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Loss of Power on Takeoff, believe Vapor Lock

mosquito

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

On the ground, my fuel pressure was pegged, 11psi or so, the highest I've ever seen, very unusual. Activating the boost pump made no change to the reading, not even a blip. Also very unusual. After pointing the plane into the wind for a few moments, I switched to avgas and tried to start. She started after a couple of sputters, and the fuel pressure instantly jumped down to normal readings, where it stayed the rest of the day.

Back at the FBO I met RWayne who helped us secure some vice grips. Thanks, Wayne! We removed the top cowl for inspection. Everything looked fine, so we waited for cooldown, fueled with avgas, and made the 200nm flight home with no issues.

The plane is an RV-6 with a carbed O320. It was approximately 85 degrees in Vidalia.

I did write up a much more detailed narrative while it was fresh in mind, as the ~20 second flight was rather memorable. I can post it if anyone is curious, maybe in the 'lessons learned' category. I'd like to think it reads at least as well as an NTSB report. This is the "executive summary."

Does the "high fuel pressure/black smoke" combo make sense? I had believed fuel pressure would show low, or even zero, with vapor lock. The black smoke/power loss clearly points to a very rich condition, which has led an a&p friend to speculate on a potential carb problem. But I've also read accounts of vapor lock causing this.

Any opinions? Thanks,

-jon
 
First off, great job making it back safe. 250' isn't much to work with.

I'm no expert, but none of that sounds like vapor lock to me. I would not fly again until you have nailed down and fixed the cause. What you described sounds like a too-rich condition possibly caused by something stuck open in the carb.

None of this is to second guess you as I'm sure you made the best decisions possible with the information you had. Best of luck resolving things!

Chris
 
I would be looking at the carb float very closely to see if it is worn enough to allow it to snag against the bowl which could cause an extremely rich condition. Check for a sloppy fit of the float and pin and for any signs of abrasion between the float and carb body. Take a close look at the needle and seat while you have the carb apart.

I don't have an explanation for the high pressure indication unless something is failing in the mechanical fuel pump.

It would seem to me that vapor lock would result in a lean condition. I've been flying 100% auto gas in a carbed O-320 for nearly two years and never have experienced the symptoms you described.
 
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Excess fuel pressure can cause the carb needle valve shut off to be pushed off its seat allowing excess fuel into the carb causing the overly rich mixture. Fuel pressure spec is 0.5 - 8 PSI max. I would replace the mechanical fuel pump.
 
Pneumatic Lock

Sounds maybe like pneumatic lock:

Fuel boils in the carb bowl, float sinks, causing an uber-rich condition. Seems to match what you had happen.

In the summer, I try to always have 50/50 avgas/mogas mix in takeoff and landing tank just incase there is some winter mogas floating around after it warms up.

I run an O-320 with a carb in my -9A.

If vapor lock is encountered in flight, the first signs would be an increase in exhaust gas temperature (EGT) readings, since fuel flow would be restricted due to vapor formation. If the vapor lock becomes severe enough the engine runs rough, much as it would if too lean, or it quits altogether. Once the engine begins to run rough, the pilot should immediately lower the nose and reduce the throttle setting to the minimum required to sustain flight. This reduces fuel flow and hence the demand placed on the fuel system. Normal throttle settings may be resumed when smooth running returns. If the airplane has a fuel boost pump and the engine begins to run rough, the boost pump should be turned on. If the pump is already on when the engine begins to run rough or quits, then the pump should be turned off. Adjusting the mixture control can also help to alleviate these conditions. Make the mixture rich to counter a vapor lock, lean for pneumatic lock. The first symptom of pneumatic lock is an increase in fuel pressure followed by rough running or engine failure. Fuel blended with ethanol is more likely to produce vapor lock than straight gasoline.
 
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Loss of Power

You might have experienced an issue with the use of automotive Winter Blend fuel on a hot day. Winter blended auto fuel has a lower vapor threshold, which is made worse by higher outside air temps. Having a carb'ed engine makes you a lot more susceptible (low pressure pump).

I use 93 octane E10 in an RV-7A with an IO-360 all the time, and have seen this near the time of year when the gas stations are switching over to summer blends...


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

On the ground, my fuel pressure was pegged, 11psi or so, the highest I've ever seen, very unusual. Activating the boost pump made no change to the reading, not even a blip. Also very unusual. After pointing the plane into the wind for a few moments, I switched to avgas and tried to start. She started after a couple of sputters, and the fuel pressure instantly jumped down to normal readings, where it stayed the rest of the day.

