Sporadic temperature gauge 3.6L

[Rohab]

Hi everyone, recently I've noticed that the temperature gauge on my 3.6L, which usually stays solidly at 90c, has started going above 100c. it appears to happen approximately at the same time, about 10-15 minutes after setting off. Once the gauge starts going past 90c it will jump up and down between that and around 110c max. It hasn't overheated yet and from what I can gather, no matter my driving behaviour, the gauge does the same thing, jumping up and down between 90 and 110c. I'm not getting any codes or temp warnings and my coolant level remains normal and unchanged. from my research my thinking is the thermostat/thermo-regulator is starting to go but I'd like some input from those more seasoned than I.

regarding the thermostat replacement, I've found a thread on the R36 section that give a part number and diagrams, but any input on the method for installation would also be appreciated.

[MIG]

What does your oil temp look like when the coolant is hot? The default way the cluster shows temp pretty much locks the temp at 90c and only moves if it goes way past that; using a scan tool, if you have access to one, will give you a more accurate reading.

It could be a few things,

Temp sensor
Thermostat
Thermo fan or fan controller
Water pump

A bidirectional scan tool can trigger the fans to test them.

[Rohab]

I did some measuring, the oil temp didn't go past 107c and the coolant temp got to about 115 max. This was a run between eight mile plains and wacol so about 45 minutes of stop/start and highway driving. the temp only really climbed past 110 after I got to those highway speeds. I didn't have the opportunity to go full throttle, but I was able to open it up a little in some parts and that temperature reading never went past that 115 mark. also just like the drives previous, the temp would drop a couple of times suddenly after it had gotten to that max temp. now that I've actually measured those temps it seems the first drop goes from 115 to about 112, then again to 108, sometimes (rarely) it'll drop to 105. rinse and repeat until I get to my destination.

I'm not sure if my scan tool can trigger the fans (it's the VW/Audi scan tool from OBD2 Australia) but they definitely work. once the gauge goes past that 90 mark they go overtime and can be heard from inside the cabin. they also still do that thing where they stay on for a minute or two after turning the engine off.

[MIG]

That oil temp is too high, mine will sit between 95-100c in normal driving and not go over 110c when doing back to back LC starts.

The next place I would be checking, is the thermostat as the OEM part should open at 90c

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[Rohab]

after much effort I managed to get the thermostat out by attacking it from underneath. I was not expecting it but it came out in 3 pieces, nothing was broken and it went back together. it seems like it's designed to be able to come apart. it was pretty crusty so I thought I would try and clean it up a little to see if that would 'free it up' and get it working well enough while I ordered a replacement. I also dunked it in some boiling water to see if there was any movement. there was none, but in hindsight I don't know if these thermostats move much if at all while they're not in the car.

I then put it back into the car (after much time and swearing) and filled it back up with coolant. once it was buttoned up I took it for the same drive that I explained in my last reply including an additional 1.5hrs over the afternoon and the temp gauge didn't budge past 90c once. I have yet to plug the scan tool in to check the specific temperatures but I think it's a good sign that I'm no longer getting the sporadic gauge movements like before. however I will be ordering a new thermostat soon as I obviously can't rely on the old part anymore.

I also have a feeling that the coolant has helped a bit as I'm not sure when it was last refreshed. it ended up taking over 5L of coolant so the majority of the capacity is now new. I'm still keeping an eye on the level as I'm not convinced that it's 100% full yet.