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Q: Over heating problems

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my g35x sedan keeps over heating it runs great and everything but i got it about 3 months ago and at first it wouldn't over heat but after a few weeks the temp gage would go up n then down like crazy it would spike but the car wouldn't leak or shake or show any signs of it overheating i rancit like that for a week and then one day it over heated in the freeway as soon as i turned off my vehicle and got off coolant leaked from the passanger side but when i refilled it from the reserve tank it didn't leak from anywhere which was weird i drove it home and it kept jumping around the gage. the following weekend it finally over heated pretty bad and drove it to a friends house quick close by and turned it off and i could hear the water bubble from the heat. i checked for leaks and nothing ! i replaced the thermo stat and it still over heats idk if it's a water pump problem or air in the system i don't want to waste money like crazy any help ? thanks

My car has 130000 miles.
My car has an automatic transmission.

Based on what you are describing, typically a Mechanic would look for a faulty water pump, faulty radiator cap, plugged up, or externally obstructed, radiator, and/or non functioning electric cooling fans. However, your description of erratic operation of the temperature gauge introduces a complexity. Normally, when a car overheats, it just overheats; the gauge is buried on the high side and you hope the operator turns the engine off quickly so as to avoid warping of the head(s) or blocks. If you are seeing a fluctuating temperature reading though, it raises the possibility of trapped air and/or possible head gasket leaks. Of course, you could have a faulty gauge or sending unit, too. You want to first purge the system of air, if there is any. Test the radiator cap (the cap has to withstand a pre-set pressure). With no trapped air and a good cap, do a leak test. If the leak test is good, run the engine and see if you get continuous heat in the passenger compartment when temperature is set to hot with blower on high (if heat indicates water pump is circulating coolant). If pump is circulating coolant, use infrared thermometer on upper and lower radiator hoses and radiator. If, via these temperature measurements, you determine that hot coolant is not flowing throughout the radiator, you may have a plugged radiator (you still have to test and evaluate the water pump further, though). A plugged radiator will easily cause overheating. If no faults are found at any stage, you have to evaluate the possibility of a head gasket leak, specifically exhaust gases leaking into the coolant, which will cause overheating. An on-site exam from a professional would probably obviate the need for some of these steps and consequently I would simply suggest scheduling an overheating diagnostic by a certified Mechanic. You needn’t worry too much. On an issue like this, so long as the cause is not related to a leaking head gasket, there is a good chance this will be a fairly mundane repair once the cause is found. If you have further questions or concerns, by all means let us know.

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