Before anyone says "Oh no - not again" rest assured I did a search of the various forums online and did not find a solution - if I missed it my apologies. Here is what I figured out it was not - thermostat , temp controlled fan system,obstruction of the radiator fins and now for the rest of the story .

Last week when we started getting temps near 90 the car overheated- turns out it was a cracked heater bypass pipe which I replaced ( really - they are made out of plastic [confused] ! ) and refilled the antifreeze via the recovery tank. For the next 3 or so days of hot weather I could smell a hint of antifreeze as before but it ran fine . Today after a short trip once again the temp guage started going toward the red during an 82 degree day-fortunately I got home before an overheat. After letting it cool I checked the reservoir tank and it looked empty or nearly so. This of course indicates a slow leak of coolant as there was no sign of puddles and no steam coming from the engine or the exhaust. In the past I would throw a container of radiator sealer into the radiator cap to fix the leaks ( I have a couple favorites but will not advertise them here) but in this case what do I do .

 

That pipe is not the only plastic piece there. The thermostat housing is also resin and a focal point of problems.

 

find a friend with a cooling system pressure tester and pressurize the system. Check for a pressure drop and visible leaks.

a empty reservoir is not always an indication of a leak. If you had any air in the system after filling it; the coolant will be drawn out of the reservoir with each thermocycle (heat up/cool down) of the engine.

vacuum filling is the best way to ensure no air remains trapped in the cooling system.