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Vapor Lock?

Ironflight

Ironflight

VAF Moderator / Line Boy
Mentor
What makes an airplane with close to a thousand hours and almost four years of experience suddenly begin to experience vapor lock after a fuel stop? I have been flying the Val for all that time and never had a single problem, until about the last two months, where I have had three experiences with it during short-turnaround stops; Twice when I had fueled, and once when I just shut down for about five minutes to drop off a passenger. I have looked over all the fuel lines and systems - nothing has changed, so I am going to have to blame it on this unbelievably hot and miserable summer we are having! Twice this happened after short local flights for fuel in the Houston area, and once over near Phoenix, where they appeared to be beta-testing the new climate from heck?.;)

Now anyone who knows me knows that I am pretty much a believer in bone-stock fuel systems. I have read too many accident reports where experimental aircraft have lost power due to fuel starvation to want to play around with that part of the equation. (Before I get a flurry of responses from those who have upgraded their systems from van?s design, let me say that I have nothing against carefully considered and well-thought-out modifications. I have a little heartburn with folks changing stuff just to be different and because they saw someone else do something similar?.that?s all!). I have a pure Van?s system with a carburetor, and in three summers, have never experienced this. Ten minutes of cooling down with the oil door open seems to relieve the problem, and once fuel is flowing - no problem at all. (Actually, since the problem manifests itself with the boost pump hammering away at high frequency, I believe the problem is with it sucking fuel out of the tanks, so I am not convinced the problem is under the cowl?.). There have been no in-flight issues, because once the fuel gets flowing, the problem disappears - and believe me, I do some serious ground power checks before take-off after experiencing this!

In all seriousness, and hoping to elicit ideas other than ?put fuel pumps in the tanks like cars do!?, I?m curious if anyone else has seen an increase in vapor lock incidents this year. Is it higher temperatures, and we have always been operating just a couple of degrees below the ?critical temp?? Is there something knew they?ve been doing to AvGas this summer? Could boost pump age have something to do with it? Am I just getting my turn-around times so short that the problem is just appearing? Any other ?out of the box? ideas?

And has anyone else noted a problem this summer, or is it just me?.?

Paul
 
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boost pump?

Perhaps a long shot that others could comment on, but if something has deteriorated in the boost pump so that it is loosing prime, it would now have to suck from the tank when hot, whereas before it might have been able to stay primed?

How about a partly obstructed tank vent that is requiring increased suction? got wasps?
 
I went to the Swift fuel forum at OSH and along with testing what they are developing, they have been testing samples of 100LL also. 100LL has over 82 ingredients in it. Their fuel will have 2. What they are finding is 100LL is not always the same from batch to batch. That could be an issue in hot weather.

The Hodges vapor pressure test kit might be in order to confirm the fuel is up to snuff with regard to is vapor pressure limits. If it does not test at over 60 kPa's something is not right.

Considering that 100LL is such a small part of all fuel produced and shipped these days, there could be some screw ups with its formulation, shipping and storage.
 
Ironflight said:
And has anyone else noted a problem this summer, or is it just me?.?
Click to expand...

Nope, it's not just you; my carbed O-360 with stock fuel system has done the exact same thing. I had it happen just once last summer during a quick turn fuel stop at Stinson Field over in San Antonio on a hot day... but so far this summer I've had it happen a handful of times. At first I thought the elec fuel pump was going bad, cavitating and making lots or noise, so I bought a new one to have just in case. Turns out the pump is fine, just a case of vapor lock. The symptoms are always the same as you described and appear after a short ground stop on a warm day. I always park with the oil door open, but apparently it's just not enough...

The last time it happened was over at Kestrel on a warm evening after taking on a load of "cheap" gas. There I was at top of the hill doing a run-up, popping, sputtering and surging with a few locals watching nearby from a golf cart... they probably thought I was crazy after making all that racket that I went ahead and took off, but with power up and lotsa fuel flowing, the problem goes away. So if the local Kestrel crowd brings up a crazy RV-8 driver departing at sunset with a rough engine, it was just me.

BTW, anyone need a new Facet boost pump, pn ES 40108? It's floating around here somewhere, new in the box...
 
