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Loss of Power Experiences - Due to vapor lock?

jrich

Active Member
Below is an explanation of the power losses we experienced and our thought process in solving it. I?m posting it here to accomplish the following objectives:

1. If the problem we?ve experienced is truly ?vapor lock?, then RV-12 owners and pilots need to be made aware of what conditions might trigger vapor lock and to consider operating the RV-12 as it was designed: Run the electric fuel pump at all times.
2. Solicit thoughts on how confident we can be that the problem we experienced is ?vapor lock? and if not, what might be the root cause of the loss of fuel pressure as the apparent cause of power loss.

Lost power and fuel pressure on (4) occasions with conditions noted:

1. 6/6/15, 9:20 a.m. began flight of approx. 2 hrs. without issue. Plane sat in sun for approx. 5 hrs. without canopy cover; note that 5 gals. of extra fuel in a jerry can was inside cabin;
6/6/15, 3:15 p.m. poured warm (> 100 deg. F) fuel from jerry can into fuel tank; 3:30 pm, 99 deg. F, 200? MSL airport elev., fuel press. during initial climb out approx. 4.1 psi., 18 mins. after eng. start and during climb out ? engine sputtered with power loss; during approx. time of power loss, Dynon data shows a relatively sharp (30 secs. ) drop in fuel press. from 3.75 to 1 psi during climb from 700? MSL to 1000? MSL and fuel press. remained abnormally low for the next 4 mins. of ?flight? back to airport with only approx. 30 secs. of normal press. (3.4 psi.) part way during that 4 min. time period and then it dropped somewhat again.

2. 6/7/15, 10:45 a.m., 92 deg. F, 200? MSL airport elev., fuel press. during initial climb out approx. 4.25 psi., 13 mins. after eng. start and during climb out, Dynon shows continual but gradual fuel press. decrease from 4.2 psi to 1 psi as altitude increased from 900? MSL to 4,300? MSL; then, an avg. fuel press. of 1.125 is maintained for approx. 2 mins. of continual climb to 5,100 MSL where engine sputtered with power loss, then, Dynon data showed continual, gradual (4 mins.) rise in fuel press. to approx. 5.4 psi and held an avg. of 5.4 psi during remainder (2.5 mins.) descent to airport.

3. 7/11/15, 8:30 a.m. began flight of 2 hrs. without issue. Plane sat in sun for approx. 5 hrs. without canopy cover; note that 5 gals. of extra fuel in a jerry can was inside cabin;
7/11/15, 2:00 p.m. poured warm (> 100 deg. F) fuel from jerry can into fuel tank; 2:30 pm, 98 deg. F, 200? MSL airport elev., fuel press. during initial climb ran 4.1 to 3.75 psi.; 18 mins. after eng. start and during climb out ? engine sputtered with power loss; during approx. time of power loss, Dynon data shows a sharp drop in fuel press. from 3.75 to 1 psi; during descent from 1800? MSL to 1480? MSL, fuel press. made a relatively sharp (40 secs.) increase from 1 psi to 4.5 psi. and remained normal (4.375 psi) for the next 4 mins. of ?flight? back to airport.

4. 7/25/15, ground testing with intent to replicate conditions (except altitude change) as in flights 1. & 3. above: 3:00 p.m., 97 deg. F, ran engine to normal oper. temp., shut down, and left plane to heat in sun with jerry can in cabin, no canopy cover;
7/25/15, 5:00 p.m., added warm (> 100 deg. F) fuel from jerry can into fuel tank; ran engine to oper. temp.; performed run-up check; ran at W.O.T. for a short period, then, 5,000 rpm for a few minutes, shut off electric fuel pump and within a minute or so, fuel flow went to zero and fuel press. went down to 1.2 psi. and shortly thereafter, engine sputtered; put electric fuel pump back on and conditions returned to normal.

