Chasing the issue - low fuel pressure on takeoff

[John C]

Scott, I am comfortable with the one electric pump.

It appears that the cavitating pump is still pushing sufficient fuel, even though the fuel pressure is low. The Bing carbs can likely work with very low pressure as long as the fuel flow is 2.5 gph.

While playing with the pressure issue, I inadvertently made a series of 3-4 takeoffs with the c/b pulled. The fuel pressure did not dive. Go figure. I don't remember having a pressure drop with the c/b pulled. Also, others have added a switch for the Facet pump. I suspect others have either left the c/b pulled or left the switch off for takeoff. Wonder what they saw.

The 4th graph shows the fuel flow as very good even though the pressure was in the 2s. I never saw a lessening of fuel flow. If the fuel flow was taking a hit, I would be worried.

Also,the pressure recovers very quickly by simply leveling off a bit and pulling the throttle a bit. For example lower the nose 5 degrees from the climb angle and pull back to 4000 rpm for a second or two.

Jim, on your other post, you experienced vapor lock. Was that on initial start up and taxi or on takeoff. I can see where pumping 1.5 gph cool fuel through the system before start would cool the entire fuel circuit. But once up and running, the system is pumping 4-5 gph keeping the system cool. ((1.5 gph for the restrictor and whatever the engine is using at idle, 3 gph perhaps))

[RFSchaller]

I have a switch. The lowest I?ve seen without cavitation on just the mechanical pump is just above 3 psi. With cavitation in hot Phoenix summers fuel pressure has taken a nosedive, but recovered to >3 psi when I turned on the pump.

[RFSchaller]

I have a switch. The lowest I?ve seen without cavitation on just the mechanical pump is just above 3 psi. With cavitation in hot Phoenix summers fuel pressure has taken a nosedive, but recovered to >3 psi when I turned on the pump.

[Piper J3]

Originally Posted by John C

Jim, on your other post, you experienced vapor lock. Was that on initial start up and taxi or on takeoff. I can see where pumping 1.5 gph cool fuel through the system before start would cool the entire fuel circuit. But once up and running, the system is pumping 4-5 gph keeping the system cool. ((1.5 gph for the restrictor and whatever the engine is using at idle, 3 gph perhaps))

John

Vapor lock happened on the ground after restarting hot engine that sat for about 15 minutes. There are many threads in this forum discussing the merits of the electric fuel pump. Van?s recommends running the electric pump all the time - it?s very good advice?

[scottmillhouse]

I also had low pressure issues on take off but only with full fuel. I had just the tank vent. Added the vent in gas cap with the idea that with full fuel take off angle may have fuel blocking the vent. Never had a problem since I drilled the hole in the fuel cap in over 250 hours. Coincidence or solution?

[John C]

Originally Posted by Piper J3

John

Vapor lock happened on the ground after restarting hot engine that sat for about 15 minutes. There are many threads in this forum discussing the merits of the electric fuel pump. Van?s recommends running the electric pump all the time - it?s very good advice?

I agree with you.

[ben barron]

Originally Posted by scottmillhouse

I also had low pressure issues on take off but only with full fuel. I had just the tank vent. Added the vent in gas cap with the idea that with full fuel take off angle may have fuel blocking the vent. Never had a problem since I drilled the hole in the fuel cap in over 250 hours. Coincidence or solution?

I reciently drilled a slightly larger hole in my gas cap after replacing the mechanical pump and sensor (without solution) and it seems to have solved my problem

[Piper J3]

I have just the vent hole in the tank filler cap per Van’s original RV-12 design. My logic is that I run 93E10 exclusively and don’t want the filler neck vent because it would provide too much air exchange in the tank when the airplane is sitting for extended periods.

Unlike Avgas, the octane will degrade in 93E10 if subjected to the atmosphere. Also the alcohol in the fuel is hydroscopic and will suck moisture out of the air.

[Charlie12]

I installed the later-design fuel tank vent line, so that is not an issue. I also added an on/off switch for the electric fuel pump, at the recommendation of instructor in my Rotax iRMT class. For two years and 175 hours I have routinely used the electric pump only for takeoff and landing and have no recollection of fuel pressures low enough to be any concern. For no apparent reason on a flight a few days ago (81 deg F ambient) I had a couple brief instances of fuel pressure low enough to trigger the audio warning, 2 psi, when the electric pump was off. One instance was at cruise, one was taxiing at the end of a 1.5 hour flight. No apparent change in engine operation. Turning on the electric pump made the pressure immediately jump back to around 5 psi.


Today I carefully did three full stop and taxi back trips around the pattern with the electric pump on and the fuel pressure stayed around 5 psi at all times. This included takeoff climb at 5200-5300 RPM. I need to experiment with this some more, and will study JC's data carefully. But for now my guess is that there is a possibility of inconsistent engine-driven pump operation due to placement of the tank unusually far aft, which would explain why the kit does not provide a switch for the electric pump. I plan on leaving mine on all the time from now on, or at least until I get a clearer understanding of what is happening.

[Piper J3]

Originally Posted by Charlie12

I also added an on/off switch for the electric fuel pump, at the recommendation of instructor in my Rotax iRMT class.

Did Rotax maintenance instructor say why he recommended shutting off electric fuel pump? Seems to me that he would have a good reason to tell someone to alter a safety feature designed by mfgr. I don't think I would want that liability...