Back at the FBO I met RWayne who helped us secure some vice grips. Thanks, Wayne! We removed the top cowl for inspection. Everything looked fine, so we waited for cooldown, fueled with avgas, and made the 200nm flight home with no issues.

The plane is an RV-6 with a carbed O320. It was approximately 85 degrees in Vidalia.

I did write up a much more detailed narrative while it was fresh in mind, as the ~20 second flight was rather memorable. I can post it if anyone is curious, maybe in the 'lessons learned' category. I'd like to think it reads at least as well as an NTSB report. This is the "executive summary."

Does the "high fuel pressure/black smoke" combo make sense? I had believed fuel pressure would show low, or even zero, with vapor lock. The black smoke/power loss clearly points to a very rich condition, which has led an a&p friend to speculate on a potential carb problem. But I've also read accounts of vapor lock causing this.

Any opinions? Thanks,

-jon
 
Excess fuel pressure can cause the carb needle valve shut off to be pushed off its seat allowing excess fuel into the carb causing the overly rich mixture. Fuel pressure spec is 0.5 - 8 PSI max. I would replace the mechanical fuel pump.

This is the first thing I've heard that ties together the black smoke and high fuel pressure. Very interesting.

I should add that I have experienced 'burps' on takeoff power with mogas exactly three times in the past, just a stumble that cleared up very quickly. There has never been so much as a hiccup with avgas. There was a rather long taxi and wait sequence prior to this departure; the p51 behind me even shut down during the wait. Hence my quick suspicion of vapor lock.

There is a bit too much fuel plumbing in front of the firewall, with about 14" of extra firesleeved line to place my fuel flow transducer. I had not burned mogas prior to installing the transducer. I intend to relocate the transducer to the cockpit to reduce fuel line exposure up front, in addition to carb inspection, etc.

Walt, do you suspect the mechanical fuel pump may have caused the incident, or are you thinking that it may have been damaged by the higher fuel pressure?

Thanks so much everyone!

-jon
 
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I agree with Walt. Sounds like the engine driven pump is failing causing high pressure which overcomes the the float and needle. When a pump fails it usually looses pressure or leaks fuel externally, or into the crank case, but I have read of circumstances where a mechanical pump failed and overpressurized a carburetor.
 
Pneumatic Lock

PeteHowell has it right. I have used a lot of auto fuel in my RV6 in the past 13 years, and I have experienced many pneumatic lock events. It can happen with avgas but is much more likely to happen with auto fuel. It usually occurs on the ground or at the first of the takeoff roll. One of the exhaust pipes on a crossover exhaust system is very close to the carb. The carb gets hot when the fuel flow is low, the fuel starts to boil, the float drops and the needle valve opens, flooding the engine with fuel. Many times on a quick fuel stop in the summer in the South I have a hard time getting the engine to restart and have to use the mixture control to keep it running until the higher fuel flow on takeoff roll cools the carb down. (I am always prepared to abort the takeoff!) Once the throttle is WOT and cool fuel flows into the carb, everything returns to normal. As a result of many such experiences, I now use avgas only in the right tank, and I land and takeoff on that tank.
"Vapor lock" is vapor in the fuel lines and fuel pump, resulting in low fuel pressure and erratic fuel flow indications.
 
This is the first thing I've heard that ties together the black smoke and high fuel pressure. Very interesting.

Walt, do you suspect the mechanical fuel pump may have caused the incident, or are you thinking that it may have been damaged by the higher fuel pressure?

Thanks so much everyone!

-jon

Yes I would suspect the mechanical FP may have caused the incident creating the higher pressure which flooded the carb.
 
Jon- I was at the Vidalia Airshow Sunday and saw your landing. Good job on getting her down!! I also have a carbed RV6. If you were taking off on 100% mogas, IMO, you definitely experienced vapor lock. Easy to duplicate the same conditions, except this time don't takeoff...just run up the power and see what happens. Then switch to other tank with 100LL and after a few minutes to let that fuel enter carb, run up power again and see what happens.
I experienced the same thing while on a XC and had to get Mogas because that was all that was available. On next fuel stop, engine stumbled right after I got the tail up. Luckily, was able to abort. Taxied back, and after a few minutes to let 100LL enter carb, did full power run-up. No problem. Took off successfully and no problems since. Just my opinion.
 