We downloaded data from the Dynon for recent flights inclusive of the ones we experienced power loss. The first flight (1. above) with the significant power loss showed after approx. 13 mins. of flight time, a sharp drop in fuel pressure and loss to about zero of fuel rate. The third flight (3. above) with power loss showed less than 18 mins. of flight time, a sharp drop in fuel pressure and loss to about zero fuel rate. We know for sure that during the third flight we had shut off the electric fuel pump just prior (approx. 30 seconds) to loss of power. We probably shut off the electric fuel pump for the first and second power loss flights because that has been our normal procedure since we've been flying the RV-12 (about 3 yrs.). Both the first and third power loss flights had nearly exact conditions: 2 hr. flight, landed, shut down with about 10 gals. of fuel in the fuel tank, a 5 gal. plastic "jerry" can of fuel in the cabin, canopy closed and entire airplane exposed to full sun in 96 degree environment for at least 2 hours. Then, added the fuel from the 5 gal. jerry can to the fuel tank. Started engine, idled between 2000 and 2500 rpm until oil temp. reached 122 degrees. Then, runup at 4000 rpm with usual checks including ignition drop check. All normal. Take off, climb, shut off electric fuel pump (absolutely sure on third flight) and then onset of significant power loss. The differences on the second flight was that there was not an already warm engine, ambient temp. was 90 degrees or less because takeoff occurred mid to late morning, fuel pressure and flow loss as recorded was gradual over a long period of time 20 - 30 mins. and power loss was not as pronounced or as severe. Again, we are not absolutely sure about whether the electric fuel pump was turned off after takeoff for flights 1. & 2. above. But, the turning off of the electric fuel pump after takeoff had been our standard operating procedure.

See next post for continuation of this.
 
Loss of Power Experiences - Due to vapor lock? continued

A person might and should ask: Why or how in the heck would someone put themselves in the position of (2) in-flight power losses after the first in-flight power loss? Here?s how this came about:

The first power loss, occurred two hours after replacing the ?bad batch? of carb. floats per the recent Rotax S.B. Immediately after the power loss event, we took the top cowl off and looked for something obvious and found nothing wrong. Since the carbs and floats were the last thing we touched, we dropped the float bowls, inspected for trash, water, etc. in the bowls, properly floating floats, and any abnormality and found none. We re-mounted the bowls, started the engine and found fuel dripping out of the air cleaner on the right carb. and jumped to the conclusion that the needle valve in the right carb. was not shutting off fuel into the bowl. So, we dropped the right carb. float bowl again, turned on the electric fuel pump to ?clear? any trash. But, we didn?t think to catch the fuel in this clearing process. We actuated the needle by hand to check that the fuel would shut off and it did. So, we re-mounted the bowl, started the engine and ran it with no leak and no indication of a problem. Thus, we thought the problem was solved and decided that since it was getting late, we?d fly out the next morning. The next morning is when flight 2. above resulted in power loss after we had cautiously circled the airport up to 5000? and when we were just leaving the area to head for home. So, we landed safely, left the plane, got a car ride home, and retrieved the plane with our trailer the next day and trailered it home. We got some advice on the situation (many thanks to Roger Lee for all his assistance). First, we thoroughly check for debris in the fuel system. So, we did that and started with the right carb., cleaning it with carb cleaner and then air pressure. We noted significant wear on the bottom end of the needle valve. There was so much wear that the retaining clip was going to be lost. Also, the mating face of the float arm bracket tab had a .006? wear depression. The arms of the float arm bracket were obviously not parallel with the carb base as required by the Rotax manual. At this point we found no trash in the carb. fuel line going to the carbs. and the right carb. had no trash. So, we?re thinking that all this wear could be the cause of why we found fuel coming out of the right carb air cleaner. We put a new needle valve and float arm in the right carb. and got the float arms very nearly exactly parallel to the carb. base. Re-assembled the carb., remounted it, turned on the electric fuel pump to check for leaks and there was no leak from the right carb. But, there was fuel dripping out of the overflow tube on the left carb. We were going to clean and inspect the parts on the left carb. anyway, so we did the same for the left carb. and found exactly the same conditions for it as for the right carb.: No trash, but a lot of wear on both the bottom of the needle valve and mating tab of the float arm bracket, and the arms of the bracket were not parallel with the carb. base. Reassembled and remounted the left carb. Turned on the electric fuel pump and no leaks anywhere. Ground ran the plane without a problem and then flew the airplane for 30 mins. circling our home base airport. So, we were thinking that our problem was due to flooding the carbs. As it turned out, this was not the problem. Take advice: If in doubt, check the Dynon data to confirm your theory. However, since we had not checked the Dynon data and had jumped to yet another false conclusion, we flew flight 3. above which is a virtual repeat of flight 1. Got a ride home, and trailered the plane back home once again. After three times, we thought we?d duplicate conditions as best we could, but with staying on the ground. We were able to get the loss of fuel pressure in 4. above when we shut the electric fuel pump off.
Since 4. above, we?ve check/tightened every fuel connection up to the mechanical fuel pump, put an air gap using zip ties between fuel lines and warm/hot coolant hoses and oil lines and wrapped each exhaust pipe with heat wrap from each cylinder head to the muffler. We?ll keep any extra fuel in the shade and try and keep the cabin ventilated or cover it. Most importantly, the plan is to always have the electric fuel pump on and in the future either eliminate the fuel pump switch or place a safety cover on it. Also, we?ll consider adding a second electric fuel pump as a back up.

