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Q: Cooling system issue

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I recently replaced a faulty radiator and blown head gasket in my vehicle after an overheat and some diagnosing of symptoms. The car ran fine after repairs, but I noticed that the Temp Guage continued to show that the engine was running cool. In addition, it was using more fuel than usual and not heating up the cabin as it used to. Checked the Thermostat and, sure enough, it was faulty and stuck in the open position. Replaced the thermostat and its housing unit, refilled the coolant and attempted to ensure that all the air was removed from the system. However, last night after running the engine for about an hour, I heard bubbling coming from underneath the hood and coolant was spraying from my reservoir. I carefully removed the cap to relieve the pressure and it continued to bubble inside. I also removed the radiator cap to relieve any pressure there and coolant just came spraying and "boiling" out. What could be the issue?
My car has an automatic transmission.

Occasionally, thermostats are defective out of the box. Unless you have bought a dealer OEM thermostat, it’s not a bad idea to test it to be sure that prior to installing it it actually works, particularly if the thermostat is located in a difficult spot. If a head gasket blows as a consequence of overheating, the head and block have to be very carefully measured for flatness. Warpage of these parts is a possible consequence of overheating. Neither the head nor the block can be out of plane more than a couple thousandths of an inch. If the flatness of the head and block were not confirmed, it is possible you could again have a leaking head gasket, in this case leaking exhaust gas into the coolant. All of the many other mundane causes of overheating have to be considered as well such as a malfunctioning water pump or issue with the drive belt, blockages in cooling passageways other than the radiator (which you of course already replaced), ignition timing off, lean operation, faulty radiator pressure cap, collapsed radiator hose and so forth. If you desire, YourMechanic can dispatch a certified mechanic to perform an overheating diagnostic. The Mechanic will get to the root cause of the issue and estimate the repair cost.

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