P

PH-SCP

Well Known Member
Hi guys,

I have been here before but I now I am getting a little desperate and need your brains.. Here's the situation.. Over the last few months I have had the engine quit on me during take-off. I start-up, taxi, do all the checks and line-up. Applying full power the kills the engine and I have a hard time starting it up again. It doesn't always happen, just occasionally.

The failure is hard to duplicate. It will operate without any problem for the next 20 flights or so and all of a sudden, the problems comes back. Just like today.

The red line in the failures is that it never occurs on the first flight of the day, always on the next leg (engine warm/hot).

The first time scared the s**t out of me and my engine overhaul company advised me to change the engine driven fuelpump. After the change, the engine ran fine until about two weeks ago when it quit on me during take-off after a prolonged taxi on the second leg of a journey. I suspected water in the fuel.
Today it happened again. Temperatures were rather high today (around 21C/70F) and after an uneventful flight the plane sat on a warm tarmac, we had coffee and cake and departed to have the engine quit again. Contributing factor was a training aircraft in front of me, blocking the taxiway. I spend 10 minutes just heating up the engine and once lined up on the runway and applying full power, it quit. I had great difficulty in starting it.

Engine-type is Lycoming O-320-D2J with a Marvel-Schebler carb. I burn 98 octane mogas.

I bought the plane from a guy in Florida and expected no problems with operating mogas in hot conditions. It burned 91 cargas in Florida all of it's life. I guess I could be very wrong here since it is vaporlock I suspect to cause the problems. As the engine cools down in the shade, it starts almost immediately and as long as it doesn't heat up at the holding point, I'm fine.

So here are my questions:

1. Do you guys agree to look into the vapor-lock question or should I consider looking at other items first ?
2. Can an O-320 run smoothly on the taxiway and show no signs of quitting due to vapor lock until full power is applied ? In other words, why doesn't the engine quit on the taxiway if vapor-lock is at hand ?
3. If we agree that vapor-lock is the cause, keeping the fuellines cool is my next task. What material must be used to insulate the fuel lines against the attacking heat ?

Really appreciate all of your inputs. Having the engine quit during the take off is turning (what's left of) my hair gray very quickly..

Thanks in advance !
 
you could try testing the vapor lock issue by adding some 100LL to get a 50% mixture. this would give you a idea if it is vapor lock. i seem to think it is something else. how about a carb float issue? is your mixture nob out further when leaned? good luck.
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I was having the same symptoms that you are describing last year. What i found is that I was running auto fuel purchased from the gas station and not mogas from the airport. The fuel purchased from the gas station was winter grade with a higher vapor pressure. I would have vapor lock issues whenever the daily temps were 70 and above with the winter grade fuel. I started running 110LL and haven't had the issue again.
 
I question if the "vapor lock" is actually the fuel is boiling in the carb bowl which then causes the float to sink which then floods the engine?
 
check out

This sounds kind of scary. Regardless of what happens with changing the fuel, I would think a thorough check of the fuel system would be in order. Just the usual stuff like make sure the fuel line is not kinked, all the B-nuts are snug, the line is not running over the header or other hot areas, the fuel line does not do any twist turveys, etc. maybe have a tech counselor look with you. With a carb setup, there is no return line, so any vapor that appears in the fuel line prior to the engine driven pump has to be pushed out by the electric pumps in the tanks, and with no return, this vapor has to wait until all the fuel ahead of it has burned. I think it would be bad if you were able to take off, and then the vapor gets in a bad spot. Please be careful.
 
fix for mogas and vapor lock

Simon,
This has been a very common serious problem with mogas and RVs. I had the same thing happen to me and searched the forum. The quickest and easiest solution that was recommended is to use Av gas in one tank for take-off and landings, and mogas for cruise. My A&P also insulated and straightened out the fuel lines. I knew it was vapor lock the second time it happen when I saw the fuel pressure go to zero even with the boost pump on when the engine stumbled on take off. When my O-320 engine was lower compression, it really didn't like 100LL (fouling plugs), but after an overhaul and increased hp and compression, the 100 LL works well. The extra cost for 100LL in one tank is very good insurance.
Cheers,
Bill McLean
RV-4 slider
Alabama
 
Wow Simon....you've had more than your fair share of issues! I don't have much to add other than I'd run a few tanks of Avgas through it and see how she behaves - and definately check over the fuel system.

Cheers,
Stein

P.S. - for those who don't know, Simon made an emergency "sully" river landing years ago in heavy IFR in rugged Canadian wilderness....quite a guy!
 
What about the plumbing.

Others have some very good suggestions, here are some of my suspicions.