I've tested 100LL a few times with my Hodges tester and have found that the tendency for vapor to form is only slightly less than 87 octane pump gas with ethanol. The margins are not anywhere close to what one might expect for 100LL.
 
Paul,
If you are opening the oil door, do you still park your nose into the wind? You may find that the firewall area cools down quicker with the oil door open and the tail pointed into the wind. The air that is being "pushed" into the lower cowl when parked tail-to-wind will amplify the chimney effect that is caused by the rising heated air and rush out of the oil door. Give it a shot.
 
Paul,

My hangar mate had an RV-6A (carburated and no funky fuel mods) that exhibited similar symptoms and he too believed it was vapor lock and spent time attempting to cool the engine driven pump, but to no avail. It seems to be that the culprit was the electric boost pump somehow blocking the fuel even though it's not supposed to do that. I wrote "seems to be" because there was no conclusive evidence to prove this point, and no "smoking gun" fuel system problems were found upon post-crash analysis. He now flies an RV-7 he built with funds from the insurance settlement.

Best of luck,
Rob
 
rocketbob said:
I've tested 100LL a few times with my Hodges tester and have found that the tendency for vapor to form is only slightly less than 87 octane pump gas with ethanol. The margins are not anywhere close to what one might expect for 100LL.
Click to expand...

I concur Bobbert. I have done the same testing and there was just barely a noticable difference between 100LL and the 92 octane I use. Maybe a needle width is all, not worth worrying about.

Paul, are you having the problem with the boost pump on?
 
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tinman said:
Paul,
If you are opening the oil door, do you still park your nose into the wind? You may find that the firewall area cools down quicker with the oil door open and the tail pointed into the wind. The air that is being "pushed" into the lower cowl when parked tail-to-wind will amplify the chimney effect that is caused by the rising heated air and rush out of the oil door. Give it a shot.
Click to expand...

I actually played around with positioning a bit, since opening the oil door doesn't do much unless you can get a breeze through it - and if you get cowl inlet air out through the oil door, that doesn't say much for the baffle seals!;)

Unfortunately, along with the high heat comes very little wind. But since the symptoms show up as a really fast boost pump before even cranking the engine, i keep thinking this is upstream of the firewall anyway.

Very interesting to note that i am not alone in noticing a difference this summer!

Paul
 
Steve

allbee said:
This is all interesting. I to have lately been having troubles with my IO360. The outside temp at about 95. I get the fuel, once I opened the oil door, but that didn't do anything. This is what happens to me. I have a purge valve, so I let it run and put fresh fuel into the lines. I keep everything running on the electric fuel pump, but I don't go rich at all. I start the engine, as I do that I push the valve shut on or open which ever way you want to call it and the engine starts right up. But, here is where I get the problem. The engine runs rough with sputters, like pulling the power off coming in to land, pops, if I lean any at all on the mixture, the engine will quit. So the only thing I can do is run close to 1500rpm until it smooths out. I also found that on one attempt I turned off the electric pump and the engine quit or wanted to. I quickly turned the electric pump back on and increased the throttle. I almost thought I had a bad manual pump. Are we getting some bad 100LL. Or at least some that is not to spec?
Click to expand...

The problem you are seeing is different to what Paul describes.

he (and others it seems) is getting true vapour lock of the electric pump.

You on the other hand are most likely getting boiling within the injector lines. This is downstream of both the pumps and by definaition is not vapour lock..Its just an irritation..One the flow comes up with the power the problem goes away.

Frank
 
Interesting problem.

This is the first I've heard of a standard electric pump vapour locking. Fourtunately it is only happening during a hot start.

I wonder if the Facet pump is being heated?..It is bolted to the firewall right?

Is the firewall insulated?

Could the pump be moved further aft and isolated from surrounding heat sources?

Are the tank screen's clean? Vents open?

This is caused by a combination of high vapour pressure/low pump inlet pressure/hight ambient temp. Which means bad fuel/ inlet restriction or a hot pump.

That was Frank's class on stating the obvious!..:)

Frank
 
When I had a carb....

