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Facet fuel pump operation (O-320)

pjc

Well Known Member
I have been testing my RV-6 with auto-gas recently and have observed some low fuel pressure events (possibly vapor lock/pump prime failures) that have never occurred with 100LL. Its a 90’s era SB RV-6 with the standard (of the time) factory Facet (cube) boost pump and Lycoming engine (O-320) driven pump in series.

The Facet pump rattles happliy when powered and seems to pump fine under ordinary conditions. To rule out a boost pump problem I have a couple questions about how these Facet pumps normally operate.

Questions:
1). The boost pump seems to rattle faster when not primed, but gets a little quieter (and slower) after it self primes when powered before engine start. Is this normal ?
2). If I let it continue to run, it continues to click (rattle) even if I don’t start the engine. There are no leaks and the fuel pressure remains 4-6psi. There are no apparent leaks. I might have expected the pressure to rise a few PSI when the carb bowl and supply piping gets full and the carb stops fuel flow. Presumably the pump might be designed to stall at this point and stop making noise. Mine doesn’t, it appears to continue to run despite hydraulic stall and no flow.
Is this normal behavior?

Peter
 
yes and yes, at least for mine...
Although, there are multiple facet pump models available.
 
Peter, I don't know if you guys have "winter blend" gas in central Florida, but I found it quite common to see lower fuel pressure when using the winter flavor than 100LL. I had a bad episode with winter blend during a hot afternoon one May and stopped using auto gas after using it for several years. I immediately noticed higher and more consistent fuel pressure after going back to 100LL.

I would still use auto gas if I could be confident of always having summer blend, but who knows what is coming out of the local pumps especially during changes in season.

Your description of the Facet pump is normal. Those pumps have been used for decades in automotive applications where they run continuously for years. They are self-regulating and will limit pressure even if there is no flow through the carb.
 
I experience similar symptoms on a hot day starting a hot engine using ethanol laced gas from local pump. More often then not. Facet aux pump is still original made circa 2004 with 5K+ hours on it (not constant run).
 
AutoGas

Peter,
There are several solutions for using auto gas in your RV. I've been using it for several years now, and over 1200 Hrs, and have found methods that solve the fuel pressure issues.
1. No 90* fittings into the mechanical engine pump. They tend to cause vapor lock. Use a curved hose instead.
2. Install an independent fuel pressure sensing system that will automatically turn ON the AUX pump if a low fuel pressure event occurs, and have it stay ON until the fuel pressure is back to normal plus some timed period (I use 10 seconds).
3. Alternately, put low pressure pumps in the wing roots that run whenever that side tank is selected. Any pressure in the lines to the engine will inhibit vapor lock situations.

These solutions will work with all seasonal blends of auto fuel.....
I'm located at KBKV if you want to go over some of these solutions...



I have been testing my RV-6 with auto-gas recently and have observed some low fuel pressure events (possibly vapor lock/pump prime failures) that have never occurred with 100LL. Its a 90’s era SB RV-6 with the standard (of the time) factory Facet (cube) boost pump and Lycoming engine (O-320) driven pump in series.

The Facet pump rattles happliy when powered and seems to pump fine under ordinary conditions. To rule out a boost pump problem I have a couple questions about how these Facet pumps normally operate.

Questions:
1). The boost pump seems to rattle faster when not primed, but gets a little quieter (and slower) after it self primes when powered before engine start. Is this normal ?
2). If I let it continue to run, it continues to click (rattle) even if I don’t start the engine. There are no leaks and the fuel pressure remains 4-6psi. There are no apparent leaks. I might have expected the pressure to rise a few PSI when the carb bowl and supply piping gets full and the carb stops fuel flow. Presumably the pump might be designed to stall at this point and stop making noise. Mine doesn’t, it appears to continue to run despite hydraulic stall and no flow.
Is this normal behavior?

Peter
 
Thanks for confirming the typical behavior of the Facet pump and the additional comments. My Facet pump has operated consistently this way but I was unable to find any confirming documentation.

The only repeatable/confirmed loss of fuel pressure situations observed during my testing are at very high deck angle full power climbs. An extended full power climb at 80-85mph IAS at 5000’ reliably results in loss of fuel pressure (possibly to zero, certainly to less than 0.5psi on my small mechanical gauge), rough running, and a significant loss of engine rpm/power. This occurs only on auto-gas and not 100LL, and does so with and without the boost pump running. Everything self primes and recovers as soon as the nose is lowered.

I have had occasional stumbles when advancing the throttle after extended ground runs in relatively hot ambient conditions on auto-gas (also never observed with 100LL). I have not observed the fuel pressure during these short events.

I have blast tubes and shrouds on the gascolator and mechanical pump. I haven’t inspected it recently, but suspect I do have 90 fittings in the fuel supply lines. I’ll look at this next.

My suspicion is that the pumps loose suction do to vaporization of the auto-gas in these challenging conditions (warm gas, limited cooling flow, and significant static suction head demand).

Peter
 
I think your analysis is correct. The issues you have observed are similar to what I've seen when winter blend is used in my RV-6 on a hot day.
 
Winter vs. Summer blends

It looks like the seasonal blend changes don't apply in longer in most areas. The EPA chart by state / county shows a Reid Vapor pressure of 9.0 psi max for most areas. By comparison I think the AvGas RVP spec is 5.5 to 7.0 max.

https://www.epa.gov/gasoline-standards/gasoline-reid-vapor-pressure

I've just started running non-ethanol 90 Octane in my low compression O320E2A and have noticed the mechanical and Facet pump pressures are slightly lower and I lean just a bit less for the same EGT. I may enrich-en my idle mixture a bit also.

Not to be compared to the Reid test, but I took a sample of non-ethanol and pulled vacuum on it at room temp and it boiled at about 5 psi absolute, while an AvGas sample boiled at about 3.4 psi absolute. Seems to compare with the spec numbers.

Based on testing of the old regular red auto fuel that I ran for years in my '68 Cardinal (O320E2D) and some notes I have, the vapor pressure back then was noticeable higher than the 9.0 psi gas we have now.
 
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