What is "normal" fuel pressure (carbureted)?

[kenpilot]

I recently removed a fuel return line on my RV-6A (carbureted). See https://vansairforce.net/threads/what-is-this-mystery-fuel-return-line-for.221249/. It accomplished the intended purpose of raising fuel pressures from the dangerously low level I had. Before, the pressures were 1-3 psi in flight (sometimes as low as 0.1 psi, triggering an alarm). After the design change, the pressure is 5-8 psi, and averages 6.8 psi in cruise. It still fluctuates, but at least it's at a safe pressure to keep the engine running.

However, now I have a carburetor leaking fuel. I already sent the carburetor in to Tempest Aero in NC. They normally test these to 6 psi, and at that pressure they could not replicate the problem. At 10 psi, they saw leaking from the float valve. They replaced the valve, and the carb is in-transit back to me now. But they were curious what engine-drivel fuel pump or fuel system plumbing I have that could be causing these unusually "high fuel pressures". They recommended I do not allow fuel pressures to go above 10 psi; otherwise the float valve could be lifted off the seat.

As far as I know, my removing that mystery line was just returning the fuel system to the stock Van's design. But their comment makes me wonder...

So, two questions, for anyone with a carbureted Lycoming O-320, with EIS data available:

1. What is the typical fuel pressure you see in flight, and what is the highest fuel pressure you've ever seen?
2. Is it normal for fuel pressure to fluctuate at all, in flight?
3. Do you have any sort of pressure-relief valve in your fuel system that would prevent pressures from going higher than 10 psi?

My specs:

• Engine: Lycoming O-320-E2A
• Carburetor: Marvel Schebler MA-4SPA (P/N 10-3678-32)
• Engine-driven pump: AC 41271 (equivalent part number to an LW-15472); from what I understand, the "low pressure" pumps for use with a carburetor are rated for 4-6 psi.
• Electric boost pump: Facet 40108; not sold anymore; Van's Aircraft says it provides 6 psi; I've seen 6-7 psi on the ground with engine off.
• Fuel pressure sensor: Grand Rapids LPS-02, connected to a Grand Rapids EIS4000 and displayed on EFIS

 

[kenpilot]

And yes, it was a real fuel leak... On the ground, engine off, I turned on the boost pump and had a puddle of fuel on the hangar floor, dripping from the airbox. On takeoff I could smell fuel. Smell went away as soon as I turned off the boost pump.

 

[krhea]

I recently removed a fuel return line on my RV-6A (carbureted). See https://vansairforce.net/threads/what-is-this-mystery-fuel-return-line-for.221249/. It accomplished the intended purpose of raising fuel pressures from the dangerously low level I had. Before, the pressures were 1-3 psi in flight (sometimes as low as 0.1 psi, triggering an alarm). After the design change, the pressure is 5-8 psi, and averages 6.8 psi in cruise. It still fluctuates, but at least it's at a safe pressure to keep the engine running.

However, now I have a carburetor leaking fuel. I already sent the carburetor in to Tempest Aero in NC. They normally test these to 6 psi, and at that pressure they could not replicate the problem. At 10 psi, they saw leaking from the float valve. They replaced the valve, and the carb is in-transit back to me now. But they were curious what engine-drivel fuel pump or fuel system plumbing I have that could be causing these unusually "high fuel pressures". They recommended I do not allow fuel pressures to go above 10 psi; otherwise the float valve could be lifted off the seat.

As far as I know, my removing that mystery line was just returning the fuel system to the stock Van's design. But their comment makes me wonder...

So, two questions, for anyone with a carbureted Lycoming O-320, with EIS data available:

1. What is the typical fuel pressure you see in flight, and what is the highest fuel pressure you've ever seen?
2. Is it normal for fuel pressure to fluctuate at all, in flight?
3. Do you have any sort of pressure-relief valve in your fuel system that would prevent pressures from going higher than 10 psi?

