Based on your photo I would say that spring and surrounding area is loaded with dirt. I could see that coating taking longer to heat the spring(higher temp) and causing it to stay on longer because of the insulation of the dirt. The photo looks white like salt not black like oil. In any case I would clean it as I described in my first post. When I cleaned mine it was clean compared to your photo, just some rust. Again you radiator is 30 years old I would check that with your IR gun once the engine is at operating temperature. I would shoot it from top to bottom in a 2 in grid and record the temps. Several mechanics have told me when the radiator start to plug up it usually happens in the center under the fan blades. If plug these readings will be cooler. You should see a delta T from top to bottom of about 50 degrees or better if your radiator is working properly. In the 10 years of ownership have you cleaned the fins or between the condenser, or chemically flushed the radiator? If you do go with a new fan stay with the OEM and make sure you have the right one. Some are HD, and some are lighter duty. Some engage at lower temps which you won't like.
