R

RVFan671

Well Known Member
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
 
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...
 
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.
 
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.
 
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
 
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
 
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
 

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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.
 
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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
 
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.

 
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.
 
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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!
 
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.
 
Walt said:
I would never run mogas in a Lycoming, definitely not worth the risk to save a couple of bucks.
Click to expand...
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?
 
airguy said:
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?
Click to expand...
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.
 
Walt said:
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.
Click to expand...
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.
 
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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.
 
Dan 57 said:
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...
Click to expand...
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
 
Godo said:
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
Click to expand...
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".


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.
 
Godo said:
Where would you get 98 ethanol free gas in Switzerland?
Click to expand...
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.
 
AirShowFan said:
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.

Click to expand...
So I retrieved the plane and made some observations which I want to get feedback on, after watching the video a second time (first time was last year, before I decided to run the non-ethonal premium).

In the video the plane is an injected engine (much higher fuel pressure required) and gets vapor lock climbing to altitude. The symptom was REDUCED fuel pressure. When I flew the plane home I ran it on the tank with about 17% AV gas in it. Temp was about 65, 5 degrees cooler then the original issue cropping up. I had fuel pump on the whole time. Started out just shy of 8psi which is what I normally observe with the pump on and on the ground, but I'm not constantly watching the thing either. I perform my run up then wait in line for takeoff. While in line I notice fuel pressure has climbed to 12psi. No vaporlock symptoms but I went ahead and advanced throttle to about 1500 or 1600 rpm to hopefully get cooler fuel flowing and drop the pressure before takeoff, which it did by bringing pressure down to 8psi again after 20 seconds or so. Departed and no issues and even flipped it over to the pure mogas tank while orbiting the field to test any abnormalities, none noted. While in cruise with fuel pump off I normally observe about 4psi which is what I saw in flight as well.

So my question is, is my INCREASE in pressure (on my carb engine) also a sign of vaporlock, or do I have a potential issue with either my electric or mechanical fuel pumps that happened to coincide with a warmer day and short turnaround time? I've read in other threads where carbs will get flooded out with too much pressure and when I had this event the plane did seem to start and run (terribly, but better then other settings) with mixture cutoff way back and a lot of throttle.
 
RVFan671 said:
So I retrieved the plane and made some observations which I want to get feedback on, after watching the video a second time (first time was last year, before I decided to run the non-ethonal premium).

In the video the plane is an injected engine (much higher fuel pressure required) and gets vapor lock climbing to altitude. The symptom was REDUCED fuel pressure. When I flew the plane home I ran it on the tank with about 17% AV gas in it. Temp was about 65, 5 degrees cooler then the original issue cropping up. I had fuel pump on the whole time. Started out just shy of 8psi which is what I normally observe with the pump on and on the ground, but I'm not constantly watching the thing either. I perform my run up then wait in line for takeoff. While in line I notice fuel pressure has climbed to 12psi. No vaporlock symptoms but I went ahead and advanced throttle to about 1500 or 1600 rpm to hopefully get cooler fuel flowing and drop the pressure before takeoff, which it did by bringing pressure down to 8psi again after 20 seconds or so. Departed and no issues and even flipped it over to the pure mogas tank while orbiting the field to test any abnormalities, none noted. While in cruise with fuel pump off I normally observe about 4psi which is what I saw in flight as well.

So my question is, is my INCREASE in pressure (on my carb engine) also a sign of vaporlock, or do I have a potential issue with either my electric or mechanical fuel pumps that happened to coincide with a warmer day and short turnaround time? I've read in other threads where carbs will get flooded out with too much pressure and when I had this event the plane did seem to start and run (terribly, but better then other settings) with mixture cutoff way back and a lot of throttle.
Click to expand...
High Fuel pressure is consistent with how I have experienced pneumatic lock - typically caused by fuel boiling in the carb bowl. I've seen this on a hot turn with winter mogas on a warm day.

