temperature issues...

I think I need a thermostat, I just don't want to go about replacing something I already got.

So, just before it started getting cold out I got to thinking that perhaps my car running cool all the time wouldn't be a good thing. It came from the south and I knew they had a cool thermostat in it. The highest it ever used to get was maybe about 3/8ths of the way up the gauge. But, often the car would run much cooler, even on the highway. I was told by some afficianados that level is where an OEM thermostat should keep the gauge reading.

So I drained just enough of my 4 month old antifreeze through the drain cock to get the thermostat out easily, swapped it, and away I go.

Now it runs hot. It doesn't steadily run hot, on the gauge. After it comes up to temperature it over shoots to just below the white line below red. It settles back down to level and then goes up again for a little while. After it's been running for some time then it will keep a constant temperature.

However, if I check the actual feel of components, the rad, and the expansion tank, they are cold. I should think that getting close to overheat these would come up in temp. Furthermore, there is never any pressure in the system. So, I replaced the expansion tank cap. That didn't help. I don't see any leaks anywhere and I'm not losing fluid (would it be possible for that drain cock to be just lose enough to vent gas but not liquid?).

Anyway... any thoughts would be appreciated. I've ordered another thermostat.

Oh, and I've checked for blowing heat. That seems to be 'ok'. I've had people tell me that the heater should blow very hot on full blast but it doesn't. It heats the car ok in -5°C but I'm worried about when it gets really cold

One final thing, in very hot weather the car did this with the old thermostat but changing the coolant 'fixed' things.

(I ruled out airflow, it can happen city or highway and I've cleaned things off... besides, it can run correctly after it's been on for awhile)

(oh, and one more thing... the idle hunts a bit when it's cold out. The engine actually has to start coming up to temp for this to occur and it's just during a very brief segment of the warm up cycle. But, I think that's unrelated. I only add that not to get that diagnosed but just in case it might be relevant.)