So, back to my original question:

How confident we can be that the problem we experienced is ?vapor lock? and if not, what might be the root cause of the loss of fuel pressure as the apparent cause of power loss?

Has anyone else experienced similar problems? Also, has anyone installed a backup (second) electric fuel pump for their RV-12?
 
Loss of Power Experiences - Due to vapor lock? cont.

FYI

RV-12 completed 2010 and had approx. 350 hours at the beginning of these issues.
 
I have a switch on my fuel pump, and I turn it off after takeoff and turn it on for landing. Today I took off at 1270 lbs with two people onboard to give a prospective RV-12 pilot a demo. It was 112F and DA 4850 Ft. As we climbed out we started getting CHT and Oil Temp alarms that did not clear (eventually reached 255F oil and 275F CHT). We decided to make a precautionary landing and the engine faltered. Turning the electric pump on we did not experience another falter before landing. I suspect it was vapor lock. I run premium autogas. BTW after a 30 min cool down I flew back to my home field 10 miles away alone and temps were high but below the alarms. I did receive a "Teperature Above Specification" message on my D-180.

I checked the ROTAX Manual, and it appears no maximums were exceeded, but from now on I will place a personal limit of 105F OAT and single occupant only limit on my 12.
 
I am assuming you are using auto gas. The problem could be a different formulation in the fuel. Auto gas is like a box of chocolates, you never know what you are going to get!
G
 
Also assuming you are using car gas. Seems to me it would be an interesting experiment to try and duplicate the conditions using 100LL.

BTW, you guys who fly in those climates must wear asbestos underwear! I can't even imagine getting in an airplane under those conditions! I guess us northern boys are just wimps!
 
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Rich,
When you mention running auto gas are you referring to premium auto gas containing ethanol or non-ethanol super unleaded?

John R,
Would you also elaborate on exactly what fuel you're using?

Thanks, Jim
 
We have those kinds of temperatures in summer too, so I'm very interested in these reports. I installed a switch, but primarily for ground operation so I can isolate the electric fuel pump easily instead of pulling a fuse. However these reports suggest that it's a good idea to leave the pump running continuously as Vans intended, at least in hot conditions and when using mogas, which is a variable product due to winter and summer fuel blends, age of fuel, additives and possible questionable quality control.
 
