Looking for a little help with a creaping temperature gauge. Following the installation of a 180 thermostat a few months ago, I noticed the temp gauge rising above 200 degrees and then coming back down to 180. I started with flushing the system, installing new upper and lower hoses, a new radiator cap and new Dextrol anti-freeze (50/50 mix). I also pressurized the system to see if there were any leaks (none). When that didn't fix the problem, I decided to roll up my sleeves and figure it out. Suspecting the cooling fans were not kicking on early enough (they do both work), I replaced the cooling fan resistor/thermostat (a device with two ceramic blocks wired together and mounted to the fan bracket). When that didn't work, I replaced the coolant temp sending unit (three wire connection) located on the block under the throttle body. It was pretty toasted and the connection side crumbled as I removed it. I thought I had finally identified the problem so with much anticipation I replaced it, insulated it from the heat of the exhaust down pipe and took the car for a test drive. It was 15 degrees outside last night and althought the temp gauge stopped rising right at the center position (200 degrees) after about 10 minutes it continued to creap up to about 220. Shortly after hitting 220, the temp quickly dropped to about 190 and started a slow creap back to 220. It continued to cycle like this until I drove it home, muttered a few curse words and stomped into my house.
Anybody got any ideas I haven't already addressed? Don't think its the gauge (don't shoot the messenger) and I still suspect the fans are not kicking on at the appropriate time. Is there another sensor somewhere which has an affect on the timing of the cooling fan activation? Thanks

What are the speeds that you were going? I would think you shouldnt get anywhere close to 220 with 15* weather outside. I havent hit 190* since October.

Well if you're driving above 30-40mph the airflow should be more than the fans would put out anyway (I don't believe the fans work at that speed anyway) so in that case it may be a t-stat issue. If you're constantly driving above about 35mph and it still does this, you have an issue. In 15* weather I would assume even 20-25mph would suffice. If you notice this at idle or below those speeds, its 'normal' with a 180 t-stat because the cooling system needs a t-stat AND fans to maintain a temp, and the fans still come on at the normal temp.

From your story it sounds obvious you were driving faster than that, so as said I'd look at replacing the t-stat. Is there any way the parts man gave you the wrong piece? T-stats do go bad, some straight off the assembly line.

yeah, it really sounds like your t stat is opening and closing over and over.