Thermostat wont open, help!

[calypsocowboy73]

Hey all, brand new to the forum, I have a Ford F-150 4x4 shortbed with a 302 in it. I have recently tried putting a new thermostat in, and it wont open up at 180f. I tested both the thermostats I have, and they tested fine. But when I put it in the truck it wont open up, all the coolant in the radiator and hoses stays cold and the block gets hot, and I have had the block up over 180f. Then when I take it back out and let it free flow, it works fine, accept obviously I don't have any heat and I cant get it up to operating temp. Everything flows fine , heater works fine but I cant put the stat in. Someone told me that it could be some sort of vapor lock , like air getting in somewhere and making a pocket behind the stat somewhere and not letting the stat open. Anyone have any insight on this, Im at a dead end. Thanks

[Ultramagdan]

It does sound like vapor lock. Drive her up on ramps and let it idle with the cap off (heater on). Watch for air bubbles and add coolant as needed until it stablizes.

[stuart1]

Do you have it installed correctly?

If so you can drill a small hole in it to let air escape and properly fill the system.
I have also heard of placing an aspirin under the element which disolves and does the same.