As already mentioned, auto fuel can be highly variable. One of those variables is the Reid vapor pressure (RVP). This can have a big influence on how easily an aircraft can experience vapor lock issues. A common problem is that refiners vary the formulation depending on the time of year. If you end up with some winter blended fuel in hotter summer temps (not at all uncommon) it can cause a lot of problems. A different RVP is used in the winter to improve vaporization in cold temps to improve cold starting performance, etc. If conditions are extreme, it might not matter what season blend you are using. Another factor can be who the fuel is purchased from. I would recommend that RV-12 owners only purchase fuel from top tier retailers (Chevron, Shell, Texaco, etc) and only ones with a high sales volume. One problem with us using premium fuel is that it is not a high volume seller at most retailers (most people only buy regular grade unless their vehicle specifically requires something else)

The actual vaporization of the fuel is effected by atmospheric pressure and temp. When an aircraft climbs, it is artificially lowering the atmospheric pressure. When any type of pump is pulling fuel through a fuel system, the inherent resistance to flow within that system will also have an influence on lowering the pressure up stream of the pump.

Because the RV-12 was designed expecting auto fuel to be used extensively, the described situation is one of the primary reasons the fuel system was designed with the aux. electric pump as close to the fuel tank outlet as possible, and configured so that it would be operating at all times.

This assures that at least one fuel pump will be operating in the coolest fuel present in the system, and the majority of the system is operating at a positive pressure.

So, based on the info you have provided I would say that all of the data points to vapor lock of some degree.
Chances are, that if at any point in the process you had drained the fuel and put in 100LL, the problem would have been gone.

Side note regarding the wear you discovered in your carb's at 350 hrs.....
This is why there is a prescribed carb inspection to be done every 200 hrs. You didn't mention it ever having been done. You are almost due for a second one.
 
fuel used

For the conditions we experienced, we were using Walmart's 93 octane with 10% ethanol. We pretty much have always used Walmart or Ractrac 93 octane fuel.

Thank you, Scott, for your input. We agree with all that you wrote. However, we are somewhat wary because others are telling us that it can't just be explained as vapor lock.
 
However, we are somewhat wary because others are telling us that it can't just be explained as vapor lock.

If you are still not sure, drain the tank and put in 10 gal. of 100LL and repeat your testing. I think you will probably have different results, but if not, then you have an additional data point. The small amount of 100LL will not effect your maint. or oil change interval.
 
We have flown before in hot weather with the same fuel. But, maybe the conditions were severe enough. We surely hadn't added hot fuel to the fuel tank before. I'm confident that adding hot fuel was a bad idea.
 
The problem could be vapor lock. Some RV-12s have also experienced similar engine sputtering due to a bad engine driven fuel pump. I am not saying your problem is the fuel pump, but it is something to consider as a possibility.
 
We have flown before in hot weather with the same fuel. But, maybe the conditions were severe enough. We surely hadn't added hot fuel to the fuel tank before. I'm confident that adding hot fuel was a bad idea.

Keep in mind with auto has you really have no idea if it's the same fuel. I won't get into shipping methods and gypsy tankers. The tester mentioned would be a nice thing to have.
G
 
I have a Peterson vapor pressure test kit (syringe & gauge) that I purchased and used when I got my STC for my Cherokee 235 to check the vapor pressure of the fuel. While I owned the Cherokee, I sampled every fuel load (non-ethanol, 87 octane) that I purchased for the acceptable vapor pressure limit which the STC allowed. How would I go about determining the acceptable vapor pressure limit for the RV-12? Would I have to log numerous data points with varying temperatures and atmospheric pressures. I'm not sure of all of the parameters that I should vary or whether I'm competent in this area to even do justice to such a study. Anyone have suggestions?
 
You may also want to check your gascolator screen, and see if your engine driven pump is subject to any recall, and make sure your facet aux pump is working properly per the PAP fuel flow test.
 
You may also want to check your gascolator screen, and see if your engine driven pump is subject to any recall, and make sure your facet aux pump is working properly per the PAP fuel flow test.

Thanks for your suggestions. Since the problems, we have opened the gascolator, took out the screen and found minimal trash. Definitely had nearly 99% of the screen available for flow. Since we suspected vapor lock due to our probable shutting off of the electric fuel pump per our past standard operating procedure, the electric fuel pump test was probably not needed. But, we went ahead and did the PAP fuel flow test on the Facet electric fuel pump to make certain that the electric fuel pump was not restricting flow and it passed the PAP pumping test. In fact, the time to pump 1 gal. was far less than required and even less than the first PAP after construction. The mech. fuel pump is the new style, Corona fuel pump that by S/N does not fall under the Rotax S.B. The Corona pump was installed (3) yrs. ago and has approx. 150 hrs. on it.

