Inlet side thermostats

[Drew99GT]

So I’ve been thinking the thermostat on my 2016 CX-5 is possibly starting to open too early/sticking open. I have OBDII data and when it’s very cold with the heater on, it’ll barely crack 180F. And when I coast down a hill, it’ll drop to 170. In the summer it stays around 190.

The cooling system is odd. Never seen one like this. I presume it has an inlet side thermostat on the UPPER hose. Every other inlet side thermostat has been on the lower hose.

Last night coming home I waited until the coolant got to 186. That’s the highest it would go. I popped the hood and felt the upper hose. Hot to the touch from the thermostat to the radiator. The bottom hose was ice cold which indicates to me the thermostat was still closed.

Anyone have insight into this design? Do I have it right? Every other car I’ve worked on with the stat on the upper hose, it’s cold until the stat opens then gets hot. But the Mazda, the upper hose is always hot.

Thanks!

 

[Rand]

Based on your observations I'd say likely functioning as designed.
There is a reason they went away from giving you a bunch of gauges and readouts.. people think they have problems when they dont.

 

[johnmyster]

It doesn't take much coolant flow to keep the upper (engine outlet, radiator inlet) hose hot. The hose itself isn't a very good radiator. It'll be hot with tiny flow or even if the thermostat is presently closed but was open in recent history.

The cold lower hose doesn't necessarily mean the thermostat was closed. It just means the radiator was doing its job and returning near-ambient coolant back to the water pump to be mixed with whatever ratio of bypass water was being dicated by the thermostat...which is functioning as a thermostatic mixing valve.

It sounds like your heater is discharging enough heat to pull the motor below operating temperature when the motor isn't working very hard. I've seen this before, and it doesn't take much to do that on a tiny gas sipper.

 

[BrendanC]

as thermal efficiency goes up the heat rejection rates go down.

 

[eljefino]

The US gets an arctic blast and BITOG gets a "wow my car sure is running cold" thread.

Cover most of your radiator with cardboard. Leave open the part in front of the fan, in case it gets stuck in traffic.

Air going under the hood, even under your oil pan, sucks a surprising amount of heat out of the engine.

My Saturn s-series used to lose lots of heat just coasting down a two-minute hill in gear with the fuel injection cut off. Cardboard helped, a little. And it got new thermostats probably every other year.

 

[Timmastertech]

Thermostat opening according to the factory service manual is 177 to 180 degrees so on a 30 degree day with the heater on it wouldnt surprise me to see it hover around there