1993 Mercury Sable. Coolant down 1oz every 4 miles. No leaks under hood or car when it is parked. Pressure test of cooling system shows OK. Chemical test of coolant shows no hydrocarbons in the coolant, therefore head gasket seems OK. What's wrong?

Check the radiator where the core meets the tanks. The core would be the aluminum part and the tanks are the plastic parts at each end of the radiator. Where these meet it is sealed and crimped. Since metal and plastic expand at different rates, over time this seal weakens and allows coolant to escape in the form of steam. This has happened to a lot of my Ford's and the fix was to replace the radiator. Check for residue or watch for steam where the tanks are sealed to the core. Keep in mind that the seal goes all the way around so you may have to unbolt the radiator and move it back to see all of the seal surface on both ends. Check the radiator where the core meets the tanks. The core would be the aluminum part and the tanks are the plastic parts at each end of the radiator. Where these meet it is sealed and crimped. Since metal and plastic expand at different rates, over time this seal weakens and allows coolant to escape in the form of steam. This has happened to a lot of my Ford's and the fix was to replace the radiator. Check for residue or watch for steam where the tanks are sealed to the core. Keep in mind that the seal goes all the way around so you may have to unbolt the radiator and move it back to see all of the seal surface on both ends.

Okay, but was the radiator actually leaking at all? Did you find any leaks as I described? The radiator cap could also allow coolant to escape if the seal on it was no good. Okay, but was the radiator actually leaking at all? Did you find any leaks as I described? The radiator cap could also allow coolant to escape if the seal on it was no good.