Thanks Cactuspilot!
 
I've dug out my test kit and found what I believe would address the testing of fuel issue. I took pictures of the instructions for estimating the Reid Vapor Pressure (RVP) along with my tester and its instructions.

If anyone is interested in the pictures, send me your email address and I'll send the pics.
 
John,

You seem to have extensively tested autogas. What are your thoughts on the variability of results?

Rich

If you are asking what I noted about the vapor pressure when I tested every load of 87 octane:

Every load that I sampled was a pass or safe load. I did not log the vapor pressure numerical results. But, I'd say that most times the readings were right around 8 psi. vacuum which is in the green color "safe region" range of 5.5 to 12.8 psi vac. on the gauge. The 8 psi. reading is, of course, in the pass or "safe region" range. And, this is all I was looking for. Keep in mind that this was for non-ethanol fuel for my Cherokee 235. I also tested a sampling of fuel before I purchase a load of fuel for the presence of ethanol. And, we never found a trace of ethanol in every advertised non-ethanol load we purchased. The issue about ethanol for the Cherokee was that its carb. was susceptible to icing if there was water in the fuel. We never experienced this problem ourselves. In fact, in the 8 yrs. that we had that plane, the engine never missed a beat. Now, back to the ethanol and water connection: Water will not readily mix with pure gasoline. However, water is readily is mixed with ethanol and if water gets in fuel containing ethanol, the ethanol absorbs the water and the water can then be in solution with gasoline and go undetected. That is unless you do the water volume test that I previously mentioned.

Regarding freshness of fuel: In the past two days, I asked RaceTrac and Raceway fuel stations how often the get deliveries of 93 octane, 10% ethanol (what I use in my RV-12 and sometimes from these stations but usually from Walmart). Each of them said that they get deliveries every 2 days. That's pretty fresh. Just FYI: They get a load of 87 octane, 10% ethanol at least every day. A while back, I asked a local tanker re-fueler how often a particular Walmart station got a load of 87 octane, 10% ethanol. He told me at least once a day and often times, twice a day.
 
Thinking out of the box when operating inflight in high OAT temp conditions

If operating in severe heat then maybe we could come up with a hot weather checklist if you incur loss of power problems.;)
Leave electric fuel pump on at all times.
Consider maintaining the carbs within the suggested maintenance schedule.
After shut down leave your oil door open to help evacuate ambient heat.
Try flooding the engine at 3/4 throttle with the choke after landing to increase fuel flow through the system.
Consider a canopy cover with you canopy cracked in the open position when parked for long period of time in high heat.
Land and add 5 or 10 gallons of 100LL to see if this eliminates the problem.
Store your extra fuel out of direct sun light in sealed containers.
Consider leaving your Master Switch on and Aux fuel pump running between flights when participating in Young Eagle Intro Flights when only shut down for 5 or 10 mins.

Others feel free to add to this list!:D

These suggestions in no way are posted here to reflect negatively toward the author of this thread but are compiled from previously posted suggestions and some added from my own observations for future pilots that may have this problem.:) And there is a some history on the forum of this problem occurring.:eek:
 
If operating in severe heat then maybe we could come up with a hot weather checklist if you incur loss of power problems.;)
1. Leave electric fuel pump on at all times.
2. Consider maintaining the carbs within the suggested maintenance schedule.
3. After shut down leave your oil door open to help evacuate ambient heat.
4. Try flooding the engine at 3/4 throttle with the choke after landing to increase fuel flow through the system.
5. Consider a canopy cover with you canopy cracked in the open position when parked for long period of time in high heat.
6. Land and add 5 or 10 gallons of 100LL to see if this eliminates the problem.
7. Store your extra fuel out of direct sun light in sealed containers.
8. Consider leaving your Master Switch on and Aux fuel pump running between flights when participating in Young Eagle Intro Flights when only shut down for 5 or 10 mins.

