Bubbling Sounds on hose that goes from engine to radiator?

How do you know what the thermostat is doing? The bubbling is a symptom of a leaking cylinder head gasket. You will often see those bubbles going into the coolant reservoir while the engine is running. It looks like the coolant is boiling, but there won't be any steam.

Your mechanic can do a chemical test at the radiator to verify the head gasket is leaking. If it is, coolant will also sneak into the combustion chamber, be burned, and go out the exhaust as white smoke.

You don't fix anything by ruining the thermostat. Only two things will cause it to not open. Either it is defective and should be replaced, or air is pooling under it from that leaking cylinder head gasket. Thermostats do not open in response to hot air. They must be hit with hot liquid. Combustion gases that leak into the cooling system commonly cause overheating because they cause the thermostat to remain closed.

I can improve on my first reply. I should have said coolant CAN sneak into the combustion chamber and be burned, but that is only one potential symptom. If the leak is small, the normal 15 pounds of pressure in the cooling system may not be enough to cause much coolant to be lost. A little may dribble into the cylinder after the engine is stopped, but that temperature and pressure go down so quickly that it could take weeks or months to see the drop in the coolant reservoir level.

I'd still have the chemical test done. It just takes a minute. With the engine running, your mechanic will draw air from the radiator or reservoir through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, that liquid will turn bright yellow.

If that bubbling is occurring before the engine gets warmed up, you know it isn't boiling. That air can only come from one place, ... The combustion chamber, due to the leaking head gasket. The chemical test at the radiator, in this case, would be a verification test, not a diagnostic test.

I'm not convinced about a thermostat being heated in a pot of boiling water. No professional has the time to do that, and I did see a mechanic next to me get fired for doing that, (it was the straw that broke the camel's back after a series of things he did after numerous warnings). A wax pellet has to melt, a thermostatic spring has to relax, then you need the push of the coolant from the water pump to push the valve open. I'd be willing to bet a new thermostat also won't open because of the air that is accumulating. If I'm right, you can run a piece of small wire through the thermostat's opening to prevent it from sealing tightly. That will allow the air bubbles to wash through and the hot liquid will get to the thermostat and cause it to open fully. Instead, a lot of import engines use a thermostat with a tiny bleed hole already in the plate. They have a little weight dangling from it. That is there to allow any air to escape so the liquid can get to the sensing element. If your old thermostat had that bleed hole and the engine still was overheating, then it may indeed be defective.

If you do indeed need a new thermostat, they only cost a couple of bucks. If you can't afford that, how do you afford to put gas in the car? Dollar-wise you're better off in the long run by putting a thermostat back in. When it's missing, the engine will run too cold and parts, mainly pistons, won't expand to fit properly. 99 percent of engine wear takes place during the few minutes the engine is warming up. Once it's at the proper temperature very little additional wear takes place. You'll eventually spend hundreds of dollars on the engine by saving a few dollars on the thermostat.

As for that chemical test to verify a leaking head gasket, many auto parts stores rent or borrow tools so you can do it yourself. Around my town most of them will let you use the tool for free but they make you buy a bottle of the special liquid. That's because the liquid will be contaminated and rendered ineffective if any coolant gets into it. That would make it unusable to the next guy. Someone would have to replace the fluid, which costs money.

You might also look for a nearby community college with an Automotive program. The instructor will likely send a student out to do the chemical test for you. It just takes a couple of minutes. Keep in mind though you must fit into their schedule. If they're in the classroom when you show up, you'll have to wait or come back on a different day. They would stop what they're doing and perform the test as a courtesy for you, but only when it doesn't interfere with their learning activities.