Weird thing happened today in this weird cold.
Had been driving about 6 miles at 50 and the car was still not warmed up fully per the gauge. It finally appeared to make it. Then about half mile later, the gauge dropped back to half up to temp. Then started to slowly work back up.
my theory is it reached operating temp, the thermostat opened up, and the coolant was so cold, it dropped the coolant temp back down. Which is weird. And potentially damaging (quick temp changes & aluminum don’t mix).
I am just guessing. I don’t know where the gauge gets its temp from? From the coolant in the engine? Or on the radiator side? Any thoughts?
im in Michigan & we r talking temps of 2 degrees, with wind chill 15 below.

 

I think that's exactly what happened, I used to see that quite often on my old Dodge diesel.

I wonder if thermostats get slower to react as they get older? I wouldn't worry about it unless it keeps happening after you get past this freezing weather.

 

Your explanation of why temp gauge acted the way it did is certainly plausible. I'm not sure where the temperature sensor is located on the 3G RDX - I looked a little on line and found conflicting info.

 

This happened once a few years ago with the 2019 RDX I had during cold temperature. Hasn’t happened with my 2023. It was really odd to see at the coolant temp reading drop and then start to rise again…

 

It would be good to know where the temp sensor is. Based off how long it takes for the gauge to move on a cold day and the slow way it climbs, I wouldn't be surprised if its not until after the radiator after the coolant has had time to go through other various heat exchangers. On mine I tend to notice the temp drop in very cold conditions if I stop, especially if I have the HVAC on high fan speed. This is quite different from a Volvo in my household that would make you think it has a super heater based off how fast the coolant temp appears to rise in that car. I suspect that sensor is probably in a hotter part of the circuit.

 

Had a similar thing happen to me the other day. 15 minutes into the drive, looked down and saw that my temp gauge dropped way down, only one or two digital notches above the bottom of the gauge range. I was also in single digit F weather, but with the added situation variable of coasting downhill for several miles (coming down a mountain pass). My heater was still blowing warm air, so I wasn't too concerned. Less heat production from low engine load + extra cooling from cold air and running the heater = lower coolant temps. 10 minutes later, after using my accelerator pedal a bit, everything was normal again.

I doubt any damage is being done by the temperature swings. The low end of the temp gauge range (once it's off the very bottom of the gauge) isn't cold like the winter air outside, just cooler than normal engine operating temperatures.

 

This does happen and is normal for RDX. Its a cold coolant getting to thermostat when it opens.
If temperature difference is very big, it could be damaging. Ive never seen this with other cars, so must be a weird design

 

With temps in single digits these days in New England, I've noticed the temp guage bob up and down slightly.
It does that less and less as the engine warms up, but it takes most of my 30mile highway commute.

 

Your assessment is probably exactly it, the ECT sensor is likely not anywhere near the radiator as that would not be an accurate temp reading.
Probably near thermostat housing.

 

It looks like there are two temperature sensors, one at the bottom of the radiator and one at the thermostat case. You're most likely right that the display is getting it's reading from the one by the thermostat, I wonder what the other one is used for?


https://www.acurapartswarehouse.com/...ature%20sensor

 

Now I'm about to jack this thing up and look! LOL

 

It’s still doing it, even now that it’s “warmed up”. I also notice the heater fan changes sound around that same time. It goes from a standard fan noise to more of a furnace roar, like it got a boost

 

For what it is worth, AI says

“The Engine Coolant Temperature (ECT) sensor on an Acura RDX, often referred to as ECT2 when located near the radiator, istypically found on the bottom of the radiator, usually on the driver's side. It is positioned below the fan housing and may require removing the lower valance and a cover to access.
Key Details regarding RDX Temperature Sensors:

• ECT2 Location (Radiator): Drivers side, bottom of the radiator.
• Access: Requires removal of the lower splash shield/valance.
• Primary Sensor (ECT1): Often located on the engine block, cylinder head, or thermostat housing.
• Symptoms of Failure: Check engine light (P2183), malfunctioning cooling fans, or engine overheating.
• Alternative Sensor: The ambient temperature sensor, which feeds the climate control system, is located behind the front bumper/grille area.”