I experienced the same thing, fast beating little facet which was obviously not pumping anything was the tell-tale sign prior to start. I cured the problem (I know you are not looking for solutions but here is one anyway) by installing a purge valve. When I built the plane I put in provisions in case I wanted and was glad I did. Simple system by teeing off the carb inlet and plumbing in a "primer" valve back to the tank. Hitting the "Purge" switch would immediately clear the problem and you could hear the pump clearing and start pumping again. Never had another problem and the system is simple and used all standard fuel components so nothing was really "experimental".

Since I switched over to the AFP FI system I now use their purge valve which basically elimates hot start issues.

I really consider this a valuable safety improvement over the "stock" Van's system, vapor lock has bent a lot of airplanes and there is a simple cure with reliable off the shelf components so why wouldn't you? (assuming you live in a hot climate like we do).

In fact I still have the valve and some hoses sitting on the shelf if you have any interest.
 
Intalling a primer purge

is a pretty good idea.

The easiest thing would be to plumb the discharge upstream of the selector valve (in say the right tank line).

Upon start select the LEFT tank and purge some fuel..That way you don't recirc the hot fuel back into the pump inlet.

Mind you this maybe sounding "funky"..But not as funky as my fuel system...:)

Frank
 
Yes but where

allbee said:
vapor lock, boiling fuel. Is there really a difference. I feel mine is boiling fuel. Lots of air in the system. Sure mine is fuel injection. I feel the fuel is the problem all the way. I'm thinking something is up with the fuel. If I don't use the purge valve, it's real hard to start. call it what you want, I think something might be up with the fuel.
Click to expand...

Vapour lock implies a pump won't pump fuel, because it is trying to pump vapour.

This is very different to localised boiling in the injector lines.

So different in fact that if one of these conditions happens in flight it could kill you.

The other one won't..Its just an irritation.

Sorry to sound like a pedantic engineer but the diiference is real and huge.

Frank
 
Has anyone tested the 100LL in their tanks for ethanol? I know it's not supposed to be there but accidents/shenanigans do happen. Those that have responded as experiencing the same problem appear to all be in Texas. It's not beyond the realm of possibility that you're all getting fuel from the same refinery.
 
frankh said:
I wonder if the Facet pump is being heated?..It is bolted to the firewall right?
Click to expand...

Interesting point Frank. I'm in Texas and haven't experienced the problem. My Facet pump is just forward of the main spar, well aft of the firewall.
 
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IIRC we are all getting fuel from the same refinery as there is only ONE in the US making 100LL. Not sure where I read this so I'd be happily corrected if anyone has facts instead of what might just seem regurgitated speculation on my part....
 
tim.randles said:
Has anyone tested the 100LL in their tanks for ethanol? I know it's not supposed to be there but accidents/shenanigans do happen. Those that have responded as experiencing the same problem appear to all be in Texas. It's not beyond the realm of possibility that you're all getting fuel from the same refinery.
Click to expand...

Tim,

Ethanol does not cause or contribute to vapor lock. I've tested E85 and it's vapor pressure came it at 60 kPa's.

Ethanol does have a down side with its corrosiveness with some materials but vapor pressure is not a problem with it.
 
fuel pressure with vapor lock?

Those of you who have this problem on a carbureted engine and have a fuel pressure gauge, what happens to indicated fuel pressure when this is going on? Does it go up because the fuel is boiling... down because the pump isn't working... or...?

--Paul
 
David-aviator said:
Tim,

Ethanol does not cause or contribute to vapor lock. I've tested E85 and it's vapor pressure came it at 60 kPa's.

Ethanol does have a down side with its corrosiveness with some materials but vapor pressure is not a problem with it.
Click to expand...

Your reply caused me to challenge my assumptions about ethanol and fuel vapor pressure. I was surprised by what I found.

Saying "Ethanol does not cause or contribute to vapor lock" isn't necessarily true. E85 according to http://www.e85fuel.com/pdf/storing-handling-e85.pdf (page 8) is anywhere from ~41-82 kPa. A readily found source ( http://www.shell.com/static/au-en/downloads/aviation/avgas_100ll_pds.pdf ) cites Shell 100LL at 44kPa. E85 can have a much higher vapor pressure than 100LL, meaning it will vaporize much more readily than 100LL.

