Acadia's Bizarre Hot/Cold Temperature Control Mystery

Having worked on a similar issue with my 2006 GMC Envoy, I can share some relevant experience. As someone with moderate car repair experience, this sounds very familiar. The symptoms you describe match exactly what I dealt with, inconsistent temperature readings and that frustrating combination of slow warm-up and overheating. After checking the cooling system thoroughly, my issue turned out to be a faulty map thermostat. The temperature sensor was giving incorrect readings to the ECU, causing these confusing symptoms. The repair involved: Complete diagnostic scan, Cooling system pressure test, Replacement of the map thermostat, System refill and bleeding Total cost was around 350 Euro including parts and labor. The shop completed everything in about 3 hours. Key signs that pointed to the map thermostat: Erratic temperature gauge behavior, Cooling fan activating at wrong times, Inconsistent heat from vents, Engine taking longer than normal to reach operating temperature Before suggesting more specific advice, could you share: Current mileage on your Acadia? Any recent cooling system repairs? Are you noticing any coolant leaks? Does the temperature fluctuate while driving or mostly at idle?

Thanks for the detailed response. My Acadia has 64905 KM on it and just had its regular service last month. No coolant leaks visible, but temperature fluctuates most noticeably during highway driving. The cooling fan seems to kick in out of nowhere, sometimes even in cool weather. After reading about your experience with the map thermostat, I checked the service records. Interestingly, the temperature sensor was never replaced during any previous maintenance. Wonder if starting with a sensor replacement might be more cost-effective before moving to a full thermostat job. The cooling system pressure seems normal, but those inconsistent readings really match what you described.

Thanks for those extra details. The symptoms youre describing continue to align closely with what I experienced with my Envoy, especially the unusual cooling fan activation. While replacing just the temperature sensor might seem more cost-effective initially, in my case this approach actually ended up costing more in the long run. After initially changing just the sensor, I still faced engine overheating issues and the same erratic temperature control behavior. The water pump and entire cooling system were working fine, but the root cause was indeed the map thermostat. These components work as a system, and when the thermostat fails, it can make the temperature sensor readings appear faulty. Based on your mileage and symptoms, particularly the fluctuating temperatures during highway driving, Id strongly recommend getting the complete thermostat assembly checked. In my repair experience, the full fix cost 350 Euro but solved all symptoms permanently. The diagnostic steps my mechanic took which might help you: Computer diagnostic scan while engine running, Temperature sensor resistance test, Cooling system pressure check at operating temperature, Thermostat operation verification The engine overheating combined with slow warm-up times is a classic sign of a failing map thermostat rather than just a sensor issue.

Thanks for sharing that perspective. Your experience definitely helps narrow down the likely causes. Given the similar symptoms and the unusual cooling fan behavior, I think you're right about going straight for the thermostat replacement rather than just the sensor. No point wasting money on partial fixes. The engine overheating issues during highway driving particularly match what you described. Since my coolant levels and pressure are good, focusing on the thermostat makes sense. Getting that 3-hour repair done for around 350 Euro seems reasonable if it fixes both the slow warm-up and overheating problems permanently. I'll schedule the diagnostic tests you listed and have them check the complete thermostat assembly. The cooling system clearly needs attention, and this sounds like the most efficient path forward. Will get this sorted before the overheating potentially causes any serious engine damage. Thanks again for walking me through your experience. Time to get this fixed properly the first time.