Vapor Lock

[craigv]

I’ve had vapor lock issues three times now and it always occurred after a flight in hot weather and then shutting down and not starting afterwards. For example, stopping for breakfast in plus 90* temps, or stopping for fuel in plus 90* temps. This is happening to other TSi’s and in particular, one in Las Vegas according to Sling.

The recommendation from Sling is to run AvGas and I hate to do that for the obvious lead issues and it’s debatable if that is a valid fear or not. I’ve decided to run full or partial AvGas in one tank for the three hottest months of the year, and use it for takeoffs, landings and parking. I’ve also rigged up a small 12v battery to a small fan, placed it in the engine compartment after parking, along with a small lithium ion battery, plus use a couple clothes pins to hold the cowling doors open. This makes an enormous difference in the heat in the engine compartment and the engine heat after a stop, but I’m not sure if this will solve the problem. Sling says they don’t know where the vapor lock is happening in the engine compartment.

Last September I was on a long cross country flying to all the lower 48 states. In Virginia I picked up some Decalin fuel additive that I’ve used successfully many times in other Rotax engines. A couple days later I stopped for fuel in Alabama on a hot day and couldn’t get the 915 started again for hours, in spite of only having AvGas in the tanks. I stopped the Decalin and haven’t had another problem while using AvGas.

This brings up another potential issue. Several people on this forum are installing external alternators on their engines. I have no idea if this will increase or decrease the possibility of getting vapor lock. It’s a lot of engine in that compact compartment and another alternator will probably add heat and change the airflow.


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[Cluemeister]

Are you running e free or 10% ethanol when this happens?

[craigv]

10% ethanol. That’s all that’s available here in California.

[PhilipRueker]

10% ethanol. That’s all that’s available here in California.

Here's a list of some Ethanol free stations for California - https://www.pure-gas.org/index.jsp?stateprov=CA

It would be worth a shot to get some to verify whether the ethanol is the problem.

[craigv]

I had no idea. Thank you.

[ibgarrett]

Not that this "may" help, but when I've flown a Bonanza F33, especially with a fellow pilot friend of mine, he would get out of the airplane and almost immediately open up the engine cowling (if it was a quick turn around) to allow the engine to cool faster to help prevent vapor lock. I'm wondering if the same held true here where opening up the access ports on the cowling would aid in this process.

On a Turbo Saratoga I flew to Osh one year it had HORRIBLE vapor lock and the only thing we could do each time we stopped was to give the engine 3hrs to cool down before it would start. Not very fun of course, but it seems to me maybe Rotax is the better source on asking the questions vs. Sling. In the case of the Saratoga I was renting it from the owner who never took the plane more than one hop - so he thought it was me and not the plane. It was the last time I took the plane on a long trip.

[lutorm]

Are the fuel lines going to the high-pressure pump insulated in the Sling? I know on the Sonex people often experience problems with the gas boiling (not vapor lock per se) causing the engine to stumble on the ground and insulating the fuel lines, fuel filter, etc, in the engine compartment seems to help a lot.

Incidentally, I haven't studied the Rotax fuel system diagram in detail but one of the reasons that electronic fuel injection is more immune to vapor lock is that it tends to circulate the hot fuel back to the tank. I'm pretty sure the fuel system in the Sling has return lines to the tanks, right? If so, and you can run the fuel pump before start, it should circulate cold fuel through the lines and avoid the problem. Is it not possible to run the high-pressure fuel pump with the engine stopped, or is the problem that it cavitates at the inlet?

[craigv]

I don’t know what percentage of the fuel lines are insulated in my engine bay, and my fuel filter isn’t,
but will check next time.

I’ve tried running both pumps for various periods of time and was never able to overcome the vapor lock. The next thing to check is if Sling’s supplementary fuel pump can overcome it…and possibly add it.

The good news is AvGas seems to solve the problem, even in Las Vegas…and just don’t use Decalin.
Opening the cowl doors and maybe a fan MIGHT solve the problem with Mogas and ethanol, I just can’t say for sure yet.

There are three airports listed in California with ethanol free Mogas but only one (Little River, KLLR) actually has it.

[okent]

I think the break point for avgas and increased oil changes is somewhere around 30% of total hours, so if you use mogas 9 months of the year you should be ok. When I travel I don't bother with decalin because of the the total hours I fly on mogas vs avgas.