More info here:

Vapor Lock Confirmed and Replicated

I need the forums help with this one. I have recently confirmed that that my RV-9A, O-320 E2A (Carbureted, FP Prop) is suffering from bouts of Vapor Lock. Early in the week after a prolonged taxi and hold I was cleared to depart the main runway. Elevation 2880, air temp 85 degrees F. All...
vansairforce.net vansairforce.net

Loss of Power on Takeoff, believe Vapor Lock

Upon departing the Vidalia, GA Onion Festival on Sunday, while on my mogas tank, I began to lose power on climbout. The airboss informed me I was trailing black smoke, and at around 250ft I suffered full loss of power. There was enough runway to get back down safely. On the ground, my fuel...
vansairforce.net vansairforce.net
 
I am following this thread. I fly a Bonanza, but vapor lock is supposedly common. I have experienced zero problems. I have been talking with a well known engine manufacturer, and I/we have come this this conclusion: Until we describe fuel temperature in degrees instead of adjectives and adverbs, we cant really know for sure. I am in the process of installing temp probes in the fuel lines so I can know for sure what the temperature is. My initial test showed the return fuel was only 2 degrees difference.
I know the Lycoming is different, but lets looks this way for a moment.
On a Bonanza, from the fuel selector forward is about 10 feet of hose. It goes from the pump to the fuel controller to the spider and back to the pump. Lets use .5 inch hose. Its actually less than 3/8, but .5 is easier math.
10 feet of hose at .5 inch will hold about .1 gallon of fuel.
This Bonanza IO-470-C uses about 13 gph in cruise.
About 25 gph on takeoff.
So...13 gph divided by 60 minutes =.216 gallons per minute.
25 gph divided by 60 minutes =.416 gpm
So...at cruise , the volume of fuel in the lines, .1 gallon, is replaced every 30 seconds.
At takeoff, the volume is replaced about every 15 seconds.
I think you will need a pretty good heat exchanger to heat fuel that fast.
But, I think the only way to know if the fuel is getting hot, and how hot, is to measure it.
I'll report back what I find.
Charlie
 
petehowell said:
High Fuel pressure is consistent with how I have experienced pneumatic lock - typically caused by fuel boiling in the carb bowl. I've seen this on a hot turn with winter mogas on a warm day.

More info here:

Vapor Lock Confirmed and Replicated

I need the forums help with this one. I have recently confirmed that that my RV-9A, O-320 E2A (Carbureted, FP Prop) is suffering from bouts of Vapor Lock. Early in the week after a prolonged taxi and hold I was cleared to depart the main runway. Elevation 2880, air temp 85 degrees F. All...
vansairforce.net vansairforce.net

Loss of Power on Takeoff, believe Vapor Lock

Upon departing the Vidalia, GA Onion Festival on Sunday, while on my mogas tank, I began to lose power on climbout. The airboss informed me I was trailing black smoke, and at around 250ft I suffered full loss of power. There was enough runway to get back down safely. On the ground, my fuel...
vansairforce.net vansairforce.net
Click to expand...

Both those threads are very helpful and I hadn't seen the 2nd one until you referenced it, so thank you. This is describing exactly what happened it seems like to me. I'll plan to move to 1 tank with >50% 100LL and pay closer attention to the fuel pressure increasing. Do you think turning the electric pump off if it is rising will help reduce the risk? I temporarily switched it off before idling at 1500-1600 to bring it from 12 to 8PSI and without the electric pump on it reduced to just over 8PSI. But I'm not sure that pump on or off, before it is actually pneumatically locking is going to have any impact. It does help to turn pump off, when it has locked and is running poorly based on that prior experience I had.
 
RVFan671 said:
Both those threads are very helpful and I hadn't seen the 2nd one until you referenced it, so thank you. This is describing exactly what happened it seems like to me. I'll plan to move to 1 tank with >50% 100LL and pay closer attention to the fuel pressure increasing. Do you think turning the electric pump off if it is rising will help reduce the risk? I temporarily switched it off before idling at 1500-1600 to bring it from 12 to 8PSI and without the electric pump on it reduced to just over 8PSI. But I'm not sure that pump on or off, before it is actually pneumatically locking is going to have any impact. It does help to turn pump off, when it has locked and is running poorly based on that prior experience I had.
Click to expand...
I have not tried turning off the pump - I just lean it to keep it running, When the cooler fuel hits, it just runs normally.

