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Loose ground, that makes sense.

A blown head gasket would be very evident, white smoke and/or engine oil looks like a milkshake. Is this your case?

Are you sure there is not an air pocket in your system? With a cold engine (NOT HOT), open the radiator cap (not the reservoir), add coolant, start the engine. Press the upper radiator hose to remove any air. Add more coolant. Wait until the fans cycle at least once, top it up and put the rad cap back on. Add to the reservoir if needed.

You said it does not overheat when the heat is on so your issue is not a gasket, your vehicle is not removing enough heat. Was the new radiator new? Is the radiator grille and ac condenser un-obstructed? Are the fans sitting against the radiator or is there a gap?

Send a pic. We are trying to help but you prefer to get a mechanic to pull the engine instead of sharing a pic...
 

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Well I'm a mechanic and there are only so many things that run a car's cooling system. First off you said the radiators good it has clean flowing fluid through it so that's not it you have your water pump run in your coolant and if that's working properly let's move on to the fans are the fans working properly check that if both fans are running when the car is hot and heat is on. All hoses are sealed and holding pressure? My next guess would be that you need to do a pressure system test on your car. Sounds to me like you have blown head gasket which does not mean that it would leak outside the car
 

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Couple things...

Your Mazda, while technically a Japanese car, is 80 - 85% Ford, including the engine - the import guy may or may not know any more than a regular mechanic.
Did it say somewhere she has the V6? I know the MZR, while very reliable, if it has an issue, it goes south very quickly. And that 2.3 is of course is Mazda's engine.
 

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You said it does not overheat when the heat is on so your issue is not a gasket, your vehicle is not removing enough heat. Was the new radiator new? Is the radiator grille and ac condenser un-obstructed? Are the fans sitting against the radiator or is there a gap?
It is stated that it doesn't overheat when the heater is on, but for how long? If the car begins to overheat while the AC is running, but to resolve the problem, the owner switches to heat, and the car engine cools, and then you arrive at the destination and shut it off, that's not the same as being able to drive 100 miles with the heater on or being able to sit in traffic for 20 minutes. Also, does it overheat with NO AC?

No one has ever replied that yes, it has been verified that the system is free of air.
 

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Also Mazda is not a Ford. Just because Ford has a 13% shareholder of stock means nothing
These year Mazdas are made with 80 - 85% Ford parts excluding some suspension work and other miscellaneous things. The engines, your standard inline 4, while the design is a Mazda L (Ford having full rights to use the MZR series), were built with Ford parts (including the block) in Mexico through 2009.

Did it say somewhere she has the V6? I know the MZR, while very reliable, if it has an issue, it goes south very quickly. And that 2.3 is of course is Mazda's engine.
Not that I can see, and I just realized that this thread is in the wrong section altogether. Fixing.
 

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And a blown head gasket is not always evident.
True but for the head gasket to be the root cause of overheating, it would mean that it is causing refrigerant to leak out of the cooling channels/orifices in the engine block & head. If it leaks, it has to go somewhere and that is either into the cylinder (white smoke), into the oil (milkshake like oil/refrigerant) or out (visible leak in engine bay).

Please educate me how else could it cause overheating without it being evident. I am always open to learn new things.
 

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I got a fan control module from a Mazda dealer and it fixed my 2003 Mazda6 6 cylinder fan problem of staying on long after shutting down the (rest of the) car.
 

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My daughter bought a 2003 Mazda 6. She thought she was getting a deal but it's the exact opposite. Problem after problem after problem. It's overheating. I have replaced the thermostat, the leaking radiator, and hoses going to them. It seemed at first to do it, but after running a while temp started jumping up again. I don't know what to do next. Water pump going out or malfunctioning? I took it to AutoZone and they checked it out, tons of problems but nothing to do with water pump. Please tell me someone else has had this problem and can't help. ANY ADVICE is greatly appreciated.
like every older mazda on earth you would probably need a L510-15-15Y COOLING FAN MODULE
 

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Can the thermostat be installed backwards on this engine? If so, could that be the problem? As for the car being mainly Ford for that year, I don't know. The 3's were all made in Japan as far as I know and I thought until more recently, the 6's were made in the USA and less quality than the 3's were. I know the 626 I had many years ago (89 model) and the Ford Probe used the same engine. If the car door sticker says it was made in Japan it's most likely a Mazda made engine. If it was made in the USA I wouldn't be sure.
 

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It was my heater core. We bipassed it and it quit overheating. But now im having any issue with it hesitating when getting up to speed. I changed my mass air flow sensor and it still hesitates when getting up to speed. What else could it be?
 

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It was my heater core. We bipassed it and it quit overheating.
Is it possible that the heater core is clogged? And that is why the car is overheating.? If you are in a place that doesn't have winter season, then you should be fine without the heater.
 

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My daughter bought a 2003 Mazda 6. She thought she was getting a deal but it's the exact opposite. Problem after problem after problem. It's overheating. I have replaced the thermostat, the leaking radiator, and hoses going to them. It seemed at first to do it, but after running a while temp started jumping up again. I don't know what to do next. Water pump going out or malfunctioning? I took it to AutoZone and they checked it out, tons of problems but nothing to do with water pump. Please tell me someone else has had this problem and can't help. ANY ADVICE is greatly appreciated.
 
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