Others feel free to add to this list!:D

These suggestions in no way are posted here to reflect negatively toward the author of this thread but are compiled from previously posted suggestions and some added from my own observations for future pilots that may have this problem.:) And there is a some history on the forum of this problem occurring.:eek:

Thanks for your input. Great suggestions except I'm not sure about 4. & 8. above. As I indicated in a previous post, I made sure there was an air gap between fuel hoses and other hotter hoses. I wrapped my exhaust pipes with heat wrap to minimize the heat gain to the engine compartment.
 
More details on fuel vapor pressure:

I tested some 93 octane, 10% ethanol gas from Walmart today at various temperatures and came up with an average Reid Vapor Pressure of 8.4 psi. for this fuel that I tested. Therefore, according to the chart that I have, when the fuel I tested reaches 98 degrees, this fuel is more volatile than avgas for the FAR part 23.961 hot fuel certification test. What exactly that means, I'm not sure. It is interesting that the chart that I have does not extend beyond approx. 105 degrees.

For reference purposes for FAA certificated aircraft:

FAR part 23.961:

"?23.961 Fuel system hot weather operation.

Each fuel system must be free from vapor lock when using fuel at its critical temperature, with respect to vapor formation, when operating the airplane in all critical operating and environmental conditions for which approval is requested. For turbine fuel, the initial temperature must be 110 ?F, −0?, +5 ?F or the maximum outside air temperature for which approval is requested, whichever is more critical.

[Doc. No. 26344, 58 FR 18972, Apr. 9, 1993; 58 FR 27060, May 6, 1993]
"

Also for reference purposes for FAA certificated aircraft:

From AC No: 23.1521-2:

"(3) A hot weather operation test (? 23.961) should be
conducted. The airplane should be tested with the fuel to the
maximum altitude for which approval is requested (see paragraph 7t
and u)."


I did no further research beyond this point. Since the RV-12 is not a certificated aircraft, it does not have to comply with any of these requirements. But, I would like to know if a similar test was performed on the RV-12 and how high a fuel temp. and altitude it was tested to.
 
But, I would like to know if a similar test was performed on the RV-12 and how high a fuel temp. and altitude it was tested to.

If by similar you mean a test that followed all of the guidance of AC No: 23.1521-2, the answer would be no.

Though there has been a lot of testing and a huge amount of operational experience gained while operating RV-12's with premium auto fuel. The factory prototype/demonstrator has been operated almost exclusively on premium auto fuel (currently at 1500 + hours) with never any vapor lock type symptoms. Before you assume that here in the Pacific North West it never gets hot, we have recently had temps of 102-103 F and the forecast is for more this next week.

But, none of this means anything when relating the info to an RV-12 that is not configured nor operated in a way the matches the certified design configuration. Yes, it is certified. Not as in type certificated....but as an S-LSA. It is certified under an FAA adopted Consensus Standard of ASTM standards. The plans supplied with the kit match the certified S-LSA version because it is technically the factory build manual, and is what allows kit builders to certify as an E-LSA (home assembled clone copy of the S-LSA)

So, if an owner has modified (such as not operating the electric fuel pump full time) their RV-12 (this would only be possible with an E-LSA or E-AB aircraft) they have crossed over into the experimental portion of their certification. In this situation, the testing burden is now technically on the person that did the modification.
 
Hi on the 01-05-2015 I have placed a thread ?Losing power during extreme heat conditions? http://www.vansairforce.com/community/showthread.php?t=121326&highlight

I thought that I have solved the problems through many trail and tests and had experts in to try and solve the problem, to no avail. I had a close shave with fate when I lost total power after a takeoff. I promised myself not to put me and my family through such a horrible experience again.

Then I came across the following explanation: with extreme heat conditions the fuel density decrease resulting in your floats not to float anymore and causes your carburetors to flood. That is why if you are fast enough and you come back on the power the engine will start running normal again all be it not at full power.

With the above explanation it was recommended that I change over to avgas.