There are some sections in the old RVators that describe the situation, not sure all of this is correct. From Page 325:

It can also result in hard starts after a "hot soak" period, very rough
running, stalling after restart, and most dangerous, engine
cutoff on takeoff. What's the story? Gasoline can boil as
low as 83-86 F. When fuel passes a check valve in any
pump it can not back up. It's trapped, so it sits and absorbs
heat. Vapor forms. In a "hot soak" situation the vapor
pressure in the fuel line between the pump and the carb
can reach as high as 30 psi. This is more than enough to
overpower the fuel inlet valve in the carburetor, causing
the bowl to overflow raw fuel into the induction system.

The result is a variety of troubles. The first is very hard
starts. If the battery holds out and the engine starts to run,
there is usually tell-tale black smoke from the exhaust
until it clears the flooded, excess fuel. The engine may
start, run a short time and die. Reason? The engine used
the fuel available in the carburetor bowl and quit. Why?
Vapor pressure in the line builds high enough to keep the
pump from delivering new fuel.

Second, the engine may start but runs very rough for
considerable time before smoothing out. Large amounts
of vapor bubbles in the fuel line can disturb fuel flow into
the carb, as well as emit large amounts of vapor to the
engine through the internal carb vents.

Cut-out on takeoff is the most upsetting. Vapor can be
trapped in the system behind liquid fuel allowing the
engine to run normally for a while. But when the vapor
reaches the carb, the show is over. The fuel system up to
the manifold is designed to handle only liquid fuel. Vapor
is a problem. To get rid of it, just get rid of the vapor.
 
Dan 57 said:
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...
Click to expand...
Yes, the carb has a bowl full of fuel and the height of this fuel is critical to how rich or lean the air fuel mixture will be flowing into the cylinders. If the fuel temp in the bowl rises above the boiling point, it boils. The rapid bubble production cause the float to bounce and dance around, preventing the needle from staying closed. All of this will over fill the bowl and cause an excessively rich mixture and the ensuing roughness that you and the OP experience. No surprise that you saw a spike in FF. THe OP saw pressure rise, but this was due to boiling inside the fuel line, though likely that the fuel in the bowl was already boiling.

Best fix if you plan to run mogas with its lower boiling point is a fuel recirc system to ensure that cool fuel is getting to the bowl before take off after a heat soak event

Larry
 
Thanks for looking into this Larry.

Having now read all the other threads about pneumatic lock, it makes sense and is the most probable cause. It ain't no plan to run Mogas, but done fact as I have almost exclusively used Mogas for the last 3 years as in more than 1K flight hours.
My system already has a fuel return line, teed at the carb inlet, as well as a dual tank selector valve. All the fuel lines are firesleeved too. That leaves the firewall installed Andair gascolator (quite far away from any heat source), the EDP, and the carb itself, as culprits.

Next to be installed is the EDP cooling shield which has been safely sitting on a shelf in my cupboard ;)
 
Dan 57 said:
That leaves the firewall installed Andair gascolator (quite far away from any heat source), the EDP, and the carb itself, as culprits.

Next to be installed is the EDP cooling shield which has been safely sitting on a shelf in my cupboard ;)
Click to expand...
I suggest putting a temperature indicating strip on your gascolator - like this one I put on my pMags to verify it is not getting too hot. The gascolactor has a lot of surface area for heat transfer.

I know some places require gascolators on low wing airplanes, but for an RV there is no gravity like a high wing to promote water gravitating to it for collection. In other words it does little more than provides the needed filter function - something that an inline filter on the cabin side of the firewall can do.

Carl
 

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Just one thought -
I've thought about a recirculation system for my Bendix fuel injection...
If I put one in, I'll make sure that the max flow rate of the recirc system is significantly less than what the pump can flow.
I don't want to have the recirc somehow inadvertently open and starve the engine of fuel.
 
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