After reading some forum threads on the issue, I was thinking that the problem was likely a bad fan control module, which on this car unfortunately comes as part of the whole fan assembly. I was planning to try removing the purple wire and checking whether the fan still ran (if I understand it right, the fan continuing to run in that case confirms that the fan control module is bad). But before I got around to trying that, the fan stopped turning on as soon as the power connector was plugged in.
I took it for a test drive up a long steep hill. The temperature climbed to mid-scale as expected and then stopped climbing. I stopped at the top of the hill and checked the fan; it wasn't running. While I was stopped and watching the temperature gauge, it suddenly (over maybe two or three seconds) jumped from mid to full scale. I headed back down the hill, hoping that the airflow through the radiator from the car moving (mostly coasting downhill) would cool it down.
While headed downhill, I fired up a code scanner (generic ELM327 1.5 Bluetooth adapter talking to the MotorData app on my Android phone) and checked the engine coolant temperature. It varied from about 186 to 192F (85 to 89C) as I drove, which if anything seems a bit on the low side. But the temperature gauge on the dash stayed pegged as if it was completely overheated.
When I got to the bottom of the hill, the gauge was still full scale. I turned the car off and the fan continued to run for almost exactly 6 minutes. I turned the key back to the run position (without starting the car). The fan started running right away. The code scanner read 178F (81C); the temperature gauge on the dash still read full scale. I started and stopped the engine; the fan ran for six minutes; the gauge remained at full scale.
I turned the key back to the run position to reconnect the code scanner; the fan immediately started running again. The code scanner showed a coolant temperature of 171F (77C). The temperature gauge started at minimum but over the next about 30 seconds it gradually rose to full scale. I turned the key off and the fan ran for another six minutes.
The fan shutting off after six minutes suggests to me that the fan control module is now working. The fan running whenever the key is on and the temperature gauge on the dash going to full scale seem to suggest that the engine coolant temperature sensor isn't working. But the temperatures shown by my code scanner make sense and don't indicate that the engine is hot. Is there more than one ECT? Should I replace the one by the thermostat?
Steve
