hi there. I own a 1998 Ford Taurus SE 3.0l v6 OHV with 150k miles. I’ve had the car for a little over 2 years and have been experiencing a problem with coolant temperature fluctuation. The coolant temp gauge shows that the car temperature decreases to about 1/4 temp on the gauge and back up to operating temp while driving. Vehicle reaches full operating temp at idle. The vehicle maintains coolant temperature while driving at a fixed speed. Temp will fluctuate in stop and go traffic, but never goes above operating range. Temp will always indicate a downward trend then back to operating range. The vehicle did fail to maintain any threshold of operating temperature once, when I replaced the thermostat. The vehicle would not warm up at all. In that instance, I decided to replace with a higher performance thermostat that automatically locked open if the vehicle were to begin overheating. This thermostat did end up locking open about 1 month after installation and had to be replaced. I tossed a cheap one back in and car hasn’t failed to get up to temp ever since. Vehicle has never over heated. It has always just had a weird fluctuation between operating temperature and about 1/4 below. Coolant was brown when I got it. Flushed system with distilled water and replaced with green coolant. Coolant has had to be drained on 2 other occasions due to unrelated circumstances where coolant hoses were in the way of other work needing done. Always replaced with new coolant and burped system on an upwards incline for about 30 minutes. Had the car tested for any leaks in the cooling system and air system. No leaks found. This problem doesn’t really seem to affect the vehicle. Like I said, I’ve been driving it this way since I’ve owned it. I’ve not had the opportunity to actually measure the coolant temperature with a scan tool. My OBD scanner does not provide live data. However, I did replace the temp sensors and the problem persists. However, it’s a minor annoyance that I’d like to get sorted if able.

thanks,

brandon