fcannas, your english is understood... and don't worry 'bout it... I can't speak a single word of Portugese, much less read or write it.
Your initial statement was that the car's losing a lot of coolant.
The implied question is: Why? or What's causing the coolant loss?
When coolant reaches normal operating temperature (approx. 180F or 80C) or greater, then removing the coolant reservior cap will always cause coolant in the reservior to "boil over" so to speak. This is normal and expected. I can explain the reason why this occurs but it's irrevelant to your question or it's answer, so I'll refrain.
To most easily discern (figure out) how coolant is being lost, you should do the following:
Run engine for awhile to heat it up nicely (80C at idle with car stationary -- not moving... without moving the car... watch engine temperature and look for water spray or leak dripping to floor or steam, or water in passenger compartment on floor.
If nothing happens for awhile while coolant temperature is at about 80C... you see no water on floor or spray of mist or water dripping anywhere, including the interior passenger & driver compartment floor, then you need to heat the coolant up to approx. 85C (185F) and keep observing for leaks on floor, mist, spray, steam coming from somewhere (where the leak's occurring).
To heat the car up to 85C place a sheet of cardboard in front of the radiator grill ... about 8.5" x 11" or standard notebook size sheet. This will reduce the cooling capability of the radiator and let coolant temperature rise toward 85C - 90C. Watch temperature gage and keep looking for leak or evidence of coolant escaping the cooling system. DON'T LET TEMPERATURE REACH THE RED MARK .... if it approaches the red mark KEEP ENGINE RUNNING and QUICKLY REMOVE THE CARDBOARD SHEET from the radiator and increase RPM to cool engine back down before shutting off the engine.
All the above must be done with car not moving (not being driven) so you can see where or begin to find out where the coolant is leaking from.
If there's no observed leaking at 85C, then increase temperature of coolant to 90C (195F) by placing another 1/2 sheet of cardboard over the radiator grill and repeat above observations You should also increase engine rpm intermittantly (occasionally).... press on accelerator pedal or use the control shaft/levers in the engine compartment. Increasing engine rpm to 2500 - 3000 or so will simulate the driving conditions and increase coolant pressures and flow.
BUT you MUST check the underside of the valve cover (cam cover) cap --- the cap you unscrew (remove) where you normally add oil to the engine. You MUST do this BEFORE you start looking for the leak source. IF the underside of the valve cover cap is creamy colored goo or you see water droplets on it, then the coolant loss is do to a crack in the block or head or head gasket. If that's the case, return car to the mechanic that rebuilt the engine and ask for your money back.