My specs:

• Engine: Lycoming O-320-E2A
• Carburetor: Marvel Schebler MA-4SPA (P/N 10-3678-32)
• Engine-driven pump: AC 41271 (equivalent part number to an LW-15472); from what I understand, the "low pressure" pumps for use with a carburetor are rated for 4-6 psi.
• Electric boost pump: Facet 40108; not sold anymore; Van's Aircraft says it provides 6 psi; I've seen 6-7 psi on the ground with engine off.

 

[krhea]

I have a similar setup, RV7, O320D2A, facet boost pump i think same number, 10-5135 carb, Advanced Flight Panel, same low pressure pump, and looking a various photos from past flights i have seen from 3-5 PSI. I recently replace pressure sender because it was reading low, confirmed with gauge, replaced sensor and all is good. I did refer to and SI about Lycoming fuel pumps and thought the minimum pressure was like 1.5 PSI but can't seem to find the document again. I would confirm pressure with a known gauge too.

Keith Rhea
Rv7
2024 supporter

 

[fixnflyguy]

I have an O320H2AD 160HP on my RV-4. Marvel Shebler MA4SPA carb, and have no mechanical pump. I run a Facet square cube boost pump as my full time pump, and have a second one as boost/back-up. With the standard old Vans fuel pressure guage (accuracy unknown) , I will get about 5 PSI static on one pump, 8ish with both. Engine running at idle, 4-5 psi on one. High power settings it will drop to about 1 PSI on one pump, goes up to 5 if I switch on boost. Cruise power settings are usually around 2-3 PSI on the guage. I have mistakenly left boost on after T/O climbout termination and ended up with carb overflow stains. These carbs do not like high pressure at all, and I don't like to let mine see over 5. It may be scary to see the pressure at 1-2 PSI, but I have never had so much as a stumble and its all it really needs.

 

[RVDan]

If you have fuel leaking while sitting engine off and fuel pump on, the float valve seat is probably an issue. There is an old Marvel Shebler SB on that. IIRC it is a grounding situation.

That said, Lycoming Operators Manual states .5 to 8 PSI with desired at 3

 

[kenpilot]

Yes, they told me that possibly the concentric edge wasn't perfect. They swapped out the valve thingy with another one they had in the shop, and that one was fine up to 10 psi. It's just weird, because this carburetor is 65 flight hours since brand new from Aircraft Spruce. But I suppose it's possible the defect was always there, but due to the fuel return line the carb had never seen more than 3 PSI before in its life.

Hmm. I think for now I'll set the EFIS alert at Max 8 PSI, so I have margin below the unwritten 10 PSI max that Tempest told me on the phone. Fortunately, I didn't actually remove the fuel return line in my aircraft. I just cut it and installed a shutoff valve, so in flight I have the ability to relieve pressure if absolutely necessary.

So it's not just my aircraft with fluctuating fuel pressure depending on phases of flight? Why would that occur? I would expect fuel pressure to either never change, or change directly relative to RPM. But mine will fluctuate throughout the cruise with nothing else changing.

 

[vic syracuse]

Fuel pressure shouldn’t be fluctuating. It will decrease sometimes with high power settings. The first thing I would do would be to hook up a direct reading fuel pressure gauge and see if it fluctuates in the air. Do you have a pulse dampening restrictor in the line to the sender?
vic

 

[kenpilot]

When I say "fluctuating", this is what I mean. It's just a gentle change periodically, not pulsing.

Do you have a pulse dampening restrictor in the line to the sender?

I'm not sure actually, as I didn't build this aircraft. I would hope they followed those instructions when the sensor was added. The sensor is at the end of a long line, so it sits much higher than the fuel pump. The fire-shielded line screws into a silver-colored thing that could possibly be a restrictor.

 

[bobnoffs]

this is just a fwi as it kind of fits the subject. i built a plane with an early jab 3300. the way it came from the factory the carb overflow leaked. when i checked the psi into the carb it was over what bing recommended so it was just doing what bing said it would do. i had a friend who was the white collar in charge of maintenance at o'hare. he said that the long and short of that was the spring in the mechanical pump was too strong. i temporarily fixed the problem with a weaker spring from ace hardware! then the west coast jab dealer started selling weaker springs. that was the end fix.

