Water/Coolant leaking from rear area of motor. Only happens when pressure and temp build up.

check oil level first. overheating can be caused by blown or leaking head gasket, ignition timing, lean fuel mixture, low coolant. take car in to shop you may incurred engine damage.

Is it low on coolant ? Do the cooling fans work ? What is the actual temperature of the engine ? what are you going by to say it's over heating ? temp Gauge ? Did you have the vehicle checked for DTC'S - diagnostic trouble codes ? White smoke then gone after 30 ,this is probably just condensation . The cooling fans on this vehicle should come on at a set temp.
The BCM and PCM/ECM command Low Speed Fans on under the following conditions:
• Engine coolant temperature exceeds approximately 106°C (223°F).
• A/C refrigerant pressure exceeds 1 310 kPa (190 psi).
• After the vehicle is shut off if the engine coolant temperature at key-off is greater than 140°C (284°F) and system voltage is more than 12 volts. The fans will stay on for approximately 3 minutes.
The BCM and PCM/ECM command High Speed Fans on under the following conditions:
• Engine coolant temperature reaches 110°C (230°F).
• A/C refrigerant pressure exceeds 1 655 kPa (240 psi).
• When certain DTCs set.

Having a professional type scan tool hooked up to view if the BCM/PCM are controlling the fans at the temp designed too and view what the actual temp is would be the proper way to diagnose . I know GM vehicles also had bad stepper motors in the insturment cluster , that give false gauge readings .
Your Water leaking under car, under rear seat area may not be a problem at all , is just water or coolant ? You must have a rear Heat A/C unit . Have you used the rear A/C ? Could be just water from the evaporator case from rear unit .
You mite do well just to have a qualified repair shop check it out .

Leaks get worse when engine is at operating temp and pressure builds. Rent a cooling system pressure tester and pressurize the system. Then you'll see the leak

Obisouvly a leak somewhere.Would start with upper and lower radiator hoses with vehicle running and at normal operating temp.This will allow the cooling system to build pressure,look for signs of wetness at hoses as well as heater core hoses from water pump to usually the firewall where they go into the passenger compartment to the heater core.

Always find the source first, The rear water ports are directly behind the rear lower intake bolts and the front ports are directly in front of the front bolts. The rear ports are cast into the lower intake manifold and are dead ends, air can get trapped in that area. In cold weather your thermostat may open slower, resulting in a pressure build up at the rear ports. Any trapped air will compress. Normal coolant system pressure occurs when the glycol is operating temperature. I would try refilling the system and getting the air out,the leak may only occur when the system is full. With a cold engine, open the radiator cap and add 50/50 mix, fill the resevoir to the low mark. Run the engine with the radiator cap off until the thermostat opens, the upper hose will be hot when the stat opens. Turn the car off and refill the radiator to the top. Sqeeze the upper hose to remove any air. fill if necessary. replace cap, run car again until it reaches operating temp. Now with a full system check for leaks. When replacing lost coolant should also have the heat at max temp. Get a good mirror to check for leaks. The outter mating surface of the rear water jacket gasket is flush with the block and the edge of the lower manifold, sometimes you can see where it blew out.

It could be a number of places. I assume you mean that the coolant is leaking. You need to get it pressure tested to find the leak if it is not "dripping" where you can see it.

CHECK FOR LEAKING WATER PUMP WEEP HOLE OR GASKET.ALSO CHECK WATER PUMP HOSE FOR LEAKS.IF ALL LOOKS GOOD.YOU COULD HAVE PINHOLE LEAKS IN RADIATOR CORE LEAKING ON FRONT OF ENGINE OR INTAKE MANIFOLD GASKET LEAKING COOLANT.

Does the vehicle overheat on the road? If it does and pushes big burps of coolant into the reserve tank then you could have a head gasket leak. Usually what happens with a head gasket leak is that the engine will often start and run normally but will soon start to push coolant out in a kind of foamy burp as the combustion gasses that accumulate in the engine block force their way out. You can have a small head gasket leak that allows combustion gasses to escape into the cooling system without getting coolant back into the engine and often the first symptoms of this kind of a leak is a motor that suddenly overheats, the heater quits blowing hot air and after a short while everything returns to normal for a day or so. As the leak worsens however it may keep pushing coolant out of the reserve tank with a cool or cold engine as the pressure builds to quickly for the reserve tank to handle it fully. Having said that if you have a perfectly good cooling system that is hot and take off the radiator cap it will spray blistering coolant coolant all over so never remove the cap from a warm or hot cooling system! Most additives won't stop a head gasket leak as there is just too much pressure for them to "stick" in the leak and they just get forced out of source of the leak. It's rare but you could have a water pump that's not circulating enough coolant around but usually you have different symptoms than this.

If you had the engine replaced and have the same leak, it may be be somewhere else. You can rent a tool from most parts stores that pressurizes the cooling system. If you get that tool, top off the coolant and follow instructions to pressurize that system, you may be able to see where it's leaking. While it's under pressure, take a flashlight or shop light and look in all the main areas. (hose connections, radiator, water pump, ect)

Good Day again,

Sorry to hear it wasn't the thermostat, Other possible causes inlcudes low coolant, water pump and or waterpump belt, radiator blockage, and radiator cap not maintaining pressure. Check you coolant level first, fill if it's low, typically if it was the pump you would devolop a coolant leak as the pressure builds in the pump assembly. Check the belt though, any squeaks while running? Also if the seal in the raditor cap is bad then no pressure can build. To test the pump remove the raditaor cap (ensure the motor is COLD!!) and start the motor w/o the cap installed, watch the coolant level as it is should be going down slowly as it pumps through the system. Also check the seal on the raditor cap for any cracks's or visible damage.

The temp sensor is a possibilty but a rare one at that.

Hope this helps again