A) Have you fire-sleeved the supply hose?

B) Your vents may be plugged?

C) The hoses may have a loose flap in the hose liner?

D) Vapor lock due to alternate fuel?

Best wishes,
 
Do you have a fuel pump shroud on the engine driven pump with a good source of cooling air? Same for the gascolator if its located forward of the firewall.

Bevan
 
If it is vapour lock it is due to the fuel boiling at the inlet to the mechanical fuel pump. I didn't see if you had the bost pump on or not.

As others have said make sure you don't have any restrictions, especially on the suction side of the boost pump and make sure the boost the system is making fuel pressure.

If you are married to the (God awful) mechanical fuel pump then add a cooling shroud and consider only runing 100LL for take off and landing.

If you finally can't stand the idea of a hydarulically flawed fuel delivery system then there are many posts that will tell you how to build a twin electric pump system that does away with the mechanical fuel pump.

Having said that, the standard system has worked well enough for many thousands of hours for most of the RV fleet. It is prone to vapour lock, especially with mogas though, and your post exactly describes why I don't run such a system.


Frank
 
Having re read your post I would concur these are classic vapour lock symptoms. If however you do have some partial restriction in the system (somewhere upstream of the mechanical fuel pump most likely) it will make the symptoms of vapour lock much worse.

Whats happening?...You are taxing to the runway under low power with almost no cooling. Temps are rising under the cowl and heat soaking the mechanical fuel pump.

You now go to full throttle and the mechanical pump sucks harder to make the full fuel flow. This then depresses the pressure upstream of the pump to below the boiling point of the now hot fuel under a partial vacuum.

The hot fuel vaporises much more easily than cold and it boils at the inlet to the pump. The pump can't pump vapour and the engine quits.

Just be glad it quits while your still on the runway!!!! Many airplanes have crashed on TO when the system decides to vapour lock 60 seconds later.

Frank
 
Try

100LL in one tank and mogas in the other. Should make it easier to troubleshoot. In the US our fuel changes from winter to summer formulations during the spring. If you have the same situation, high vapor pressure "winter" fuel could easily be the problem. At idle, your fuel flows are much lower, resulting in less tendency for vapor lock... At full throttle fuel flows, the tendency to vapor lock is at it's greatest.
 
My experiences with this issue

I built an RV6 with the 160 hp Lycoming and carb set up and flew it for 900 hours using mostly MoGas. (I also used MoGas in several other planes for years.) I also had issues, exactly as you describe them, and it turned out to be vapor lock. I made sure everything was installed correctly, that the fuel lines were not too close to the exhaust and that all fuel lines were fire sleeved. After all of this, there are two things that will actually address the problem.
One is, mix about 10-20% 100LL with the MoGas and the problem will go away.
The other is to install a vapor return line in your fuel system. This consists of installing a Tee fitting between the fuel line and the carb, with the open leg of the Tee fitting pointing up. (This should be right at the carb.) Then run a small (1/8-1/4") line back to a cool source of fuel, such as the fuel selector valve or the tank. You can put a restrictor fitting at the Tee where the return line connects to limit the amount of fuel running back, but a 1/8-1/4" line should work fine, since a lot of fuel isn't being sent back. At any rate, this allows the vapor to get out of the system and recondense at the cooler fuel source. (The fuel return line works fine whether you use MoGas or 100LL.)
As stated in previous posts, there are a lot of factors going on in a vapor lock scenario and they can be weather related. Considering winter vs summer blends of gas, along with differences of temperature from day to day, it can be difficult to determine what the real problem is. As someone stated, you could use 100LL gas for a while to determine if you have any other engine related issues, but one of the two methods I listed above should cure the vapor lock issue.
You can google on "vapor return lines" and get lots of hits about how to do this, as well as other people's opinions.
ECI discusses return lines as it relates to their Fuel Injection systems, see http://www.eci.aero/exp/RV6,7,9.pdf and it is basically the same thing.
(I only relate the above information as it worked for me. Others will surely have differing opinions.)
Good luck.
 
Steve F. has hit the nail on the head! :) And... please do not install what they advertise as a cooling shroud for the fuel pump! When on the ground running with minimal air passing through it makes the problem even worse. The fuel pump gets totally heat soaked. The shroud also hides the fuel pump, and that is not a good thing as the lines and fittings need to be inspected during pre-flight and maintenance.
 
Yes it is vapour lock. I ran 91 Octane mogas in my RV6 with exactly the same results.

Vapour lock is created in the fuel pump, not the fuel lines. Failure always occurred on the second run of the day, after the $100 hamburger is eaten.

