over heating

Ok I have chased this problem till I just finally need some help before I drive it off the boat ramp lol I have a 92 mazda 2600i my problem is it over heating at idle I have replaced the thermostat and the radiator cap flushed radiator put in new prestone 50/50 coolant and it sat in the yard and idled and never over heated get in it the next day it was 94 degrees here so I was driving around went to a drive thru and while i was there I had to turn on the heat to keep it from over heating it seems to lose water doesnt seem to be a leak anywhere and no water in the oil so one where is the water going and two why does it continue to over heat at idle also as a side not there is no over flow resevoir and the line oming off the neck of the radiator for it is capped off

fan clutch could be weak...
im having somewhat the same problem, doesnt heat up in the winter, and over heats in the summer, ive changed water pump, thermostat cap and checked the fan, no clue

i wonder if it might be a clogged heater core that could be causing those problems
check the core who knows

its not good for the over flow to be capped off.... its there incase the system builds up to much pressure, thats the only place it has to release before blowing out a hose or worse, if you dont have the factory overflow reservoir you can use just about anything that will catch the water (its there to keep the antifreeze from going onto the ground, which is very toxic it animals and the environment). have you kept an eye on the exhaust when its over heating? hold your hand where the exhausts comes out and see if your get a build up of moisture on your hand.

Well made me a overflow tank out of a corona beer bottle lol but anyway It turned out the waterpump died I pulled the hose of the top of the radiator and started the truck not even a dribble moved out the radiator which should be a pretty good indication that the water pump isnt working thanks for the replies