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Q: temprature went from hot to cold while driving on 2008 Mazda Mazda5

My car started showing hot on guage.when I would check engine all looked well fan was running fluids were good. Proceeded to drive and after a day the temp went from normal to hot and came back down went hot and then went all the way cold while driving. The engine light came on so I pulled over. Checked under hood and the coolant was coming from the radiator back into the coolant resevoir. Which was causing the resevoir to overfill. Coolant started to come from cap on resevoir. I'm not sure if the radiator is clogged or if its the water pump or the themastat. Please help so I can be more clear of what the problem may be. Thank you. Eric
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I have two suspicions. One possibility, that the thermostat is failing and causing the engine to get hot and blow coolant into the reservoir, that causing the gauge to show cold when the coolant level is below the sensor. The other worse possibility is that the head gasket's failing, causing the same symptoms as I've described above for a failing t-stat. You can get a technician to check for combustion products in the cooling system which would indicate a blown head gasket. If you work on the car yourself, maybe replace the thermostat first. Don't allow anybody to sell you a 'cooler' t-stat, that won't help you. You want the engine to run at the right temp so get the specified t-stat.
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