Would like to learn more on vapor lock

[RVFan671]

So I think I had vapor lock yesterday but wanted to hear others thoughts on the topic as well, since I wasn't anticipating it given the conditions (mainly the temp which I thought would generally need to be warmer to cause it).

I'm flying an O-360A1A (carb) with 92 octane non-ethanol. I flew to another airport about 10 or 12 minutes and landed and shut down for 5 minutes. Fired up just fine and taxi over to take off. Taxi was about 2 minutes I'd estimate. As I am just approaching the hold short I thought I might have had a single stumble but not very clear. I ask for takeoff clearance aftera few seconds and apply small power to get onto the runway and that is when it started to barely run.

Tried to give it a little more fuel and lean it but nothing helped and it died before rolling to centerline. I had already asked for canceling clearance but took some time to finally start up and run, with a lot of missing to get back to an FBO to have them add 2.8 gallons of 100LL to my 16 gallon tank of mogas.

Throughout the time it was running rough I had 8psi registered on my mechanical fuel pressure gauge and my electric pump running. My CHT are never very hot in flight (barely over 300 on climbs) and were only 270 during the event. I tried many of things to get it to improve such as leaning to almost idle cutoff and turning off the electric pump temporarily. Each of those two things seemed to help it run marginally better as I taxied to the FBO. One other unique thing was when I used idle cutoff to shut it down, it dieseled / kept running for over a minute, probably 2. Tried giving it more throttle, turning off electric boost pump and even turning the fuel selector to off (but it wouldn't have run dry during the duration of the event). I think it finally shut off after a couple quick blast of throttle then back all the way off.

Let it sit for over 30 minutes with the oil door open and dipstick unscrewed. Then it fired up fine (still mogas since it takes time to get the new mix through lines? But started to slightly miss but as I taxied down to what was going to be a nice long run up with higher settings for a minute or two based on recommendations I've read on the past to clear the vapor it seemed to be running better and I was taxiing at 1500rpm (brakes don't like that as much though). Then, all the sudden full electrical failure. I believe the reason for it is my shorai doesn't have all the protections and it wasn't happy with all the draw down and rapid charge from alternator. I mention it because I didn't end up doing my long run up to verify all clear and confirming (as best I could) it was vapor lock. Now I left the plane and got a ride home and ordered a battery which is a week out.

Summary event statistics

• Carb
• Temp was 70 degrees at the metar 30 minutes before the event and cooling (next metar was 66)
• 5 minute turn on the ground
• CHTs only 270 during the event
• Fuel pressure maintained normal for my plane (just shy of 8psi with electric pump on)
• 92 non-ethanol which was purchased less then 4 weeks ago (probably about 2 or 3) so tail end of March
• Have gascolator
• No water or contaminates found when I sumped before flight or before taking the 2.8g of 100LL

 

[Dan 57]

Well, I have an identical configuration, O-360A1A (carb) but running 98 non-ethanol fuel. I've now had 3 cases when I lined up, applied full throttle and the engine started dying... reducing power and going thru half throttle the engine picks-up again. Slightly increasing throttle all good, long runway and takeoff continued. Normal operation thereafter.
My FP hovers around 5-6 PSI.
I have to add that I have a fuel return line returning excess pumped fuel to the respective tank.

Engine data thru Flysto showed a sudden increase in FF during the event, which has me wonder if the float or needle is getting stuck open.
Your case sounds similar to mine, and having engine data available would be of some help in troubleshooting. Yes, your case might have been vapor lock induced, and my guess you will find all normal when returning to your plane.
Until next time...

 

[RandomSquawk]

Watching this thread, as I'm curious about the likelihood of vapor lock with NON-ethanol mogas vs ethanol. I'm aware of 2 low-pressure events recently, both with non-ethanol unleaded.

 

[Freemasm]

sounds like VL. The typical comments about checking line routing and insulation are appropriate.

Two other quick points: regarding the “dieseling”, check your carb bowl tightness. A leak there can cause this -> air getting sucked in making an explosive mixture even when the throttle closed. There was a Lycoming SB on such many years back IIRC

Regarding FF indication step change high. This is what would happen if a turbine or any spinning element type flow meter was suddenly exposed to vapor vs liquid.

I personally wouldn’t fly either aircraft until I had solved these issues. Keep us informed.