How can a standard engine driven fuel pump provide pressure above that provided by its mainspring? Can someone please explain the mechanics?

Jon, where is your fuel pressure transducer located? The usual tee off the engine pump outlet fitting?
 
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I have to agree with Dan. The Mechanical FP is only a lever sliding on an eccentric lobe that pushes a on a diaphram and chamber with check valves.

It can't over pressure
 
How can a standard engine driven fuel pump provide pressure above that provided by its mainspring? Can someone please explain the mechanics?

Jon, where is your fuel pressure transducer located? The usual tee off the engine pump outlet fitting?

Good question Dan. I have not personally seen this failure mode with a mechanical pump on an aircraft, but the local FBO told me that had this scenario on a Cherokee 140 once where a new pump fixed the issue. I have read of similar reports in the past as well.

I have experienced multiple AC Delco mechanical fuel pump failures on small block Chevy engines in my old truck. One of these failed pumps would cause intermittent erratic pressure causing fuel starvation most of the time, but did cause the float bowls to overflow and cause power loss and run so rich that the exhaust looked like an over fueled diesel truck.
 
STC Knowledge

Todd has forgotten more on this topic than I will ever know.....

Pete:

I read through the comments on the website your link led me to, it? pneumatic lock, no question about it. The trailing black smoke confirms that the engine was running too rich. I cannot exactly explain the mechanics of what is happening with the float and needle other than to say they are overpowered by the foam much as the float would be overpowered by a fuel pump that gave too much pressure.

In a pneumatic lock the fuel boils when it hits the carb, it foams up and pegs out the pressure gage, and the engine dies because it?s too rich. Fuel at 85F is at the most crucial temp for vapor and/or pneumatic lock. The better an airplane performs the more critical these issues are and with the RV?s performance I cannot say I am surprised.

In an untested non-STC?d airplane like the RV, if one is going to use automotive gasoline then the autogas should be in one tank for cruise and another tank with avgas in it should be used for takeoff and landing.

Feel free to post this letter on the Vans Air Force forum.

Regards,

Todd L. Petersen
Petersen Aviation, Inc.
984 K Road
Minden, NE 68959
308-832-2200
autofuelstc.com
 
Ahhh, but how can a pump be the problem?

IIRC, the mechanical force of the pump lever enlarges the diaphragm chamber, and fuel enters the pump. Squeezing the diaphragm chamber is a function of a spring, not lever force. The size of the spring sets fuel pressure.

I can only think of one way to generate pressure higher than provided by the pump spring, and that would be vapor pressure. However, seeing 11 psi or more on a fuel pressure gauge tapped into the pump-to-carb line would require a blocked carb inlet (the fuel pump's inlet check valve would prevent reverse flow). The blockage could be the result of trash, or in this case, because the float bowl was full enough to firmly close the float needle valve. My conclusion is that the high fuel pressure was probably an after-the-event item, not a pump issue.
 
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One more thought........

Gary Z. had some interesting thoughts here that seem to apply and were very helpful to me with hot starts:

(Text copied from linked post)

Not all hot starts are the same. I check the fuel pressure before starting to decide whether I'll use plan B or plan C for starting. Plan A is for cold starts.

Plan B: Check fuel pressure and if below 5 psi a normal warm start ensues; Mixture to about the middle of travel, throttle cracked about 1/2 inch, left mag on, press the starter button. Starts every time, right mag now on.

Plan C: Check fuel pressure and if 5 or above a hot start ensues; Mixture to ICO, throttle cracked about 1/2 inch, left mag on, press starter button. After several blade swings the engine will start. As soon as the engine starts the right mag is switched on. Keep the engine running with the mixture only, Do NOT TOUCH the Throttle. After it starts push the mixture in to keep it running. It may try to die at this point so the mixture goes to ICO to keep it from quitting, advance the mixture when it catches. This is a mixture dance to keep it running until the fuel pressure drops below 5 psi and then the boost pump is turned on and the mixture set for taxi.

Fuel pressure is my clue. The fuel is boiling somewhere and causing a super rich mixture if the pressure is 5 or above. I have seen it as high as 7 or 8 psi and when that high it takes a longer dance with the mixture to keep it running. Once the pressure stabilizes to normal values the engine runs like it was a warm start.

I suspect that the fuel is boiling the carb bowl which floats the float to better seal the needle valve allowing the high pressure reading in the fuel line. The boiling fuel in the bowl may spill some into the carb throat coupled with the fuel vapor causes the rich mixture. Hence, the mixture at ICO for the hot start.

