Seams like you have some air bubbles in the system. Definitely remove the cap and let the car warm up with it off and bleed air from the bleeder screw.

Thermostats are a on or off sort of part. Meaning they either work or they don't If they fail they either fail open or closed if they fail open your car wont reach operating temp if its anywhere below 70 degrees outside.

If it fails closed it will obviously over heat. An easy way to test the thermostat is to remove it and place it in boiling water. If it opens its good if not you know its bad.

Good luck!

Jason

 

To burp my 2.8 after a G12 flush, I had to take the cap off and get the coolant up to temp and then hold the RPMs at 3k-4k until the system burped the air and suddenly sucked all the coolant out of the reservoir. When the coolant light triggered, I immediately shut off the motor and re-filled the reservoir.

All set from there on. Two of the three bleeder screws on my car were seized, but this worked.

 

did you do this with the bleeder screw opened up? cuz i did it but with the screw tight just the reservoir open and bubbles would come up and alot of water too and then i turn it of and all the water would go down and ill have to refill it again. i already wasted 2 gallons