My Understanding is that tey injected Propane into their test tank to try and prove their theory, and even that didn't work.
You might notice that they didn't ground all those planes as one would assume they should.
I don't know where you picked up that understanding, but it's mistaken. EDL at CalTech (who are a bunch of scientists who know about explosions, or, if you are a conspiracy tool, a part of the vast conspiracy) were able to make many explosions, including some really spectacular ones.
This is their site: http://www.galcit.caltech.edu/EDL/, this is their Jet A explosions home page: http://www.galcit.caltech.edu/EDL/projects/JetA/background.html, and this is their "misconceptions" page which is probably a good place for people who have only heard the conspiracy side of things to begin: http://www.galcit.caltech.edu/EDL/projects/JetA/misconceptions.html
The "Misconceptions" page addresses the myth you refer to, an article of faith in the toolosphere, and provides links which include, among other things, pictures of the explosions that somebody told you didn't happen. Video too! You might notice that they didn't ground all those planes as one would assume they should.
As I have said, over 40 Airworthiness Directives were issued as a result of this accident. Grounding is not required if an equivalent level of safety can be provided in other ways (for instance, by keeping enough fuel in the tank so that the fuel-air mixture is always too rich, and not ever inflammable). While no fleets of aircraft were grounded, all Boeings had their operational procedures changed, and over time, all that had Kapton insulation (among others) had extensive, expensive rebuilds. If you've ever rewired a house, think what rewiring all the systems in a 747, with at least four different voltages, entails!
d.o.l.
Criminal Number 18F