is my thermostat not opening?

i was going to work today and pull over white castle drive through to get some burgers. i was sit idling waiting for my food and notice the coolant temp was up at the middle mark which is pretty high. ok let me explain on my stock coolant gauge before i get you confuse. i don't know how to read this damn line on the gauge, but anyway the lines start 1/4, 1/2, 3/4. between 1/4 and 1/2 mark would be around 180* which is where the needle suppose to be on. i was sit idling no more than 5 minutes and the needle was moving little pass the 1/2 mark. the temp outside was 53* today. during the summer time the gauge never gone up to the 1/2 mark before. i turn on the heater and it seem to drop down little bit, but still little high. when i got to work the needle was up close to the 1/2 mark. does this sound like the thermostat is not opening properly? should i go ahead and change the 'tstat.

i had a similar problem it ended up that my overflow tank cap was cracked causing air pockets in my radiator which caused it to go to about the same temp

now that you brought up about the overflow tank that might be the problem. the overflow tank hose or around there on mine is leaking too. i think when i put the stainless steel sleeve on the hose i over tight the clamp and it must cracked the beak on the tank where the hose go in. either that or the hose itself must leak cause when i put my finger under it i get coolant on my hand.

Like you said, you have no idea how hot the car actually is because the temp gauge on the dash is both highly inaccurate and hard to read. Best bet is to have the car scanned to determine what temp the car is really at. My gauge is so slow to move compared to the actual reading by the PCM, it's crazy.

normally when a Tstat sticks.... the temp rises...and rises. and when it finally does open you will see the temp gauge go WAY down.

if ti does not open... you would be able to hear te coolant boil out and go into the over flow tank...and that would be pretty loud. I have had a few stick in my day. also the smell of coolant everywhere gives it away.

check the caps first!

Check and see if your fans are cutting on also.

my GP ... ALWAYS stays at the 1/2 mark! nothing ever higher though.

i think i found the problem. i change the overflow tank hose and refill coolant. i used 1.5 gallon. i don't know how i lost that much coolant. drove around town for about an hour and the temp gauge stay on the 1/4 line which is about normal where it's suppose to be. thanks guys for the input!