YMMV
 
Pneumatic Lock

The fuel "boils" when it hits the carburetor. In the fuel lines it's under pressure from the pump, plus it's trying to boil off due to the fuel temp and vapor pressure, it can't do so until it gets to the carb, and the carb is so hot that the fuel goes to foam when it enters. Think of a warm can of coke, shake it and open it and when it hits atmospheric, it explodes. This is not unlike what happens when an airplane develops pneumatic lock. Fuel pressure pegs out and the engine either surges or dies because it's too rich. Constantly adjusting the mixture can help you limp to an airport. The critical thing here is the vapor pressure of the fuel which is less than in times past, but still higher than aviation gasoline. Spring is the time of year when it's most likely to be a problem. 85F and 110F are critical fuel temperatures, and 85F is more critical than 110F because the light ends start cooking off at that temp. By the time you've reached 110F most (but not all) of the ingredients that cause vapor lock have boiled off.
 
The exact same think happened to me when I left Sun & Fun last Tuesday. Only made it about 20 feet in the air. Had a report of black smoke but no time to look at the fuel pressure. Landed and put AV gas in and all was well. The gas in my plane and the op's most likely came from the same tanker truck as we live within a couple miles of each other. So either two fuel pumps had issues on two different engines with radically different lives or they both had issues run the same gas in similar ground conditions.
 
Ahhh, but how can a pump be the problem?

IIRC, the mechanical force of the pump lever enlarges the diaphragm chamber, and fuel enters the pump. Squeezing the diaphragm chamber is a function of a spring, not lever force. The size of the spring sets fuel pressure.

I can only think of one way to generate pressure higher than provided by the pump spring, and that would be vapor pressure. However, seeing 11 psi or more on a fuel pressure gauge tapped into the pump-to-carb line would require a blocked carb inlet (the fuel pump's inlet check valve would prevent reverse flow). The blockage could be the result of trash, or in this case, because the float bowl was full enough to firmly close the float needle valve. My conclusion is that the high fuel pressure was probably an after-the-event item, not a pump issue.

I see you're point regarding the diaphragm spring actually creating fuel pressure. Having said that, a blockage would not cause high pressure since the diaphragm spring would not be able to build addtional pressure. I can see where fuel vapor pressure could cause the fuel pressure in the float bowl to overcome the float and needle and therefore cause high pressure in the fuel line and the resulting high pressure indication.
 
How can a standard engine driven fuel pump provide pressure above that provided by its mainspring? Can someone please explain the mechanics?

Jon, where is your fuel pressure transducer located? The usual tee off the engine pump outlet fitting?

Hi Dan,

Yes, that's where it is.

Thanks everyone for the great input. It's fascinating that mgaffney had almost the exact issue, just days before me, on gas that was almost certainly from the same tanker truck, as there are limited sources of ethanol-free here in western NC. His plane is a carbed -6a, o360. His copilot/passenger has informed me it was 87 degrees in Lakeland during his incident.

-jon
 
Mogas

Based on 1400+ hours using ethanol laced car gas in my RV-8 for nearly every flight for the last 9 years, I strongly recommend against using car fuel near the ground.....period.

My experience has led to this procedure: Use mogas only in cruise flight below 60% HP and let the engine temps cool down after climb out before using it.

Winter blend ethanol laced fuel in warm temperatures applied to a hot O-360 is a a guaranteed vapor lock event in my plane (engine turns off).
 
Mogas and carbs ?

Great information guys, appreciated. We fly a C172 with STC on mogas without problems. So, I am assuming the incremental stress comes from the confines of an RV cowl. Would a fuel recirculion line on my -3 make any difference regarding the hot bowl boiling the fuel?
 
Boiling mogas is trapped between the fuel pump outlet check valve and the carby float valve, it continues to boil, the pressure increases to a point where the float valve is overcome and the excessive pressure is applied to the fuel in the float chamber ejecting it into the main jet leading to an over rich mixture and black smoke and engine quitting.

The pump is fine - but it simply does not have the ability to generate sufficient pressure to overcome that generated by the boiling fuel on the other side of the check valve. So it is incapable of pumping fuel while this overpressure exists.

The pressure is only relieved by cooling the fuel line below the boiling point of its lowest temperature fraction, or by some active pressure relief mechanism such as a purge valve.
 
