I purchased an Expedition the summer before last. well everything went fine up til a few weeks ago when the radiator stopped flowing. I changed out the radiator and flushed the block out with water. Pulled the coolant tank out and had to use gasoline to clean out three types of radiator block. washed it out with soap and water put everything back together took it out for a good test drive and it passed with flying colors.
Now the problem the next day I took it to the store and the new thermostat was stuck closed! it did free it self up when the temp skyrockted and I shut it down for a few minutes. What kind of flush is out there to get rid of the leftover sealant that seems to still be in the engine block?

 

Nothing will remove it all and if it did you would find out why it was placed there. You may have to run without a thermostat until enough sealant leaches out and you can flush again and reinstall new thermostat. Heater core is probably not putting out much heat anyway for MO. Watch intake manifold for leaks.

 

You might have had an air bubble in the system , rather than a stuck thermostat.
The purging of air in these can be problematic if not done properly.

 

What is the proper method? By the way the year of the truck is 2000.

 

IMHO, take the radiator to a radiator shop, have it boiled out, pressure & flow tested and repaired if needed. Done & fixed.

 

The radiator is brand new along with the thermostat I cleaned out the coolant tank with gasoline since it was out. the engine block seems to be the problem all the left over radiator block sitting in the cavities. I'm not about to use gas in the engine block to get the rest out so for now I'm still scratching my head as to what to use. I'm going to take out the thermostat and put some small dents where it seats to prevent the radiator block from sealing it shut and put it back in until I figure out how to get the garbage out of the system. So I am still open to ideas to get this stuff out.

 

I have heard , never used , vinegar and distilled water is good for cleaning .
Honestly , I have had very good results using commercially available radiator flush.
In your case , I would remove the radiator and thermostat, make a loop to connect the radiator hoses , and run it up to temperature and drain.
The key is to reverse flush .
Also make a loop to delete the heater core as well to prevent clogging that.
You will need to do this cycle several times .

 

Either a radiator shop can use a mild corrosive to clean out the engine (its not expensive) or you can buy a few gallons of off the shelf vinegar (5% solution) , fill the radiator...run the engine just long enough to circulate...let sit overnight...remove lower radiator hose & drain/flush.