What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

unstable fuel pressure

J

jeff beckley

Well Known Member
On my RV7 with an O-360 I have been noticing low fuel pressures on climb out.
With the electric fuel pump on I see 4 to 5 lbs which is fine but if I were to turn off the pump then I would see around 1 to 2 lbs until I leveled off then the mechanical pump will show 4 to 5.
For what it is worth I have been also noticing a slight roughness in the engine between 2300 and 2450 rpms with intermittent misses when I have leaned the mixture.
I get good idle and good climb power and it runs smooth at any setting above 2450rpm and under 2300rpm.
One last note. I have also noticed that there is a slight hesitation in engine acceleration when I push the throttle when the engine is running at 1200-1400rpm.
I might have two distinct problems here. Fuel pressure and carb problems.
Any ideas?
 
Common

My O-360 behaves the same way. It concerned me at first quite a bit, but i've since been informed by many out there that this is normal. The engine runs fine with as low as a pound of pressure.

The 'reason' for it, i've been told (no proof mind you) is that the pressure transducer was not designed for the rapid changes our engines experience during climb out. Whether that's true or not, I don't know. I do know that in 3 yrs and 400 hrs, it's never been an issue.
 
It's very typical to see fuel pressure variations during climb and altitude. As long as you have at least 0.5 psi you are OK with a carby engine, per Lycoming.

Your hesitation could be a slightly lean carb adjustment. Try resetting your idle "mixture" screw per the Lycoming instructions.

Roberta
 
Fuel PSI

I've checked the fuel pump PSI with a mechanical gauge and the Van's Electric, both at the same measuring point...Mechanical gauge=6.5 PSI, Van's Electric gauge= 3.5 PSI...is the transducer/Van's Gauge that far off...are other people finding this discrepancy...ground running the engine...runs fine...Carb is not flooding/leaning...almost ready to fly ...SOON!!!! O320 H2AD with Facet Electric pumps.
your comments appreciated....billy d
 
Billy.
Do a fuel flow test (gph) take the fuel line off at the carb, measure vs time.
You should see that the facet pump pumps plenty of fuel.
I have reviewed my Dynon in flight fuel pressures and they range from 2.1-5.7 psi. I don't think the actual pressure varies that much,and I don't worry about it
o-320 D2J
 
I can tell you this. It is way spooky to look over at your fuel gauge on climb out and see it reporting 1 pound. If it is vans gauge/transducer then I think a phone call is in order.
 
I can tell you this. It is way spooky to look over at your fuel gauge on climb out and see it reporting 1 pound.
Click to expand...

It's even spookier to see the fuel pressure display on your RMI uMonitor flashing "0" since I have the alarm threshold set at one psi. But the engine has been running happily for over 800 hrs and as has been stated many times before, the "low" reading is not at all unusual. Many pressure tranducers just can't resolve low pressure very well.
 
Fuel Pressure voodoo..

Anyone else ever noticed how the indicated fuel pressure follows the throttle setting??? WOT, no pressure, half throttle and it returns like magic!
 
Is there any chance that that long length of hose up to the sender can get an air lock? Wonder if it would help to crack the upper fitting and bleed it?
 
One PSI is twice what Lyc. recommends at minimum. My neighbor's Tailwind with 0-320 runs fine with no fuel pump at all. The fuel tank is behind the firewall, leaving very little pressure head from gravity feed.

Roberta
 
No Gravity Feed Here

robertahegy said:
One PSI is twice what Lyc. recommends at minimum. My neighbor's Tailwind with 0-320 runs fine with no fuel pump at all. The fuel tank is behind the firewall, leaving very little pressure head from gravity feed.

Roberta
Click to expand...

Roberta,

Those minimum numbers are for gravity feed systems. For engines with fuel
pumps, the pumps need to be monitored for output via some reliable type of pressure instrument. Having two fuel pumps does you little good if you can't reliably check their functionality both on the ground and in the air.

Debris can become lodged in the pump check valves, causing low or no output pressure. I wouldn't fly without reliable fuel pressure readings.
 