Since I have changed to avgas I did not have any incidents again.
I fly from very high altitudes at extreme heat conditions and it is my believe that autogas or mogas is very susceptible to heat and altitude and will not give the performance we require from it at these conditions.
I am now a happy avgas flier.:)
 
More about using mogas:

The gasoline providers are allowed by the EPA to dispense "winter blend" gasolines generally from October through April. Winter blend gasolines have a higher Reid Vapor Pressure (RVP) than "summer blend" gasolines. The concern for us, especially RV-12 fliers, is that the winter blend is more volitale than the summer blend. That means that the winter blend gasoline will vaporize or flash to a vapor more easily than a summer blend. As the temperature increases in warmer weather, the winter blend is more likely to vaporize than the summer blend. I mention this because if you have winter blend gas in your tank and you use it in summer you are more susceptible to "vapor lock" (vapor instead of liquid in fuel lines and carb. bowls) and thus loss of power. This becomes even more likely at higher altitude. The higher altitude means less atmospheric pressure on the top surface of the fuel in the fuel tank and thus less pressure in the fuel system, which makes it easier for the suction side of the mechanical fuel pump to vaporize the fuel.

There is a table published by the EPA that shows the maximum allowable RVP during the warmer months generally (there are exceptions) the period from May 1 through Sept. 15 by state and in many cases further broken down by county. This table is online at: https://www.epa.gov/gasoline-standards/gasoline-reid-vapor-pressure

Be cautious and consider the following conditions in what I consider the order of importance for creating vapor lock:

1. Electric fuel pump not running. This pump should always be on and the fuel pressure checked prior to each engine start up so that you know that it is working and that the pressure looks about right (approx. 2 psi).
2. Hot fuel in the fuel tank &/or hot fuel lines in a hot cabin due to high temp. ambient conditions &/or the sun baking the cabin interior, etc.
3. Hot engine compartment due to heat soaking after flight &/or high temp. ambient conditions. This can be reduced after shutdown and before next fight by opening the oil/coolant inspection door on top of the cowling to allow cooler air from below to be drawn upward around the engine by the warmer engine compartment air convecting and rising out of the open cowl door.
4. High altitude &/or high density altitude due to high temp.
5. Low fuel level in fuel tank. This will affect the pressure at the suction side of the mechanical fuel pump. The lower the level in the fuel tank the less pressure on the suction side of the mech. fuel pump.
6. Steep climbs will cause the fuel level and thus the pressure at the fuel tank outlet to be reduced slightly with the nose up pitch of the airplane and thus also the fuel tank. Remember, you never want to take off with less than 4 gals. of fuel because a high nose attitude may cause the fuel outlet to become un-ported (no fuel draining into the fuel suction side connection on the front of the fuel tank because it is higher than the fuel level).
 
Partial loss of power - higher ambient temps.

In this discussion in some cases i suggest what is being missed /overlooked is the fuel is boiling as it enters the fuel bowl. The fuel stays in a liquid state whilst in the fuel lines due to the raised boiling point which is a result of the pressure increase from the fuel pumps. (the same principle as the engine cooling system uses a pressure cap to raise the boiling point of the engine coolant to keep it in a liquid state) when the under under cowl temps are high and low fuel volatility conditions occur the engine runs rich, not lean and looses power. This is due the rapid expansion of the fuel entering the fuel bowl and some of the fuel boiling / vaporising. The air pressure (normally atmospheric pressure) inside the fuel bowl increases, as the small bowl vent cannot equalise the fuel bowl pressure to the atmospheric pressure outside of the fuel bowl quickly enough. This additional pressure inside the fuel bowl forces additional fuel through the main jet enriching the air fuel mixture usually causing (in my experience), a fuel smell quickly followed by partial loss of power. Fuel pumps on or off makes no difference to this situation occurring if the fuel volatility is low and the fuel bowl temperatures are quite high.
In my case i found turning the electric fuel pump off and on as the engine response changed and lowering the nose then slowly reducing the power i was able to increase airspeed/ airflow cooling to the engine compartment and stabilise the engine operation to get myself better options in case the situation worsened or the engine stopped. It was an extremely uncomfortable few seconds (Felt like minutes) when it happened!

