Weird cooling issue
 

Can anyone help me out with a weird cooling issue I'm having? Intermittently I have some cooling issues, the temp starts to climb, fans are running. Then as fast as it stops working, it starts cooling again and the temp goes back to normal. I can't determine what causes it when it happens, but I does happen more when it's hot out. When the temp starts climbing, the A/C acts up at the same time. So the a/c stops blowing cold air. When it starts cooling again the a/c starts blowing cold again. I had the radiator replaced last year as a mechanic diagnosed that as the issue, but I had the issue Monday morning, it's not even hot. I've never experienced such an intermittent cooling issue before. Someone mentioned it could be a failing thermostat, or perhaps air in the coolant system. How to check those?

A thermostat would do it. They're cheap enough to replace, but once you have it out of the car, you can test it in a pot of hot water. I think air would have worked itself out by now. Does it happen sitting at idle? While driving? In traffic? Does the car still have the plastic engine undertray? You really need that for effective cooling.

You'll want to minimize the numbers of times that temperature arrow moves past middle, it's calibrated such that right of middle is potential engine damage zone.

Thanks for the reply Loki, sorry it took so long to replay been crazy at work.

It's happened in all the situations you mentioned. Most often it happens at idle or city speed traffic on warmer days, although it did get hot once on the highway. Most times it starts to creep up, then returns to normal. Still have the plastic tray on the bottom.

I did the 20 brake pedal stomp/ecu reset thing, and it's actually not done it since then, but I've only driven maybe 10 times since.

Was thinking it might be the thermostat, but though they would have replaced it with the radiator. I'm going to go ahead and search up a how to and replace it.

Thanks again!

A couple common concerns to check:

-Thermostat, if this device is not reliably opening at the intended temperature, it could cause greater swings in temperature. These are wear items and should be replaced whenever the coolant is flushed. I suggest sticking with Mazda OEM to assure the most longevity. These should be good for five years without any issue, but I prefer to change coolant and thermostats every two years or 30,000 miles.

-Coolant leaks, if there is a slight leak at a coupler, gasket, or break in the coolant bottle, it can cause loss of fluid that allows air pockets to form. It can also reduce the operating pressure of the cooling system, lowering the boiling point. Tracking fluid loss and/or pressure testing the system will confirm no issue here.

-Pressure cap, this could be failing to hold the correct system pressure, allowing for coolant boil at lower temps, or coolant loss via the overflow. Coolant loss can be an indicator, accompanied with evidence of coolant burping into the engine bay from the overflow line. I suggest replacement of the pressure cap whenever the pressure tank is replaced, something that should be considered every five years or 50,000 miles.

-Fan control concerns, the fan relays can fail to allow high-speed operation, leading to high temperatures and even overheating at low driving speeds. An indicator of this might be that you never experience high temperatures when moving greater than 35mph, only when stuck in traffic or traveling at low speeds.

It is unlikely that you have a coolant seal failure given that there has been no mention of white smoke from the exhaust, consistent overheating, not just running hot, and no mention of hard-starts.

The fact that the A/C and coolant temps are misbehaving at the same time and then fine at the same time makes me think this is an airflow issue (since the condenser is mounted to the front of the radiator).

I can't see how a thermostat would cause a problem with the A/C system.

I would check your radiator fans and verify that the plastic tray that runs from the from lip to the bottom of the radiator is still present.

So things have deteriorated quickly since last month. It's become apparent that I have a coolant seal failure. Dealer says new engine is 6800, not happening. I guess a reman might be an option?

A reman costs around $3000 + freight (anywhere from $125 - $500). You'll have to ship your old motor back to the place you bought your reman from (another $125 - $500) or pay a $1000 core charge.
When I bought mine, I found a dealer about 3 hours away and I dropped my old motor off when I picked up the new one.

Plan for an extra $1000 for incidentals, tools, fluids, etc. That's a worst-case scenario.

If you want somebody else to do the labor, add in at least another $1200 (probably more).