Vapor Lock with 912iS. Most probable location?

RW,

Here in the USA the1.0 PSI RVP waiver for ethanol fuels only applies to summer fuels, which is less critical because even with the waiver the limit is 9 PSI in some states like CA, and 10 in less restrictive states. Not as good as AVGAS, but acceptable.

In the transition and winter months these restrictions are lifted and all fuels are expected to meet the ASTM class C or D standard. The class C standard applies to transitional months and targets 11.5 PSI.  The class D standard is for the summer months and targets 13.5 PSI. The exact dates for transition months and summer months are defined differently by various jurisdictions.  Knowing what you are getting can be difficult. I buy my fuel in batches of 40 gallons and I test the RVP when I buy in the transition months. It’s an easy test done at home once you learn how. Here in the USA I am eager to see the Swift UL94 product available at more locations so we can be done with all this. 

Rotax Wizard wrote:

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

That is interesting.....

Jeff and Paul...

Look soon to see the Swift 100R on the market.  It has an ASTM standard and currently is being tested independently at Rotax for suitability for their engines.  Until we get some widespread acceptance and market studies it is always an uphill battle to get away from auto fuels.  As you both know auto fuels are not reliable in that they are not constant from one state to another, indeed even within the state there are regional differences depending on the time of year.  

The worst part of auto fuels is the release of wavers to the suppliers for winter fuels to be sold into the summer months.  That one is a literal killer in that vapour lock, fuels with very high RVP in summer conditions, can cause havoc to your fuel system.  I applaud you Jeff for testing your fuel, it is indeed the only way to know what the rating is.  Virtually no bulk supplier will tell you even if they know. 

utube vid 2018 crash in Texas  https://www.youtube.com/watch?v=1xcCpZDSoiU

Cheers

OK, some food for thought...I talked about wavers, look out we have a real mess coming this year with the demand to lower prices of fuel.  This has happened before, it is political football and could cause you a lot of problems.  Now would be a good time to see if you can source Swift UL94 or another alternative to bad blends of fuel with high RVP numbers.

Cheers

https://www.reuters.com/business/energy/us-poised-waive-summer-gasoline-regulations-ease-prices-sources-say-2026-03-18/

I had what appears to be a similar issue last Friday.  I was flying my Flight Design LSA F2 with 912is engine back to Sedona with temperature in the mid-80s.  I was about 70 minutes into my flight and got low fuel pressure warning and lost a lot of engine power.  I had both fuel pumps on and no electrical issues.  I was on descending with about 3,000 RPMs. 

Situation corrected itself after 5-10 seconds (although it felt a lot longer!) so I was able to complete normal landing.  I haven't been able to replicate issue doing long run-ups on the ramp. 

I was using 100% Mogas (91 with 10% Ethanol and probably the winter blend) so I assume it's the same vapor lock issue that Paul experienced.  I'm switching to 50/50 Mogas/Avgas and maybe even 100% Avgas and just do more frequent oil changes.