Q: Car overheating and not accelerating

As far as the thermostat causing the motor to sound muffled, I personally have never heard of such a thing. Your thermostat controls when the flow of coolant in your cooling system starts, allowing the coolant to flow and cycle through the radiator being cooled by the fans. This, in turn, cools the block and head through the coolant jackets. If the vehicle temperature is low the thermostat will stay closed not allowing the recirculation of coolant to occur in turn continuously heating the coolant up. To truly know whether or not the coolant temperature sensor is falsely increasing the temperature on the gauge or not an automotive scanner should be hooked up and the actual output coming from the powertrain control monitor that monitors the engine coolant temp can be viewed. The lack of acceleration could be caused from the vehicle actually overheating which could have lead to serious engine damage.

A proper cooling system diagnostic should be performed by a qualified technician. A diagnostic like this can show whether the system holds the correct system pressure, whether or not the fans operate, what the actual temperature of the engine is, and what memory fault codes stored in the powertrain module. The mechanic can survey all this information to pinpoint the cause of your overheating issue and come up with the right repair.