There is an interesting discussion of ethanol-blended gasoline and evaporation characteristics at http://ethanolproducer.com/article.jsp?article_id=2729&q=&page=1 starting at the bottom of that page under the heading "E10 Emissions Dogleg." That brief discussion suggests that a low level of ethanol contamination could actually cause more vapor problems than a heavy blend like E85. If the E85 vapor pressure you measured was 60 kPa, which is already around 35% higher than what Shell claims for their 100LL avgas, and a light blend of gasohol is more volatile than E85, contaminated 100LL should be very suseptible to vaporization. One last source ( http://www.afdc.energy.gov/afdc/vehicles/emissions_e10.html):
This is largely because at low-blend levels, ethanol increases the Reid vapor pressure (RVP) of gasoline (making it evaporate more easily) and makes the vapor more permeable through fuel lines.
Click to expand...

I like VAF, it makes me think about things... :)
 
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vapor lock, or...?

frankh said:
Go to nearly zero by definition.
Frank
Click to expand...

I asked because a while back I had a hot weather engine stoppage (on the ground, after a long slow taxi and long wait in the runup area with no wind) that I thought was vapor lock. But I looked at the fuel pressure at the time and it was high (9 psi or something like that, normal is around 5). So was heat-expanded fuel causing overpressure and flooding? Makes me wonder if maybe some hot weather problems might be that instead of vapor lock.

--Paul
 
Paul,

Regarding your observation that the problem seems to be upstream of the electric pump - is there some sort of check valve in it? In other words, why wouldn't boiling fuel in front of the firewall push fuel (vapors) backwards through the system after you've shut down? (It would seem that the pump would need a check valve, but I've never even had one in my hands.) Then, when you turn on the pump, it has to increase the pressure firewall forward enough to re-liquefy the vapors. Only then can fuel move forward, assuming the float is full and the needle valve closed.

Without knowing anything about the expected distribution of 100LL vapor pressure over the last three years, it is tough to say if you are just experiencing a lower vapor pressure period than you have before or if indeed something has changed with the airplane. The latter seems unlikely, but I've sure seen a lot of "unlikely" things indeed be causes...

There were a couple posts in this thread which have been deleted regarding vapor lock, or what is called that, in an injected plane. In this particular case, it was stated that turning the boost pump on stopped the problem for him, which indicates the problem was indeed vapor lock, and not just boiling in the injector lines. The pressure in the injector lines would not be changed by turning on the boost pump, unless there was a problem in the servo.
 
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Check valve?

Hmmm, after additional thought - perhaps if the boost pumps for carbs indeed do have a check valve, perhaps yours is leaking a little? This would perhaps allow vapor up front to push fuel backwards.
 
frankh said:
This is the first I've heard of a standard electric pump vapour locking. Fortunately it is only happening during a hot start.

I wonder if the Facet pump is being heated?..It is bolted to the firewall right?

Is the firewall insulated?

Could the pump be moved further aft and isolated from surrounding heat sources?

Frank
Click to expand...

Actually Frank, in the -8 with a Carb, the fuel pump is int eh cabin on the left sidewall, after of the firewall about 10 inches, so it's not firewall heat, or related to it (my left foot is still attached and it is right next to the pump).

The suggestion of a purge valve is a good one, and in fact - I have one - it's my primer solenoid, which is downstream of the fuel pump. I decided to try it the other day when I had this problem on a scorching ramp in San Antonio, and the result was inconclusive because a Citation was cranking up right next to me,and I couldn't hear myself think, much less what the pump was doing. I quit and tried five minutes later, and got a good start. (I'd hate to add an additional purge valve on the theory that what isn't there can't fail....but I'm not totally against it Walt - thanks for the idea!).

I'd love to know more about the inerds of th electric pump Alex - I wonder if anyone has a drawing or dissection photos of the Facet "cube"?

And the theory that it is Texas fuel, well, I've been to both the west coast and Minnesota in the past month - bought fuel all over.

