Lets talk your LP.
If you replaced the ECO and Tstat set and it continues to run UNTIL the ECO trips, then either you have a bad tstat (VERY UNCOMMON for a new to fail), or the tstat is NOT secured to the metal of the tank to accurately sense the temp. LONG shot would be you installed a EXT set of tstat and ECO. The EXT tstat is 160 degrees, but does NOT have the same connections on the tstat. You would have had to modify the connections to make it work. I would also wait and see how hot the temp gets and see if the ECO trips it off or the Tstat finally opens at a hotter temp. Doug