Great information guys, appreciated. We fly a C172 with STC on mogas without problems. So, I am assuming the incremental stress comes from the confines of an RV cowl. Would a fuel recirculion line on my -3 make any difference regarding the hot bowl boiling the fuel?

Yes, on my O320 6A, my circulating valve is on my left kick panel. I can reach down and open it and hold my hand on the valve and feel the temp change as the fuel is circulated. The "T" is right at the carb inlet and dumps into the left tank. Open the valve and pressure is gone.... fuel pump will then push fuel through the bypass till it is closed.
 
Gasman question .

I was thinking of a continuous recirc like FI uses. Calibrate the office to maintain 5psi with either pump. If you run the plumbing, why not?

One answer Is managing fuel level. You would need a fancy valve to coordinate the tank you are using😒
 
I was thinking of a continuous recirc like FI uses. Calibrate the office to maintain 5psi with either pump. If you run the plumbing, why not?

One answer Is managing fuel level. You would need a fancy valve to coordinate the tank you are using😒
Adair makes a 6 port valve, the return fuel returns to the tank selected for feed.
 
I've experienced the onset of vapor lock in my T-18, which has a single fuel tank behind the firewall, an electric and a mechanical fuel pump. Sitting in the sun, heat soaking at a summer airshow with winter blend mogas was the scene, also I still had the winter block off plate in front of the oil cooler. The take off was OK, but during the flight the oil temp gauge showed 210F and the engine started to run rough. Extra attention to the mixture control was required to keep it running, so we landed to look things over, drain some mogas and add some avgas. Later I removed the oil cooler cover plate as I more fully understood what had happened.
On my RV-8, I use dual electric automotive fuel pumps. Fuel pressure is controlled by a return regulator. Unused (potentially heated) fuel is returned to the tank via the selector valve.
Why don't I have the good old reliable mechanical fuel pump? The engine mounted fuel pump is heated by the hot engine. Fuel passing through it must get heated.
I also have AFP fuel injection, fuel pressure to it is about 40 PSI. That helps prevent vapor lock as well.
To avoid hot start problems from flooding, I always shut down the engine via a brief run at about 1400 RPM, then shut off the fuel pump. The entire fuel system pressure is purged thru the injectors, then the engine dies. you can't do this with an engine driven pump.
I do need to keep an eye on the alternator voltage, if the charge stops, I'll have limited time to land before the electric fuel pump runs the battery down.
You get a little, you give a little.
I use mostly high octane ethanol free mogas. I could used ethanol-laced mogas, the AFP injection, fuel pumps and regulator are rated for it. I worry about ethanol effecting the proseal that keeps the fuel inside the tanks. I top off with Avgas at away airports.
 
Vaporization-Induced Overpressure

Boiling mogas is trapped between the fuel pump outlet check valve and the carby float valve, it continues to boil, the pressure increases to a point where the float valve is overcome .....

The pressure is only relieved by cooling the fuel line below the boiling point of its lowest temperature fraction, or by some active pressure relief mechanism such as a purge valve.

Exactly, Doug. Inadvertantly, the OP is doing a fractional distillation in the fuel line. The winter fuel has some components with low boiling points- makes your car start easier in cold temps, but also makes vaporization in a hot soaked fuel line more likely. And, as Dan said, the fuel pump springs sets pump pressure. Boiling fuel in the line and the resulting high pressure has the pump diaphragm pushed as far toward the crankcase side as possible and the spring is not stiff enough to move the diaphragm until fuel side pressure drops.

Interesting failure mode. One might call it vaporization induced overpressure.

The other mode is fuel starvation. No pressure and no fuel as in when all the fuel pumps have to work with is vapor, and the vapor compresses and doesn't work it's way through the system- vapor lock.

Both failure modes are a very good reason for a fuel recirc line. It may only be needed to evacuate the fuel lines post hot soak. When the airplane has continuous fuel flow and air moving through the cowl, the fuel may never get hot enough.
 
I was thinking of a continuous recirc like FI uses. Calibrate the office to maintain 5psi with either pump. If you run the plumbing, why not?

One answer Is managing fuel level. You would need a fancy valve to coordinate the tank you are using��

This is what I have on my 70 OLDS 442! using a mallory 140 GPH 15 psi fuel pump near the tank, adjustable fuel presure regulator in engine comp, one line in two lines out on regulator, one to the carb and the other has a restricted return to tank, with engine running psi is adjusted to 7 psi. cool fuel recirc and when ign is off, all presure is released.
 
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