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Discussion Starter · #1 ·
Now that the weather is warming up I am having some puzzling issues with my '08 Mazda 3 2.0L.

It started as a suspected AC issue. After 10 - 15 minutes of city driving and at idle my AC is short cycling to the point of blowing warm air. However, on open interstate about about 50 mph the AC works fine. I suspected low refrigerant but a set of AC manifold gauges showed normal pressures. That suggests fan, but the fan is operating while the AC is on. I also wondered if a pressure switch went bad.

However, I then noticed that the very low sensitivity coolant heat gauge on the cluster sometimes ticks up to maybe the 2/3 position from it's normal 1/2 mark. That's never happened before to my memory. I threw on an OBD2 scanner and drove around in slow traffic on a very hot day and saw temps as high as 238 within 5 - 10 minutes (and the AC started to short cycle). It wasn't enough to throw a CEL but--that isn't a normal temp for this car so far as I can tell. Testing on the interstate got it down to 220 and the AC came back on. The coolant hose to the radiator was extremely hot after the test drive but the return hose via the thermostat was only ambient temp.

It seems like the thermostat is broken in the closed position, but I'm not certain of that because I thought this car's thermostat normally fails in the open position (the thermostat is OEM and the car has about 140K miles) and the AC cutout is strange. Does anyone have experience with a similar issue? Does it sound plausible that the ECU keeps cutting power to the AC compressor because of the heat or that excessive engine bay heat means the condenser isn't working right? The compressor isn't seized up, the belt turns freely, and at idle with the AC on (before it starts short cycling) idle is a normal 750 rpms. All thoughts welcome!
 

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Discussion Starter · #3 ·
I don't have an IR gun but I have felt the hoses and there is a massive temp difference between them when warmed up. There is also no flow activity visible in the expansion reservoir (although the level did rise). A test yesterday got temps to 240 in 10 minutes of moderate street driving (with no AC use), which I believe is just under the level of throwing a CEL.
 

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Discussion Starter · #5 ·
Thanks for the radiator flow test idea--I would hate to go through the trouble of pulling and replacing the thermostat just to find it's something else. A you suggest, the thermostat is in a tough location--most people end up removing the belt tensioner for better access.
 

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Discussion Starter · #9 ·
I'm going to tackle the thermostat replacement today. I'll let you guys know if that solves my overheating and AC woes. I have seen only a couple cases in these forums of the Mazda 3 thermostat getting stuck closed, so it'll be another data point for others if that turns out to be the case.
 

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Discussion Starter · #10 ·
Well, the replacement thermostat seems to have fixed both the overheating and the associated AC problem. I'll push it harder tomorrow in the heat, but meanwhile the boiling test of the old thermostat and the newly correct engine temps seem conclusive that the old part was stuck closed.
 
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