Bubbles in Return Fuel Stream

Definitely a good idea to use up that winter blend before the weather gets too warm...

Winter blend fuels can be up to 15 RVP (PSI) from mid September to Mid April, as soon as hot weather hits winter fuel causes problems.  Summer auto fuels are way down to 9 and 7 PSI and not the issue.   If you're flying in these shoulder months the best Idea is to blend in 50% Avgas.  Avgas is 7 RVP year round and that will pretty much deal with the vapor pressure issue for most aircraft.  If you use Swift UL94 it is 9 RVP I believe.  Be careful with the type 912i and 915i engines, even with fuel injection at 45 PSI, (3 Bar) the return line remember is not under pressure once it leaves the regulator at the end of the line heading back to tank.  That rapid pressure release will always show bubbles in hot weather.  it is important that your pickup point within your tank can not circulate those bubbles back into the pumps, makes for a bad day.  

Cheers

In CA it is common to end up with a warm winter or early spring day where you can get in trouble using the winter Mogas.  You pull in somewhere for a two hour lunch and meanwhile the sun is warming the wings. Then you return to the plane and have 90 or 100 degree fuel in the tanks.  In my airplane when this happens you can hear the difference in the sound of the pumps as they are in the early stage of cavitation - they are louder and higher pitch, but still making good pressure.  

Summer fuel starts here on May 1, but I guess you don't really know which stations will be in immediate compliance. Even though I have an investment in self-fueling equipment to meet my airports requirements for that, this event has me thinking of just running Avgas full time.  That's the only way you are really sure what you are getting.  25 hour oil changes and the other maintenance penalties don't bother me as much as the engine stopping mid flight.  It's really a shame that there is not a commonly sold unleaded aviation grade fuel.  

Pump inlet pressure should closely follow the altitude.
My spreadsheet shows that RVP13 Fuel at ~4500ft would have a critical temperature right around 95°F in agreement with your findings.
You may find the attached useful.  Use with caution.

Thanks Bill, that's a fantastic spreadsheet.   I was working on a simlar sheet, but I had extracted the TVP values from curves published on the internet, mostly because I was struggling with the formulas. 

I want to thank everyone who replied.  With all this great feedback I feel a lot more confident in my ability to recognize and avoid fuel vapor problems going forward.