Scott's comments about "changing to 100LL and the problem would have most likely disappeared" in my case and experience was 100% correct. I drained all the ULP and added 100LL Avgas and fuel pressure fluctuations and partial power loss all immediately disappeared, even at ambient temps at 40DegC plus. I also trialed with the electric pump off (after reaching a safe height) in climb power attitude and settings with no power loss issues or fuel pressure fluctuations.
My limiting factor now is coolant and oil temps when operating in high ambient temps.

The fix, In my case,

With the fuel quality we had available last summer (We only have one fuel station in town) was to either run a minimum of 30% Avgas with the unleaded fuel if ambient temperature is likely to be above 35 degC or stop start operation was to occur, i.e. taking friends for a ride with short engine shut downs between flights. This year the fuel quality has been much better, I've still been adding Avgas prior to purchasing a Ray Hodges (Pederson Aviation) voltility tester to check fuel vapour point is safe, as well as minimal flying due to other commitments.
Why not 100% Avgas? Reason - Fuel cost and lack of availability.
 
This thread has brought to our attention a number of important issues regarding this dreaded loss of power on takeoff and in flight due to vapor lock. A big thanks to all contributors!

An aspect of this issue that is specific to the RV-12 is the possibility to unknowingly takeoff with fuel at elevated temperature due to the greenhouse effect created by a closed canopy while the airplane is parked for several hours in the sun. One may think that opening the canopy for a few minutes before takeoff will solve the problem, not realizing that it would take a much longer time to cool down 10 or 15 gallons inside the fuel tank. Someone earlier in this thread mentioned the importance of covering the canopy in this situation and I could not agree more. One year ago it happened to me during a quite moderate temperature (70 F) but sunny Spring day. I mistakenly attributed the loss of fuel pressure on takeoff to the mechanical fuel pump, I now know better! I have to admit that I had two aggravating factors: I was using a 91 Octane Mogas of dubious origin and I did not have the electric fuel pump on. I was lucky enough to get the "low fuel pressure" alarm just before rotation.

I will never ever again:
- Park in the sun without my canopy cover and the canopy crack open
- Switch the fuel pump off from engine start to shut down
- Use Mogas from a non brand station
 
FWIW, I have been flying my 12 for 210 hours in South Florida, and am tied down. It gets hot to very hot! I have a cover for overnight tie-down, but do not cover the canopy for the lunch trips, 1-2 hours on the ramp. I use Costco 93 exclusively, and to date, have never experienced and throttle hesitation, vapor lock, or similar issue.
 
Vapour Lock on takeoff can be deadly.
I seem to remember Vapour Lock on takeoff killed 2 people here in Melbourne, Australia about late 1970's in a certified aircraft (a Victa Airtourer, powered by a Continental O-200 or Lycoming O-235, using AVGAS).
 
FWIW, I have been flying my 12 for 210 hours in South Florida, and am tied down. It gets hot to very hot! I have a cover for overnight tie-down, but do not cover the canopy for the lunch trips, 1-2 hours on the ramp. I use Costco 93 exclusively, and to date, have never experienced and throttle hesitation, vapor lock, or similar issue.

Regarding gas I should have been more specific. I was using Ethanol free gas which became scarcer lately in my area. I had to switch to an ethanol free station that had only 91 octanes gas while I had always been using 93 Ethanol Free gas. I have since investigated this issue further and found that the ethanol free gas suppliers I looked at mention that this kind of fuel is not to be used in aviation. I suspect that the reason is that they are after the market of older boat engines and classical cars. For this market a vapor lock is less likely to happen and is just an annoyance when it happens. Since there is no such disclaimer by suppliers of ethanol gas I have decided to use only premium 93 ethanol brand name gas. The brand is not that important as it all comes from the same source and each brand puts its own additives including the ethanol. Using a brand name station guaranties a minimum quality and volume on the distribution side which is what you get also at Costco.
 
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