Burping Coolant: How Do You Know When The Thermostat is Open? 2015 CX-5

[anyman]

First the TL;DR
2015 CX5 with 98k. This fall I started doing my own oil changes and noticed my coolant reservoir was completely empty. Not knowing much at the time I filled the reservoir halfway and topped off the radiator with Xerex coolant (the “Asian vehicles” green stuff) and called it a day. I closely monitored the levels as I was worried about a coolant leak but they maintained over 2500 miles.

A couple of weekends ago I noticed the level in the reservoir had risen slightly. Opened the radiator cap and couldn’t see coolant. So I added maybe a couple of ounces or so to top it back off. Realizing I had air in my system I assume there was still air trapped somewhere (maybe the heater core…it’s finally gotten cold enough to blast the heat so maybe it opened more to let out more trapped air in the heater core?).

I bought a funnel kit to burp the system. It’s in the low 30s here and I know I need to run the engine for a while to get the thermostat to open up fully.

So here we are at my actual question:
How do I know when the thermostat is open? If ambient temps are in the 30s will it eventually open up at some point while idling? Are the radiator fans spinning up a sign the thermostat has opened?

 

[theblooms]

The thermostat is rated for 195. When it hits that temp, it opens. So just monitor your temps. If you don't have a scan tool that can measure temps, just use a cheap infrared thermometer aimed at the thermostat housing.

 

[anyman]

Awesome. I have a Bluetooth scan tool. Haven’t used it in ages. I’ll have to dust it off and see if it works with the Mazda.

 

[anyman]

Quick update on this. Did the process a few days ago. Everything went great but it took FOREVER for my thermostat to open due to it being in the high 30s here. I ultimately had to rev the engine to get it to open but having the rpms up also helped purged the air out of the system a little faster too. I did 2k rpms for maybe 4 minutes and then 2500-3k rpms for another 4 minutes. Got a decent amount of air out once the thermostat opened and then progressively the bubbles subsided and became more and more intermittent.

Ultimately it took an hour (yeah, I know). I used my bluetooth scanner and noticed the fans (and thermostat) would kick on when the coolant temp hit closer to 200-205 degrees. This would've been much easier to achieve in warmer months. Just happy to get it over with.

 

[Jmaz]

I haven't checked on my CX-5, but on previous cars , with the radiator cap removed I could see coolant flowing past the filler neck indicating normal flow out of bottom radiator hose, through engine block and out of block through thermostat and then top radiator hose back into radiator.

 

[anyman]

The fill port on the CX5 (at least my model year) has a little extension or neck so it’s not sitting directly over the radiator…maybe more problematic to burp? My garage floor is at a bit of a slope. I didn’t jack the front of the car up. But the bubbles subsided mostly. There were extremely tiny bubbles coming out intermittently towards the end but after an hour at it I just wrapped things up. My guess is that those smaller ones will eventually work their way to the expansion reservoir over time with the expansion/contraction cycles…maybe?

 

[Jmaz]

The fill port on the CX5 (at least my model year) has a little extension or neck so it’s not sitting directly over the radiator…maybe more problematic to burp? My garage floor is at a bit of a slope. I didn’t jack the front of the car up. But the bubbles subsided mostly. There were extremely tiny bubbles coming out intermittently towards the end but after an hour at it I just wrapped things up. My guess is that those smaller ones will eventually work their way to the expansion reservoir over time with the expansion/contraction cycles…maybe?

You're absolutely correct, I stand corrected. I forgot about the extension of the filler neck and not being able to monitor coolant flow like in the "old days". After a coolant changeout or replacing coolant as part of a job, I would just check the radiator level on a cold engine for a few days and add until the level didn't change. Any trapped air works itself out through normal operation.