Possible Vapor lock?

You can buy those ethanol test kits from Amazon Ebay or Aircraft Spruce. I had two guys at my field that used to test every batch of fuel. In Arizona we use 10% ethanol. Fuel is shipped to most cities via pipe line with no ethanol. There when it is put in trucks to be delivered it gets its ethanol. In all the years of testing these guys found 6% - 7% was normal with only a few batches up to 8%. Since then I checked around and darn few places in the country are at their full ethanol percentage. If you use 15% I would be your real concentration when tested is lower. Most cities say up to 10% or 15%.

Hello,  I have a 915is in a sling TSI and here is my experience which may be helpful.  In the Sling TSI we have added a small boost pump in the cabin to help push the fuel to the Rotax pumps to stay away from this fueling problem.   Twice on a hot day in the sun in michigan the fuel was heated in the wings and during climb out with both rotax pumps on the fuel pressure dropped into the yellow,  below 42 psi.  It did this on climb out down low so not fun,  i reduced the power setting and the fuel pressure came back up to 42.1 psi,  slowly climbed into cooler air,  at 6000 feet the fuel in the tanks cooled so after 15 minutes the fuel pressure raised up to 43.5 psi to the point where i could turn one pump off, and the pressure dropped to 42.6 which is still lower than i usually see but ok for smooth engine running.  

I have a 100 gallon fuel tank and run 90 gallons of 91 ethanol free fuel and put in 10 gallons of 100LL to help with the fuel vaporization issue.  To date this has worked well for me,  winter time no issues any time with auto fuel because it is cool out. 

When i built the Sling tsi i used larger dash 6 aluminum fuel lines and put as few bends in the lines as i could.  But there are a bunch of 90 degree fittings coming out of my fuel selector which may be where my problem originates.  I have the added small fuel booster pump which is between the fuel selector and the rotax pumps that should help.  This booster pump only makes 6 psi i am not sure if this is enough pressure to condenser any vapor before it gets to the rotax pumps.

I talked to Phil Lockwood for an hour a few years ago at KOSH, and he mentioned in florida that some aircraft with the rotax will not run at all in the heat of summer on auto fuel.  The lack of quality of the auto fuel is a real problem in the USA also.  I am running a mix of 100ll with the addition of the correct amount of Decalin added to the 100LL,  this has worked well for me so far.   My son has a rv12 with a ULS engine and has not had problems but he does not have a fuel pressure readout in the cockpit which we will be adding soon.  

if you want to avoid the problem all together you can run 100% decalin,  but then you run the risk of lead build up in your end and it costs more.  So for now i am doing to run some 100ll as a good compromise. 

I have a 915iS. My aircraft operators manual calls for 100ll at altitudes above 6,000’

I’m glad this topic is getting some contribution from the 915 owners as well as 912is owners, hopefully this will help us validate and fully understand this issue.

The 912is installation manual does address this in a couple of different paragraphs, so Rotax acknowledges a fuel supply problem can occur with high vapor pressure fuels. 

Section 73-00-00, page 9, lists the fuel pump maximum absorption capacity as “up to -5.8 PSI compared to ambient, depending on the vapor pressure of the fuel used”.   

Section 73-00-00, page 13, includes this paragraph:

In case of supply problems of the fuel pump, the fuel tank should be emptied and filled with AVGAS. If the problem does not occur at the next test (with AVGAS), then this is a sign of formation of vapor bubbles when using MOGAS (or auto fuel).  

Simple answer...do not use auto fuel of any kind in hot  temperatures or high altitude.  The issue if Reid Vapor pressures of the fuel is not sufficient in these conditions and vapor formation in the fuel can lead to some serious problems.  

Current problems with fuel supply due to the world conditions have opened up some serious problems for those using auto fuels in that the requirements from the EPA to switch to reduced Reid vapor pressure levels for summer have been waved in many states.  This creates a lot of problems and at altitude can lead to vapor formation.  Indeed in testing even on the ground with winter fuels in hot conditions vapor formation can occur.  At the very least blend 100LL with your auto fuels for the time being and if you have fluctuating fuel pressures try switching over to 100LL before you spend a lot of time replacing parts. 

https://www.epa.gov/gasoline-standards/gasoline-reid-vapor-pressure

Cheers