The engine needs to be cool or cold when checking leakdown. When checking the fuel pressure, the pump is running continuously, and the regulator is regulating the pressure, and the fuel is constantly circulating through the lines. When the pump shuts off for checking leakdown, the fuel in the lines doesn't move. Heat radiating off the engine will cause the pressure to rise. If/when the pressure rises above regulated pressure, the regulator will send the excess pressurized fuel back to the tank and skew the results of the leakdown test.

Fuel pressure and leakdown are critical on this engine, a couple of psi can be the difference between starting and running properly, or dead in the water. Ambient temperature and the condition of engine tune plays large part as well. A well tuned warm engine may start and run fairly decent with ~58psi at 80F. A poorly tuned cold engine might not start at all with 58psi at 40F.

 

Vapor pressures vs temp that different under compression after atomized to cause a no start?

 

Jeff, are you thinking vapor lock? The pump pressurizes the lines, injectors, (poppets if equipped) and up to the regulator valve plate. When the valve plate opens, any bubbles are bled out instantly. That's the beauty of fuel injection, no more vapor lock