolant. I have a new tank for 4 years. I just had the flushing a month ago. The car runs great.

What's the sympotom of bad head gasket?

If you think you have a bad head gasket, check for the following.

Look for white smoke out of the tail pipe (steam) and check your dip stick. If you see a milky white oil on the dip stick, kiss a head gasket good-bye.

You can tell whether it is the front or rear gasket by pulling the spark plugs and if you find a wet plug, you now know which head gasket blew.

Thanks you for the explanation. No smoke at all.