2008 RX-8, 6-speed automatic, stock, good condition, no known mechanical issues or leaks, I think around 80K miles. Have owned it for several years, but not the original owners. This is technically my mom's car, but she's elderly and living with me, and I just use it to drive her around occasionally since she has a hard time getting into/out of my own car. Have not had any temp related issues whatsoever until a couple months ago when I was taking her to a dr. appt. and noticed the temp would quickly start to rise when going up grades (middle of summer, so 100+ degree temps outside). Immediately pulled over when the temp quickly climbed from the usual spot just below halfway to about 3/4 in the span of about 1 minute. Once pulled over it immediately stopped climbing and quickly cooled back down in a few mins engine idling (and AC off at that point of course). No overheating issues whatsoever even on 110 degree days with the AC on or off, as long as it's driven on level ground. No issues at all in heavy stop and go traffic, the needle stays steady just below halfway, regardless of outside temps. But as soon as I start up an even moderate incline the needle pretty quickly starts to move up. Routinely check/fill fluids and coolant, and last oil change/fluid service was maybe 1,000 miles ago. The car only gets driven a couple times a month typically these days, usually just around town.

So, bottom line, no overheating at all regardless of external temps, except on inclines when the engine is under load. The car was not allowed to overheat past 3/4, and immediately pulled over when it got that high. I topped off the coolant as soon as I noticed the problem that day (it took a moderate amount), but the problem persisted on any inclines, even hills on the freeway at freeway speeds. I've read that these cars have two fans, and I know at least one fan is working as I can hear it running after I turn the car off on warm days. I assume one or both only kick in when needed, so not sure how to check to make sure both are working. I searched of course before posting, but only found one or two threads related to overheating specifically on inclines only, it doesn't seem to be a common overheating problem. Haven't checked codes (I have a cheap basic scanner), but haven't had any check engine lights either.

I have a good radiator/cooling system shop nearby that did the radiator, water pump, thermostat and hoses on my Vette recently, but they're not cheap, and I'd like to try and at least narrow down the problem before taking it anywhere. No good/trustworthy Mazda dealerships or mechanics around that I know of. If there's anything that I can reasonably check/troubleshoot myself please let me know. I'm disabled so I can't climb under the car or anything and I'm no mechanic. Thanks in advance for any constructive help.

 

hey Edenney,

No easy answer here so we'll go over a few things.
Coolant: are you using pre-mixed anti freeze? could try running simply more distilled water as you don't need the anti freeze based on the temps you are describing and straight water has better cooling characteristics.
temperature gauge; unfortunately Mazda gave us a 'dummy' gauge in the car that isnt' actually connected to anything other than the PCM. When the car determines it's at operating temperature it goes to the same spot and basically sits at that spot until you exceed a certain temperature (I forget the exact number but it's somewhere around 210°+) which means the car is already starting to overheat. 230+ = Danger Will Robinson.

this could be an airflow issue.
do you know if the radiator is OEM? Mazda lined the radiator with foam to help force air to run through the radiator without escaping past it and that foam can eventually degrade or some installers don't know to reuse it when they install aftermarket radiators. When going uphill in elevated temperatures that lack of direct airflow to the radiator will become more prominent which might be why you're seeing it there as opposed to on the highway or speed.

if the water pump were failing you might see a bit of coolant leaking but you haven't seen that so its safe to assume that is probably okay.
t-stat: could be starting to fail.
if you're mechanically inclined I believe there's videos on here to show how to test it.