Paul
 
tim.randles said:
Your reply caused me to challenge my assumptions about ethanol and fuel vapor pressure. I was surprised by what I found.

Saying "Ethanol does not cause or contribute to vapor lock" isn't necessarily true. E85 according to http://www.e85fuel.com/pdf/storing-handling-e85.pdf (page 8) is anywhere from ~41-82 kPa. A readily found source ( http://www.shell.com/static/au-en/downloads/aviation/avgas_100ll_pds.pdf ) cites Shell 100LL at 44kPa. E85 can have a much higher vapor pressure than 100LL, meaning it will vaporize much more readily than 100LL.

There is an interesting discussion of ethanol-blended gasoline and evaporation characteristics at http://ethanolproducer.com/article.jsp?article_id=2729&q=&page=1 starting at the bottom of that page under the heading "E10 Emissions Dogleg." That brief discussion suggests that a low level of ethanol contamination could actually cause more vapor problems than a heavy blend like E85. If the E85 vapor pressure you measured was 60 kPa, which is already around 35% higher than what Shell claims for their 100LL avgas, and a light blend of gasohol is more volatile than E85, contaminated 100LL should be very suseptible to vaporization. One last source ( http://www.afdc.energy.gov/afdc/vehicles/emissions_e10.html):


I like VAF, it makes me think about things... :)
Click to expand...

Me too. And sometimes I get easily confused by facts by experts.

Here is what kPa means to aviators using mogas and the approved method of testing for vapor pressure limits.

The kPa vapor pressure numbers I am familiar with are off the Hodges test kit meter. They are in a vacuum so the higher the number the better. As you go down the scale, somewhere around 34 there is a red line. What that means is a fuel testing at 34 kPa is good for flight to just over 10,000' where it will boil. You arrive at that altitude by subtracting 34 from 101 (SL kPa) and going into a standard atmospheric pressure chart with 67 kPa.

The number I saw for the E85 sample was 60 on the Hodges meter. 101-60=41. 41 kPa on the standard chart is about 22,500' or about the same as 100LL usually tests for.

That's all I know about the subject. :)
 
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Maybe its the carburetor?

Ironflight said:
What makes an airplane with close to a thousand hours and almost four years of experience suddenly begin to experience vapor lock after a fuel stop?
Click to expand...

Paul,

I had something similar happen several times in the last year, always on hot days. Never happened previously in the first few years operation. I was able to overcome the problem by starting with mixture full lean, then pushing to rich as the engine fired. This seems to work every time.

My theory is that this is a carb problem and not vapor lock. Even a hot engine is still going to have fuel sitting in the carb bowl, left over from the previous run. This fuel should get sucked into to the engine and permit it to run at least temporarily, regardless of what the rest of the fuel system is doing.

I suspect that the idle system is delivering a too-rich mixture during hot starts, although I can't explain why. The carb manual mentions that the idle fuel is mixed with air to make an "emulsion" in the idle fuel channel; maybe this emulsion gets richer when its hot? Something could have happened to the carb to make the emulsion richer than normal? (blocked vent?)

Then again it could be global warming...
 
David-aviator said:
Me too. And sometimes I get easily confused by facts by experts.

Here is what kPa means to aviators using mogas and the approved method of testing for vapor pressure limits.

The kPa vapor pressure numbers I am familiar with are off the Hodges test kit meter. They are in a vacuum so the higher the number the better. As you go down the scale, somewhere around 34 there is a red line. What that means is a fuel testing at 34 kPa is good for flight to just over 10,000' where it will boil. You arrive at that altitude by subtracting 34 from 101 (SL kPa) and going into a standard atmospheric pressure chart with 67 kPa.

The number I saw for the E85 sample was 60 on the Hodges meter. 101-60=41. 41 kPa on the standard chart is about 22,500' or about the same as 100LL usually tests for.

That's all I know about the subject. :)
Click to expand...

Ah! Thanks for the clarification of methods. I was only familiar with RVP given my background and unaware of the Hodges kit. It's important to know the technique used to arrive at some value, not just the units of measurement. Although that can be an issue as well. Mars Climate Orbiter anyone? :rolleyes:

Thanks for sharing some knowledge Dave.
 
