Car Tech 101: Car temp ok when moving, but too high when sitting still = problem with fans, or fan/airflow. Whether it's a Ferrari or a Honda. The temp is lower when moving because the airflow is doing the job that the fans aren't. You may have other issues with leaks, belts, water pump, etc - I've not seen your car, so can't say. But I'll point back to my first sentence as a starting point.
My thoughts exactly except: Yesterday: While driving slowly in stop go traffic, temps rose rather high. I pulled out of traffic and went another direction at normal speed - temps came back to normal. Verdict: fair or airflow. Today: Gauge shows 90C, radiator shows max 66C and both fans turned on. Temps kept rising (idling at rest) and fans turned off while temp kept rising. Then later while driving at normal (~50mph) speeds, temps rose rather high, and then plummeted rapidly (in 2 seconds) to normal temps. Verdict - not fan or airflow. It's not as clear cut as it seems.
That sounds a bit like air bubbles in the coolant. Rapid shifts on the temp gauge could be air pockets passing the sensor. That can also impact the sensor that controls the fans. Air in the system could be a leak, somewhere, low coolant in the tank, or a bleed valve not shut tight ... or air in the heater core that was released when the heater was turned on. I had to replace my radiator bleed valve, as the old one had trouble sealing. (And then I wound up having to replace the radiator, when it sprung a leak.) Diagnosing cooling problems needs to be fairly methodical.
I have an EZBleed screw I can install, and could bleed the system of air if it's there. Might be a Sunday project after other commitments.
As has been noted, there are several possibilities, from very mild to major. Frankly, since the car was recently serviced, I would contact the mechanic involved and get his views, especially if the car did NOT exhibit these symptoms before servicing AND the service included ANYTHING involving the cooling system. I'm sure the mechanic would be interested in hearing about the problem and would wish to take a look at the car and fix it..
"Then later while driving at normal (~50mph) speeds, temps rose rather high, and then plummeted rapidly (in 2 seconds) to normal temps." Well...this is a classic symptom of a blown head gasket. But I learned from experience on my 328 that an improper rad cap can cause the same symptom due to the way the cooling system is designed/location of the coolant temp sensor. In addition to the symptoms described: Are you getting any coolant "spitting" from the expansion tank vent? Are you losing coolant in some other way? IOW, is the coolant level when cold today the same as it was when cold a week ago?
I've looked at both the oil and water, and no evidence of either in the other. And Brian replaced the radiator cap a couple of months ago as he didn't like the one in there. Not noticed any spitting, nor any loss of fluids. There are no signs of fluid loss on my garage floor, and I've looked and not seen any fluid from spots I've parked
"In my limited mechanical experience I'm thinking it's thermostat, sender or gauge." Could be, of course - Easy enough to check that with an IR temp gun - just shoot the Tstat box and see if the change in temp readings as the engine warms up correlate between the gauge and the IR gun. Don't worry about specific readings, you just want to see that the gauge goes up as the IR gun temp goes up and goes down as the IR gun shows it going down.
I don't believe the last service touched anything in the cooling system - warm up regulator and oil change. Other than the radiator cap, the cooling system is untouched in my ownership.
I think perhaps tomorrow I will install the new EZbleed screw in the thermostat housing, and then use the IR gun to monitor this, while following Birdman's bleeding tutorial. If nothing else I might be able to eliminate air bubbles from Brian's list when I hand it over.
Rather than the quick bleed screw, get one of these installed. Get the one with attachments for correct thread size. https://www.ebay.com/i/283647709587?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=283647709587&targetid=313776934696&device=m&mktype=pla&googleloc=9009971&poi=&campaignid=1669934606&mkgroupid=65058347459&rlsatarget=pla-313776934696&abcId=1123856&merchantid=6296724&gclid=EAIaIQobChMIjrnM6LOt5QIVuIVaBR2C7Q0KEAQYASABEgJDzvD_BwE This may solve your problem and at the very least, it will tell you if it’s a air-in-the-system issue. The 3x8 series is famous for sucking air back in the system at shut off from what I’ve read.
It wasn't when it was new. People didn't have to continually take their 3x8's to the service dept to have the systems bled. That tells us that sucking air back into the system is a result either from leaks in the system or incorrect replacement parts (radiator cap, for example).
"Whatever the reason, the auto bleeder works." I can't help but think that if it eliminates the symptoms, it's disguising a problem, not fixing it. Admittedly, trapped air could simply be due to the system not being properly bled and the bleeder allows the trapped air to leave. In that case, of course, the problem it is "fixing" is a one-time maintenance-related issue, not a problem that is constantly allowing air into the cooling system.
I totally get your point. I’ve had this issue as have many others through the years. My system has been vacuum tested and has no leaks. It’s like the car sucks air into the system through the overflow hose when the car gets turned off. Just my hypothesis and others have written about the same thing in the past. One guy installed a canister full of coolant and kept his over flow hose submerged in it for the reason.
34 years. Only time it sucked air back in is when it blew some coolant out due to a bad radiator cap. Never had a problem bleeding the system either. Never needed to "rebleed" over and over.