 

[YellowJacket RV9]

I had an issue with carb flooding that I traced back to a faulty mechanical fuel pump that somehow failed to a higher pressure. I don't remember the exact pressures but they were way above spec. I did have the carb rebuilt as well simply because of age and to make sure no damage had been done to it, but a new pump fixed the issue, and was a pretty easy install thanks to VAF.

 

[Sam Buchanan]

So it's not just my aircraft with fluctuating fuel pressure depending on phases of flight? Why would that occur? I would expect fuel pressure to either never change, or change directly relative to RPM. But mine will fluctuate throughout the cruise with nothing else changing.

After a few hundred hours on a new fuel pump I started seeing inconsistent pressure in cruise flight. This was different from the often-reported low pressure indication during a climb due to sender difficulties in keeping up with changing ambient air pressure. After a couple of instances of rpm loss in flight I replaced the pump and steady cruise fuel pressure was restored.

By the way, I would plug the bypass port on your fuel pump fitting and ditch the entire return line system. It isn't needed in a properly functioning carbed aircraft and just adds additional failure points.

 

[kenpilot]

Thanks for the advice, everyone. I'll have to investigate the fuel pump spring. The mechanical pump has 275 hours on it since new.

I did have the carb rebuilt as well simply because of age

I didn't do an official "rebuild", but they did inspect a lot of the internals for me. I needed to send the carb in regardless, because of some unrelated issues that were beyond my experience to fix.

By the way, I would plug the bypass port on your fuel pump fitting and ditch the entire return line system.

Totally agree! That is the plan. My installation of a shutoff valve, kept in the OFF position except for emergencies, was a temporary solution until I could reach 100% confidence that the line could safely be removed.

 

[Sam Buchanan]

Thanks for the advice, everyone. I'll have to investigate the fuel pump spring. The mechanical pump has 275 hours on it since new.

I wouldn't attempt a diagnosis or mod of the fuel pump, this is too critical a component for informal engineering. If a fuel pump is suspect it is prudent to replace it with a pump that has been officially blessed.

 

[kenpilot]

Thank you for the reminder.

 

[penguin]

I flew a n O-320 powered 6A for nearly 1000 hrs. The fuel pressure was usually stable(ish) but did occasionally fluctuate down to almost , particularly in a climb or at a high (8k or so) cruise. The engine kept running smoothly all the time. This was with 2 different engine monitors, GRT and AFS. I removed the low pressure warning as it was serving no purpose. My conclusion was the engine wanted flow rather than pressure, indeed as you have found out too much pressure into a carb can be worse than no pressure. I think, but couldn't prove, the pressure senders were struggling at very low pressures. I could not find the reason for the lower than usual pressure, the boost pump always brought it up to 4 or 5 psi. As the engine kept running happily eventually I accepted the behaviour as a feature of the aircraft.
Pete

 

[Flyingnorth]

I flew a n O-320 powered 6A for nearly 1000 hrs. The fuel pressure was usually stable(ish) but did occasionally fluctuate down to almost , particularly in a climb or at a high (8k or so) cruise. The engine kept running smoothly all the time. This was with 2 different engine monitors, GRT and AFS. I removed the low pressure warning as it was serving no purpose. My conclusion was the engine wanted flow rather than pressure, indeed as you have found out too much pressure into a carb can be worse than no pressure. I think, but couldn't prove, the pressure senders were struggling at very low pressures. I could not find the reason for the lower than usual pressure, the boost pump always brought it up to 4 or 5 psi. As the engine kept running happily eventually I accepted the behaviour as a feature of the aircraft.
Pete

Glad to see your reply. My Superior 0-360 fuel pressure will bleed off almost to zero (engine runs fine) as I climb, leaning brings it back to the range of 3-4 . i changed out the fuel pump, no change.
terry Reece

 

[airtractor8]

Thanks for the advice, everyone. I'll have to investigate the fuel pump spring. The mechanical pump has 275 hours on it since new.


I didn't do an official "rebuild", but they did inspect a lot of the internals for me. I needed to send the carb in regardless, because of some unrelated issues that were beyond my experience to fix.


