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I have a 2007 Mazda 3 that over heats only when it’s cold out. I live in Florida so it’s only cold for a few months, but regardless, something is wrong. Iv replaced the Radiator, Radiator Fan, Coolant Flange, Thermostat and sensor, Coolant Reservoir, and 2 main hoses. And thought it did the trick until the cold season came back. It’s goes up with acceleration, goes down when I throw into neutral or am idling. Heat works fine until overheat start then no hot air. Nothing seems to correlate with the fluctuating temp, except for accelerating. Iv been lookin for two years online trying to find an answer(stalled so long because Florida is 80 to 90 degrees 9 months out the year, thus not causing any issues until next year). This system has no radiator cap, just reservoir, no bleed valve. And before you ask….I haven’t seen any coolant leak all year nor coolant level drop. Iv tried bleeding or burping by jacking the front,taking of reservoir cap, and letting car warm up to temp with cap off. And even revved at 2500 rpm’s for 5mins, same with 3000 rpm’s to try and push any air in there. Squeezed hoses and overflow. Any suggestions??? I don’t wanna blow a head gasket(hoping I already haven’t) Any takers , I need this 😂
 

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Yo. This is ignoring the fact that it just started now that it's "cold", but my first guess is it's the water pump/pulley on your waterpump. Temps go up when you're giving it gas, the pump should be pushing more coolant at the same time but that seems like it might not be happening.
Any other symptoms? Like if it's really the cold that's doing it you'd probably have other symptoms like BIG mpg change, rough idles especially on cold starts etc.

I'm guessing you changed the cap with the reservoir? I don't trust those things especially the cheap ones from autozone/NAPA, and if you really want to cover your bases I would re-burp and once its at temp just watch the resevoir for air coming up for like 10 15 minutes, maybe the cap is letting air into the system. It seems like you followed the directions but still need to ask: you didn't let the reservoir go under "min" when you were burping it? Easy way to let air in the system. Even if all that checks out, the caps are still so cheap i would get a new one and drive around in it and see if that fixes it, that's saved me hundreds of $$$ before.
 

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your reservoir cap on that plastic tank is also the pressure cap... says the psi amount right on it... plus the reservoir is already at the highest point for bleeding air out of the system. just high heat and full blast while getting the operating temps up should do it

test the cap for pressure via getting that free loaner pressure test kit from your local Autozone or similar parts store


don't forget to check the drive belt and tensioner as well since it does power the water pump.

dunno how to test for water flow with this type of system though
 
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