Vapor Lock with 912iS. Most probable location?

Hi Paul,
It’s my opinion the only segment of the fuel system that will vaporize on the 912iS is before the pump. It’s best if the system can be designed so that there is gravity flow to the inlet side of the pump, or at least very little pressure drop required to pull fuel to the inlet. Winter fuel and hot weather don’t mix.

Another consideration for the injected engines is the temperature of the fuel in the tank itself. Since the vaporization happens before the pump, hot fuel in the tanks from sitting on the ramp in the sun is a factor. The fuel can easily reach 120 degrees in the wing tanks sitting in the sun.  Then you may climb to altitude before the fuel in the tanks has cooled, and experience the vapor lock you describe.  Of course in your neck of the woods you are starting at higher altitude.  

This happened to me just one time after leaving the plane in direct sun for a three hour lunch on rare 90 deg March day in Vacaville CA. I had winter fuel at that time.  After that I installed a temperature probe that wraps around an aluminum fuel line at the output of the selector valve and displays on my EFIS. I’ve seen temps of 120 deg F coming right out of the wing tank, and that would be a no-go for me with winter fuel.  I mix 50/50 with Avgas when only winter fuel is available.  

Hi Paul...yes get rid of any winter fuel.  Better yet avoid it altogether.  You hit the nail on the head with your assessment.  Winter blend, high altitude and hot temperatures.  In testing we have seen that fuel (winter blend) boil while you were doing fuel transfer with a fuel can.  It would bubble as you pour it into the tank.  If you must use auto fuel at least use a better octane.  Blend it as you noted with some Avgas, it can be leaded or unleaded.  (100LL or Swift fuel) 

Cheers

Be careful not to conflate octane with Reid Vapor Pressure (RVP), they are unrelated and a higher octane won’t do anything to reduce vapor lock. We have a local fuel distributor that sells a 94 octane ethanol free gasoline year around for off road use. Light sport and experimental owners were initially excited about this fuel until people started having vapor lock problems. We tested this fuel and it turned out to have an RVP of 14! For comparison most winter fuel is about 13, summer fuel about 8, and avgas always 6-7. 

Yes for now with winter gas, we are blending 100LL and 91 auto plus limiting our OAT at airport to 70 F.

Rotax has said it pretty clearly about blending for such situations. My bad. After 8000 hours with the 912iS, got the perfect storm. Additionally with the fuel tanks cooking all day in the hot sun, rather than fly in the morning, this did not help.

Jeff, yes RVP is not the same as octane.  The point with Ethanol blends is that the statutory allowance in E10 is 1 point higher RVP simply because it is a higher oxygen content.  Winter fuels with no ethanol will be 1 point lower.  

https://www.epa.gov/gasoline-standards/volatility-regulations-gasoline-and-alcohol-blends

Cheers