One possible cure is to install a fitting, with restricted orifice, on the fuel line where it enters the carburetor. This line returns to the fuel tank and will bleed off the vapour. I did have this setup and still had two failures on the takeoff roll, I then opened up the orifice to bleed 12 litres per hour and have not had a problem since. I forget the orifice size, quite small, less than 1/16".

And to answer one of your questions, Yes the engine ran very smooth on taxi and run up, the failure only occurred at takeoff power, on the run. I then restarted the engine and took off almost immediately without further problems.

In Canada this occurs when the fuel mix is changed from winter to summer mix by the gas Companies. I did a vapour pressure test of the summer mix and it was the same as 100 low lead avgas.

So my procedure is to use mogas during summer, fall and winter and avgas during spring

Peter
 
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takeoff acceleration also a factor

In addition to all the factors mentioned above, the takeoff acceleration increases the pressure required by the boost pump to "lift" the fuel from the tank to the engine-driven pump.

During taxi, the electric pump would only be required to overcome the hydrostatic pressure due to the difference in level from the tank to the mechanical pump, which isn't much pressure. So little that the mechanical pump can successfully "suck" the fuel up, but not so reliably with bubbly fuel at high temps.

I now understand from recently and for the first time flying an RV-7 that the takeoff acceleration is quite brisk. That acceleration is in a forward direction and since the engine-driven pump is way ahead of the tank, the pressure required to deliver liquid fuel from the tank to the front end of the plane is greatly increased during takeoff acceleration and climbout. This factor alone could explain the loss of power without even requiring exceptionally hot fuel if the boost pump is on the weak side. Recall that this is all happening at maximum fuel flow unlike taxi or a normal runup.

I am gradually coming around to Frank's philosophy of viewing the ahead-of-firewall gascolator as the "fuel heater" and the engine-driven pump as the "fuel boiler". In addition to tight engine cowls and low fuel tanks these aircraft permit high takeoff accelerations and steep climbout deck angles. All of these factors add up to boiling fuel. When you add unpredicatable fuel formulations resulting in unknown vapor pressures, I think the end conclusion must be that trying to "suck" fuel up from any height at all with a hot, little pump, is a marginal proposition at best.
 
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I installed a purge system on my carbed O360 as you can see below. Consisted of a restrictor fitting from the carb inlet (there is a plug there so easy to install) and a fuel primer valve mounted on the firewall with a return back to the tank (plumbed in during construction). It was VERY effective (you can hear when the little facet pump is vapor locked by its rapid clicking).

I have since switched to AFP FI with a purge valve. I am a big believer in purging hot fuel in the FWF fuel system (I had a friend crash just after getting airborne due to vapor lock).

IMG_4385a.jpg
 
While vapor lock is a likely candidate do not exclude the fact that at high power settings you could be sucking air somewhere in the fuel system or the real possibility of a flap or partial flap in a fuel line.

Does it happen regardless of what tank you select?

I agree with the one comment above. This is serious and you neeed to examine every inch of your fuel system before the next flight.

One day it will occur during climbout rather than on the runway.
 
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Many thanks

Hello guys,

Thanks however so much for the great advice and hints you all gave.
I am convinced that my problem is vapor lock and I will address this issue by first firesleeving the fuel lines and install heat shielding as much as practicable.

I always use the electrical pump during take-off and landing.
The strange thing is that when it happened to me for the first time, switching the electrical pump on during take-off, killed the engine. Turning the pump off kept the engine alive.... Can anyone explain that ? It sounds like things happen in reverse when the fuel boils...

Also, I will start buying avgas (at $ 14,50/USG !!!!:mad:) to mix with mogas and experiment how much avgas it takes to keep vapor lock away. I will run avgas (or an effective mix) in the right tank and use that for take-off and landing and mogas in the left tank to cruise.

I have serious doubts about installing a shroud around the mechanical fuelpump. As mentioned, in my specific case vapor lock occurs during taxi and during take-off. A shroud sounds great but adds another layer around the pump and there is no airflow during taxi and run up to effectively cool the pump. In my particular case its sounds like a bad idea to install a shroud.

A purge system sounds very effective and it gets rid of all the hot fuel in the lines. After firesleeving and testing with avgas, installing a purge system is on the list.

@Stein... Nice to hear from you again. Thanks for the great support in the past. I still rather have an engine quit at 11000 ft than at 11 ft. This issue must be resolved quickly, specially since I am allready operating in God given "extra time".. :D

I will let you all know how I am progressing.
Thanks a million for the great input and please don't stop if you feel that there is more to report on this issue !
 
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