 

[Charlie Sisk]

I had this conversation a few days ago with a well known engine builder, he builds for Bonanza's, Cessna's, etc. We discussed vaporlock on my K-35 Bonanza. He stated that he has fixed most of these issues by replacing the magnetos or adding EI. As we talked further, I ask why, if folks think they are getting vaporlock from high fuel temps, why not measure the temperature of the fuel ? He was silent a moment then said "thats a good idea". I attached a temperature probe to the return fuel line and it was within 2 degrees F of outside air temp when in cruise. I am thinking of a more permanent install. I believe it would make a great trouble shooting tool for fuel related issues.
Charlie

 

[RVFan671]

Based on the current comments so far, here is more info.

• No return line
• Fire sleeve for all fuel lines inside FWF
• Noticed a small fuel stain on carb bowl in past so I will for sure tighten the bowl a little, although this has been around before the 10hr or so on the mogas
• No engine monitoring reporting
• Fuel flow gauge isn't working either. I need to replace the cube

 

[Redbud]

I had vapor issues. Here are pics of before and after. Added a heat shield and more insulation to fuel line. No more issues. Use 100ll

 

[Freemasm]

It would be a good troubleshooting data point but there’s a lot more going on. Fuel type, once through vs constant flow, which point in system (boost suction, main suction, main pressure side). A comprehensive layout review considering all operating regimes, etc is needed. Ultimately the owners need confidence/safety across the whole spectrum. Hopefully they keep us informed.

Edit =. If this was happening with your boost pump on, that would suggest your problems begin on the suction side of your boost pump. This actually makes things worse. Not trying to be an alarmist but I truly wouldn’t fly that aircraft, especially with MoGas for the time being.

 

[Carl Froehlich]

I chased my tail on a buddy’s RV-8 that had the same symptoms including dieseling engine when taking the mixture to cutout. He was running only 100LL. On examination of the fuel system I found:
- air leak on the engine driven fuel pump
- way too many 90 degree fuel fittings
- the fuel lines themselves seemed fine

The fix:
- Replaced the fuel pump
- Remove four 90 degree fuel fittings forward of the firewall (mostly associated with the gascolator, an inline filter and less than optimal fuel routing).

After these changes the problem was completely resolved (and I ended up with a bag of used 3/8” 90 degree AN fittings). On this RV I attribute the majority of the problem to the fuel pump air leak. This was a rotary type engine driven fuel pump and hard to see. I did a careful watch on it while someone else turned on the boost pump (engine off) and found the leak.

Working similar problems on other RVs I find fuel line air leaks to be something to look at along with fuel line routing and restrictive fittings. On my RVs the only 90 degree fuel fitting is on the output of the engine driven fuel pump (to accommodate the fuel pressure take off point). I use an inline AFP fuel filter on the cabin side and other than my first build, no gascolator.

Carl

 

[AirShowFan]

A terrific source of info about vapor lock is the following video. It contains two presentations, one by Ron Singh (who experienced vapor lock in his RV-12 and investigated the heck out of it) and then one by Rian Johnson from Van's.

 

[Walt]

I would never run mogas in a Lycoming, definitely not worth the risk to save a couple of bucks.
Back in the day when I ran a carb I used a purge line and a primer valve to circulate hot fuel back to the tank, even with 100LL you can get VL.
I still use a purge valve today with my AFP FI, purging the hot fuel prior to start is a wonderful thing.

 

[sailvi767]

It’s the time of year when gas stations are switching from winter to summer blend fuel. Always seems Like a big increase in vapor lock issues. Keep in mind that gas sold at stations might not be exactly what you think you’re getting. A example is the recent CA issues where stations were buying very cheap gas. The reason it was cheap is it was reclaimed from wrecked cars in junkyards. Gas station fuel is like a box of Chocolates, you never know what your going to get!

 

[petehowell]

Search for the threads on pneumatic lock here on VAF - fuel boiling in the carb bowl, sinking the float and causing an over-rich condition, sometimes showing high FF and pressure. Winter MOGAS on a warm day can cause this to happen. Good to have 100LL or a mix in one tank for ground ops and Takeoff/Landing.

This may not be what is happening here, but worth considering.

 

[airguy]

I would never run mogas in a Lycoming, definitely not worth the risk to save a couple of bucks.

Careful Walt - your inner curmudgeon is showing!

In all seriousness though I'm curious to hear your reasons and what you believe may/will go wrong. Will you stop flying if 100LL goes off the market?

 

[Walt]

Careful Walt - your inner curmudgeon is showing!

