How do you know you is the thermostat is causing overheating?

This is the primary cause of engine overheating. Possible leak points include hoses, the radiator, water pump, thermostat housing, heater core, head gasket, freeze plugs, automatic transmission oil cooler, cylinder heads and block. Perform a pressure test.

Common Causes of Overheating ' ASC Industries, Inc.

asc-ind.com/cooling-system-information/common-causes-of-overheating/ https://www.google.co.uk/webhp?sourceid=chrome-instant&rlz=1C1CHBD_en-GBGB719GB719&ion=1&espv=2&ie=UTF-8#q=engine%20overheats

Hello Martin, I have seen someone install a thermostat upside down . This causes the thermostat to not open . Was the engine overheating before you replaced the water pump and thermostat ? If the engine has been excessively overheated, this could cause a blown head gasket .

Check your antifreeze level and your thermostat. Low antifreeze will cause overheating, sticky thermostat will also cause overheating.
Check your water pump. The internal pump fans may have come off

sounds like coolant and that vehicle overheated or now has a leak ---and replacing plugs usually doesnt correct an overheating problem and if told had bad thermostat [ which will cause overheating ] i would have system tested for leaks then look into thermostat as a problem to your overheating problem

Stick with the OEM headgasket from the toyota parts department, you are correct in assuming that there is another problem that caused the overheating in the first place. Overheating is usually caused by an external coolant leak or a blockage in the cooling system, possibly a partially restricted radiator, stuck thermostat.

Most common overheats are caused by low coolant due to a leak somewhere in the cooling system. Second to that is a bad thermostat, obstructed radiator (internally or externally). A cooling fan that is not working can do that too. Less frequently, you can develop cooling system leaks inside the engine...cylinder head gaskets or intake manifold gaskets will do that.Even less frequently that that is a crack in a cylinder head. Overheating problems can "snowball" due to continued driving. A bad thermostat can be an original cause but can lead to multiple failures if you continue driving and overheating. Same applies to all I've listed as "common" causes. Check everything over carefully and if you have a specific question about anything you find, just ask!!!

remove the thermostat and place in a pan of water a boil and wait to see if the thermostat opens , as for the noise , get the thermostat sorted out and see if still makes the noise , could just have been caused by the overheating, one thing at a time so you know what the problem is

CHECK COOLANT LEVEL FIRST BUT IT SOUND LIKE FAULTY THERMOSTAT OR LEAKING HEAD WILL ALSO CAUSE OVERHEATING WHILE DRIVING.I WOULD REPLACE THERMOSTAT AND RADIATOR PRESSURE CAP FIRST.THAT WAY YOU KNOW IF HEAD GASKET LEAKING.CHECK ENGINE OIL IF IT LOOKS LIKE MILK SHAKE HEAD GASKET BLOWN.

Replace the thermostat. It will be fine after that. I know thats what everyone says anytime there is an overheating problem but in this case with the way the system works and it has been working fine up to this point, it's gonna be the thermostat.

The most common problem with overheating is the thermostat. It becomes plugged and won't allow enough coolant to pass through the system. Another cause could be old coolant. Flushing the coolant periodically (when the coolant appears to have rust in it or has faded in color) is a good idea to keep the antifreeze/coolant up to peak levels. Bost are worth looking into but I'd be surprised (barring any leaks in the sytem) if the thermostat wasn't the culprit here. Best of luck.
Greg