Originally Posted by Ironflight
What makes an airplane with close to a thousand hours and almost four years of experience suddenly begin to experience vapor lock after a fuel stop?



There needs to be a starting point to find out what is going on with Paul's airplane. Perhaps checking the fuel would be a logical place to start. It needs to be tested for vapor pressure and ethanol. We know 100LL will become vapor at 130F. If the quality of the fuel is not up to standards, that number will be lower.

It is a pain in the butt to have to do it, but testing the fuel does give some comfort when the numbers come in where they should be.

I do not believe it is a mechanical issue with the fuel system, it is a temperature issue with the fuel for whatever reason.
 
AlexPeterson said:
Hmmm, after additional thought - perhaps if the boost pumps for carbs indeed do have a check valve, perhaps yours is leaking a little? This would perhaps allow vapor up front to push fuel backwards.
Click to expand...

Maybe Alex is on to something. If the fuel pump check valve is allowing fuel to leak back through and evacuating some of the fuel line forward of the pump it would make vaporization more likely. Would this be evident during normal operation as a slightly lower than normal fuel pressure?
 
How do we know...

I read through the posts and didn't see this suggestion (or maybe I read right past it).

How do we know that the fuel hasn't boiled out of the carb bowl via the vent or into the sump to begin with? You turn the facet pump on and it happily and quickly refills the bowl until the needle valve shuts off and facet pump stops/slows.

Not being familiar with these carbs, I don't know if this experiment can be done. But, is it easy to drain the bowl itself? I know, it involves cowl removal, cool down, etc. But it may provide a piece of the puzzle.

Anyways, just a thought...
 
pump heat is a contributor?

frankh said:
I wonder if the Facet pump is being heated?..It is bolted to the firewall right?
Click to expand...
I got to thinking about Frank's idea about a hot pump and it started to make more sense. I once had a car that worked perfectly until the mechanical fuel pump failed. The original pump was bolted to the aluminum cylinder head (think HOT) and had a phenolic spacer of some sort. The replacement pump had a different geometry, and did not use the spacer; it attached directly to the head. You guessed it - vapor lock city. Usually when sitting at a traffic light. It was unfixable (no original style pumps existed) - so I put an electric pump in series. When it would cough I'd hit the switch.

On my RV, the facet pump is bolted to the insde of the SS firewall down low in the center, right where the hot air exits the cowl. Seems to me that the pump could be getting heated significantly by the firewall.

I wonder if someone could stick a thermocouple on the pump body? That would be easy to do and determine how hot it gets. I'm not sure how hot would be too hot, but it would be interesting to know.

If it was hot, I bet a 1/4 inch or so plastic spacer between firewall and pump body would help. I may look into this idea anyway. I'd only have to re-make 1 fuel line :(
 
Yes

I'm thinking this could very well be vapour being pushed back through the pump.

A spring check valve won't necessarily work in a vapour situation anyway..i.e if there is a spring check in the pump it could still elt vapour go backwards.

Sounds lie an experiment is in order..I.e shut the selector valve to off (can you do that?..I don't use one so I don't honestly know)..And also try the the purge valve to see if the pump re-primes.

Frank
 
Carb bowl

David_Nelson said:
How do we know that the fuel hasn't boiled out of the carb bowl via the vent or into the sump to begin with? You turn the facet pump on and it happily and quickly refills the bowl until the needle valve shuts off and facet pump stops/slows.

Not being familiar with these carbs, I don't know if this experiment can be done. But, is it easy to drain the bowl itself? I know, it involves cowl removal, cool down, etc. But it may provide a piece of the puzzle.

Anyways, just a thought...
Click to expand...

I've removed my carb recently (after warm-weather operation) and there was still lots of fuel in the bowl. It would be a pain to pull the drain plug, but easier to check by pumping the throttle a few times (with the engine cold). If there's fuel in the bowl it will be squirted out by the accelerator pump and drain back down through the airbox.
 