Totally agree! That is the plan. My installation of a shutoff valve, kept in the OFF position except for emergencies, was a temporary solution until I could reach 100% confidence that the line could safely be removed.

It seems your fuel return line may have been a copy of this system featured in an article of "14 years of the RVator" 1980-1993. If thats the case you should find a .030 sized restrictor orifice in the fitting at the carb. Perhaps that wasn't fitted and to much fuel is bypassing?
Van himself didn't like the system much at the time by the look of his comment below the article.
One thing of note is the builders use of hose tail/barb type hose fittings and rubber hose for the line. Gives me the shivers looking at it.

 

[vic syracuse]

It doesn’t look like you have a restrictor fitting in the pressure line to the sensor.

 

[kenpilot]

It seems your fuel return line may have been a copy of this system featured in an article of "14 years of the RVator" 1980-1993. If thats the case you should find a .030 sized restrictor orifice in the fitting at the carb. Perhaps that wasn't fitted and to much fuel is bypassing?
Van himself didn't like the system much at the time by the look of his comment below the article.
One thing of note is the builders use of hose tail/barb type hose fittings and rubber hose for the line. Gives me the shivers looking at it.

Hey, that's the first plausible explanation I've ever heard for why this airplane has the extra line. This one was a 1996 kit, and first flight in 2002. And yes it is a rubber hose, with a single-rib fitting and hose clamp, and without any fire sleeve. The main line is fire sleeved, at least.

I hadn't thought about it until just now, but if this was about vapor, then it's about heat. Is it possible that my installation of a shutoff valve, without fully removing the line, actually makes it worse? Now the fuel in the return line is motionless, so is providing no coolng effect for the rubber hose itself.

It doesn’t look like you have a restrictor fitting in the pressure line to the sensor.

But, Vic, isn't the purpose of the restrictor on the sensor line just to eliminate the quick pressure pulses? I don't disagree that it should exist, but how would affect long-term pressure reading throughout the flight?

 

[kenpilot]

Perhaps that wasn't fitted and to much fuel is bypassing?

I would have to take it apart to find out, so I probably won't know until later in the year when I fully remove the line.

But even if there is a restrictor, I definitely don't like how it was all routed. The fuel flow "red box" was in the loop, so it was measuring both the fuel used by the engine and also the fuel returning through the line. The installers had configured the EIS with a huge calibration factor to try to get the fuel flow at cruise to appear correct, but then the fuel usage at idle appeared way too high, and fuel usage would also fluctuate during a flight without explanation. That RVator article doesn't appear to show where the red cube should have been.

airtractor8, could you post (or DM me) the entirety of that article? I'd like to include it in my documentation of this project.

 

[airtractor8]

I hadn't thought about it until just now, but if this was about vapor, then it's about heat. Is it possible that my installation of a shutoff valve, without fully removing the line, actually makes it worse? Now the fuel in the return line is motionless, so is providing no coolng effect for the rubber hose itself.

Oh its all about fuel vapor for sure. There is no other reason in a carb type fuel system to have it there. Its just a pity the builder did not choose to use standard AN flaired fittings and good aircraft quility firesleved hose. It is part of the fuel system after all. I also question the use of a brass fitting hanging of the carb like that as well. Steel with steel fittings would have been my choice. Each to his own. It is an experimental after all.

 

[airtractor8]

But even if there is a restrictor, I definitely don't like how it was all routed. The fuel flow "red box" was in the loop, so it was measuring both the fuel used by the engine and also the fuel returning through the line. The installers had configured the EIS with a huge calibration factor to try to get the fuel flow at cruise to appear correct, but then the fuel usage at idle appeared way too high, and fuel usage would also fluctuate during a flight without explanation. That RVator article doesn't appear to show where the red cube should have been.

Its likely the "red cube" was not invented when this article was written sometime in the 80's perhaps. It was all old school then!!

airtractor8, could you post (or DM me) the entirety of that article? I'd like to include it in my documentation of this project.

Sure. Here it is for you.

 

[kenpilot]

Thank you. I appreciate it. You've given me a lot to think about here.