In all seriousness though I'm curious to hear your reasons and what you believe may/will go wrong. Will you stop flying if 100LL goes off the market?

When 100LL goes off the market there will be a replacement.
Detonation and vapor lock are serious hazards, I’m not willing to take the risk to save a few dollars..
I have seen melted pistons from detonation, and the stats are full of vapor lock as probable cause.

 

[airguy]

When 100LL goes off the market there will be a replacement.
Detonation and vapor lock are serious hazards, I’m not willing to take the risk to save a few dollars..
I have seen melted pistons from detonation, and the stats are full of vapor lock as probable cause.

Agreed on both counts. Do you honestly believe that 100LL is the ONLY way to solve those problems?

The automotive world solved them long ago. There is no practical reason we can't do it in aviation as well - particularly easy to do for experimentals versus certified. Some of us are already doing it, with enough time in the air to prove it's not a fluke.

Nothing wrong with being unwilling to lead the charge, I'm just asking the question.

 

[RV10inOz]

This is why I discourage the use of togas, especially coming out of winter. This is not uncommon.

Let's look at some facts. RVP matters and you really need a significant volume of avgas to "fix" the RVP problem, so why bother? You can get this same result in flight not just on the ground. So again why bother?

Yes you can mitigate some of these technical challenges with pumps etc but fuel boiling off in the carby bowl is hard to treat.


Walt, so that we do not perpetuate OWT's, vapour lock is not likely to be causing detonation however detonation due to advanced timing, low octane fuel and or turbocharged engines and the above or not rich enough at high power can readily damage spark plug ceramics that lead to preignition and then melted pistons.

Either way conforming engines on conforming fuel is the way to go and that is my preference.

Lastly even avgas, conforming avgas, in a low wing plane, on a hot day with hot fuel passing roughly 5000' and climbing, the suction pump is sucking up hill with less ambient pressure behind the fuel, vapour bubbles do appear. Electric boost pumps are there for a reason. Obviously high wing do not suffer this as easily.

 

[Godo]

Well, I have an identical configuration, O-360A1A (carb) but running 98 non-ethanol fuel. I've now had 3 cases when I lined up, applied full throttle and the engine started dying... reducing power and going thru half throttle the engine picks-up again. Slightly increasing throttle all good, long runway and takeoff continued. Normal operation thereafter.
My FP hovers around 5-6 PSI.
I have to add that I have a fuel return line returning excess pumped fuel to the respective tank.

Engine data thru Flysto showed a sudden increase in FF during the event, which has me wonder if the float or needle is getting stuck open.
Your case sounds similar to mine, and having engine data available would be of some help in troubleshooting. Yes, your case might have been vapor lock induced, and my guess you will find all normal when returning to your plane.
Until next time...

Dan,
Where would you get 98 ethanol free gas in Switzerland?
I assumed that all EU 5% or 10% were the only mogas available accross Europe.

Godo
RV6/12 in Barcelona

 

[rv8ch]

Dan,
Where would you get 98 ethanol free gas in Switzerland?
I assumed that all EU 5% or 10% were the only mogas available accross Europe.

Godo
RV6/12 in Barcelona

Dan is lucky that his home airport sells Mogas and I believe it does not have ethanol.

I've used BP premium which they claim does not have "bioethanol" - seems like they are trying to say it does not have ethanol, but it could be "weasel words".

https://www.bp.com/fr_ch/switzerland/retail/carburants/bp-ultimate-sans-plomb-98.html

I don't know if they change the blend in winter/summer, and have kind of lost confidence in this pump fuel after noticing that my fuel caps are a bit harder to take off when I use the BP premium, and when I switch back to UL91 it gets better - indicates to me that there is something different in there. I have not noticed this when using Mogas supplied at an airfield, but my sample size is very small for airport Mogas.

I have also noticed a very slight tendency to vapor lock with BP premium fuel on hot days, and running the boost pump resolves it. Never had a problem in the air with any fuel.

 

[Dan 57]

Where would you get 98 ethanol free gas in Switzerland?

On my base LSZF.
They call it UL98, though when I queried I was told it is car gas, Super as it's called, same as from the station. I have not tested it for ethanol, but was told it is free of it.
Last time I for sure had ethanol in Cargas was in Scotland, and though the engine luckily did its job as usual, as Mickey describes above, the fuel caps were felt hard to open since the o-rings had swelled up some.