The evidence does seem to support a theory of "vapor expansion pushes liquid fuel back through the pump toward the tanks". Theory requires an experiment to prove or disprove. I'd suggest a direct observation experiment; simply plumb in a temporary short length of clear polyurethane fuel line at the pump inlet. Ground run, get the system hot, observe the line.

Only one thing bothers me about the above theory, but perhaps I didn't fully understand the information. Paul, can you clarify? You report a hot start problem, carbed engine, standard diaphram pump, Facet pump. At shutdown, the float bowl, lines, and pumps should all be full of liquid fuel. Heat soaking may boil the fuel in the engine driven pump and/or the lines and/or the Facet pump....but as noted above, the float bowl should remain full. The engine should hit when you crank, although it may die some seconds later as bowl level drops without incoming fuel. Is the hot start problem "no combustion at all" or "fires up and then quits"?
 
DanH said:
Only one thing bothers me about the above theory, but perhaps I didn't fully understand the information. Paul, can you clarify? You report a hot start problem, carbed engine, standard diaphram pump, Facet pump. At shutdown, the float bowl, lines, and pumps should all be full of liquid fuel. Heat soaking may boil the fuel in the engine driven pump and/or the lines and/or the Facet pump....but as noted above, the float bowl should remain full. The engine should hit when you crank, although it may die some seconds later as bowl level drops without incoming fuel. Is the hot start problem "no combustion at all" or "fires up and then quits"?
Click to expand...

I've experienced what I believe to be the same issue on my carbed 0-320 on hot days with quick turnarounds. The engine acts fuel starved and won't fire.

The "fix" has been to pump the throttle a time or two with the engine turning. Once the engine fires, it runs fine.
 
Kyle Boatright said:
I've experienced what I believe to be the same issue on my carbed 0-320 on hot days with quick turnarounds. The engine acts fuel starved and won't fire.

The "fix" has been to pump the throttle a time or two with the engine turning. Once the engine fires, it runs fine.
Click to expand...

Yes, the engine appears to be fuel starved. You can get an occasional "puff" out of it, but pumping the throttle doesn't help at that point - still no fie. The real tell-tale not to forget is that the Facet pump is clearly running way faster than it does when liquid-filled - and it is a good six-8 inches behind the firewall. Bone stick, Van's designed fuel system.

I'm still glad to hear I am not the only one, because this is a strong argument against there being something wrong with my particular aircraft. Still an interesting problem. It really would be nice to have someone who has the ability to really lab test a sample of this summer's fuel do it.

Paul
 
You could do it

Simply get a syringe and a pressure guage and make sure everything is at 100F (the RVP reid vapour pressure is measured at 100F) and pull a vacuum till it boils.

Then of course one needs to find the spec for the RVP of the fuel.

I could easily see the vapour could expand to push a few feet along a 3/8th tube.

The other thought is presumably there is an overflow on the carb?..If so this is going to boil off as well..So when you come to start there is say half the fuel gone from the float bowl, plus the facet is full of vapour and won't pump very well

Hence hard to start.

Frank
 
Maybe two problems?

Ironflight said:
Yes, the engine appears to be fuel starved. You can get an occasional "puff" out of it, but pumping the throttle doesn't help at that point - still no fie. The real tell-tale not to forget is that the Facet pump is clearly running way faster than it does when liquid-filled - and it is a good six-8 inches behind the firewall.

Paul
Click to expand...

I had trouble starting today, after a quick-turn for fuel on a hot morning. Then engine was off for about 10-15 minutes. Wouldn't start using the "pump on, mixture rich, no prime" procedure. It did start easily with the mixture full lean, then advanced slowly to rich as the engine fired. Unfortunately I wasn't paying attention to what the pump was doing.

Some fuel could be boiling out of the carb bowl, but seems unlikely all of it would (I didn't see any puddles). The idle port is just above the throttle plate, designed to be in an area of strong vacuum when starting. Maybe pulling this vacuum is causing the already hot fuel to boil into the carb throat, making the mixture too rich? Flooded start could produce some puffs.

This doesn't explain the Facet pump running fast, but the above explanation doesn't exclude the presence of vapor lock in the fuel lines (there could be two different problems, both heat related).
 
