Q: I installed new thermostats in my 2007 Nissan Altima 2.5l shortly after the car started to overheat so I put back the old t-stats

The best thing to do first is put the new thermostats back in the engine. Old thermostats are damaged when they come in contact with the ozone layer outside when they have been running in the engine for a long time and can stick. Once the new thermostats are back run the engine and see when the temperature is around operating temperature and check the lower and upper radiator hoses. If the hoses are both hard and about the same temperature, then the water pump is working. If you are not sure if the water pump is working or not, shut the engine off and remove the thermostats and drain the coolant. Then keep the upper hose removed from the engine and put a water hose in the port where the thermostat goes. Then turn on the water and let the water flow out of the engine. Then start the engine and throttle it up and down. If the water shoots fast through the upper radiator hose and then flows slow, then the water pump is working. If the water has the same flow, then the water pump is not working and needs replaced. Then at that point, either the head gasket is burned causing the engine to overheat or the temperature sending unit has failed. If you need further assistance with your engine overheating, then seek out a professional, such as one from Your Mechanic, to help you.