Loss of Power on Takeoff, believe Vapor Lock

[mgaffney]

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.

[Mike H]

Originally Posted by DanH

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.

[mosquito]

Originally Posted by DanH

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

[chuckwn]

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).

[Larry DeCamp]

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?

[Doug]

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.

[gasman]

Originally Posted by Larry DeCamp

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.

[Larry DeCamp]

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😒

[David-aviator]

Originally Posted by Larry DeCamp

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.

[SHIPCHIEF]

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.