How about pre-heated fuel? Paul's fuel tanks are red which causes solar heating. Could the fuel be getting hot enough in the tanks to cause this issue? Do we have any "white wingers" having this issue?
 
Hot summer blues

Paul: Being a desert rat, I understand your situation. I firesleeved all fuel lines ahead of the firewall, and things improved.
TN
 
Tom Navar said:
Paul: Being a desert rat, I understand your situation. I firesleeved all fuel lines ahead of the firewall, and things improved.
TN
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Thanks Tom! Good to see that you're still monitoring....unfortunately - I've already got all the fuel lines covered - have since day one. It has made me wonder if the insulating qualities of firesleeve deteriorate with time - but checking the sleeve, it all looks good.

We actually had a thunderstorm here in Houston this afternoon that cooled things down a bit - like down to 90. Seemed almost pleasant in the hangar at that temp...

Paul
 
From WAAAY out in Left Field..........

Ironflight said:
Yes, the engine appears to be fuel starved. You can get an occasional "puff" out of it, but pumping the throttle doesn't help at that point - still no fie. The real tell-tale not to forget is that the Facet pump is clearly running way faster than it does when liquid-filled - and it is a good six-8 inches behind the firewall. Bone stick, Van's designed fuel system............. Paul
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Paul, I had a starting problem with my carb'ed O-320 that appeared to be a fuel problem, as well. After several months of wondering, a friend correctly diagnosed the left mag impulse coupler as the culprit. It would function (click) when the prop was pulled through, but at cranking speed, it didn't work properly. When we listened carefully during cranking, we could hear that it wasn't clicking with any regularity at all. Result, lots of cranking, but no popping noises. It did get worse when the engine/mag was hot.

And BTW, my Facet pump seems to speed up noticeably when the fuel flow stops, and the pump is holding pressure with fuel in the line. No experience with what it does if there is no fuel to pump.

Hey, I said this was from Left Field!:rolleyes:
 
Hey, when I was a (no-sport-talent) kid, they always put me WAY, WAY out in left field.....;)

The Val is coming up on 1,000 hours - about time for my 500 hour Mag inspections!

Paul
 
What is the mechanism of 'vapour lock'?

I simply don't understand the mechanism that is in play to cause the vapour lock issue.

Casting aside the possible mag issue, I think it is safe to assume that 'vaporised fuel' is a problem. Just too many reports of normally functioning aircraft failing in hot conditions.

I am sceptical that it is a problem with drawing fuel from the fuel tank with the negative pressure exceeding the VP of the fuel - possibly at 22k feet this might become an issue, but at sea level, tanks near full and with the Facit pump mounted inside the cabin, I doubt it.

Considering that the issue arrises usually with a hot engine - I believe the problem is vapourisation in the fuel line between the fuel pump and carby. If this pressure exceeds the max pressure that the fuel pump can generate - it will prevent the pump from passing fuel.

I can understand that if the float bowl is full and the needle valve has shut off the fuel inlet to the carby how this pressure might be retained in the fuel lines.

What I do not understand is why the condition persists during the starting procedure and why the fuel flow to the engine from the carby bowl could be inhibited. It is interesting that pumping the throttle helped in one persons experience - this would transfer fuel via the accelerator pump. Also, fitting the primer purge supports this as well.

The instant the carbs needle valve opens I would expect the vapour pressure to be released into the bowl chamber and out via some vapour purge oin the carby. Is there such a path in the carby?

I would have also expected that fuel pressure indications would be high in this instance rather than zero.

The information does not fit together for me.

Doug Gray
BTW Left field is where you dream about building airplanes and other wonderful magic.
 
Lets not over analyze this...

I never had a problem starting mine, it was just the facet pump running fast and not pumping (sounded like a dry pump) that occured. After installing the purge valve it took just a few seconds or less of purge before you heard the pump slow down and start pumping again like normal. I sumised the fuel was boiling in the gascolator and fuel lines FWF and vapor pushed back to the pump and caused it to vapor lock, no where else for the vapor to go. Until you release the vapor pressure fwd of the pump either by running or by purging the situation would continue.
 
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