where is thermostat located

how to change thermostat
can someone tell me how to change thermostat in 1995 transport SE 3.1 ....this is what is happening.....when engine gets hot and i stop the engine, white sudzy stuff comes out of the top of the radiator overflow jug...now this is when i turn engine off...also noticing that heat in van works sometimes and not others...the water and antifreeze in the overflow jug looks like its bubbling or bowling...can someone help...am i doing the right thing by changing the thermostat and where is it in the van...thank you.

ps i already put new rad cp on...thanks

THis lets me thing of air in the cooling circuit, thus airlock. Air in the heater core , even hot, does not carry a lot of heat. And changing the thermostat will not do better....and....I hate to write this given all what you posted about this car....may be it is not "air" , but CO2.......you know, the stuff they put in coke to make it bubble...that byproduct of combustion......exhaust I mean....ehhh....blown head gasket.......
If this is the case, take off the radiator cap, make sure it is filled to the rim, and let the engine run...and look: stray exhaust would pop bubbles and bubbles from the radiator....
Sometimes (?!?) removing the valve covers and tightening the head bolts will alleviate the problem.

Anyway, I wish I'm mistaken; thermostat is near distributor

www.avigex.ca/xport/thermostat31.jpg

and the bolt on the top of the housing (not shown on drawing??) is what one removes when filling up the coolant to let the air escape .