Those minimum numbers are for gravity feed systems. For engines with fuel
pumps, the pumps need to be monitored for output via some reliable type of pressure instrument. Having two fuel pumps does you little good if you can't reliably check their functionality both on the ground and in the air.

Debris can become lodged in the pump check valves, causing low or no output pressure. I wouldn't fly without reliable fuel pressure readings.
Click to expand...

Maybe some clarification is in order. I've always assumed the 0.5 psi requirement is a function of what the carb needs in order to maintain the proper volume of fuel in the bowl during full power climb....regardless of whether the system is gravity or pump fed.

Checking the pumps is easily accomplished during runup. Inflight, turning on the electric pump will cause the pressure to rise, regardless of what was originally indicated. If the pressure doesn't go up when the pump switch is flipped, and/or the ampmeter doesn't indicate additional load, then we can assume the pump is bad.

We are talking about carbed engines. Fuel injection is another matter and proper pressure monitoring is something you want to be concerned about. I'm comfortable flying my carbureted RV-6 in spite of the inconsistent fuel pressure indications.
 
Sam,

What are your color-coded range markings on your fuel pressure guage?
 
Last edited:
What are your color-coded range markings on your fuel pressure guage?
Click to expand...

You're gonna love this.....there aren't any. The fuel pressure indicator is part of the Rocky Mountain uMonitor which is a b/w LCD display. I have the unit set to alarm at <1 psi.

panel_07-06.jpg


Larger version of this image is here:

http://thervjournal.com/panel_07-06_big.jpg
 
Last edited:
Trust your instruments

Roberta/Sam........

In as much as both of you are EAA Tech counselors I would like to agree with you, but ignoring engine instrument indications is no solution. Please bear in mind that the gentlemen who started this thread is also having "issues" with his engine, as well as indication problems. I think you should reconsider your advice.

Roberta, idle mixture settings only affect idle, with the butterfly closed, and have no effect what so ever on acceleration. Once the high speed jet comes alive, it is non-functional. Sounds like a float level issue to me, which of course might be fuel pressure/volume related.
 
Roberta/Sam........
In as much as both of you are EAA Tech counselors I would like to agree with you, but ignoring engine instrument indications is no solution. Please bear in mind that the gentlemen who started this thread is also having "issues" with his engine, as well as indication problems. I think you should reconsider your advice.
Click to expand...

John, we aren't "ignoring instrument indications", we just recognize the fact that the pressure tranducers we use with carbed engines aren't consistent at very low pressures. Roberta and I didn't just discover this fact, it has been well documented in the RV community for years. It also comes as a surprise to many pilots as to how little fuel pressure needs to be supplied to the carb in order for the engine to run as designed.

And....if your RV-9A has a carbed engine with a remote pressure transducer, you will eventually discover exactly what we are talking about, and like us, you will learn to safely fly the plane in spite of the fuel flow indicator inconsistencies.

I know all this flies in the face of your well-known reputation for looking down on our experimental panels, but this flow issue is just the way it has been for a long time with inexpensive, low pressure transducers.

I have never recommended anyone, especially the original poster, to ignore instrument instruments, and I suspect the same could be said for Roberta. But if every RV with a carb and fluctuating fuel flow indications was grounded, then a huge percentage of the fleet would be taken out of action. And we are talking about planes that have and will continue to safely rack up thousands of hours. This is one of those cases where we learn the idiosyncrasies of the system and work within them.

Good luck with your project, and if you use fuel injection you will never see what we are discussing. :)
 
Last edited:
Sam Buchanan said:
John, we aren't "ignoring instrument indications", we just recognize the fact that the pressure tranducers we use with carbed engines aren't consistent at very low pressures. Roberta and I didn't just discover this fact, it has been well documented in the RV community for years. It also comes as a surprise to many pilots as to how little fuel pressure needs to be supplied to the carb in order for the engine to run as designed.

And....if your RV-9A has a carbed engine with a remote pressure transducer, you will eventually discover exactly what we are talking about, and like us, you will learn to safely fly the plane in spite of the fuel flow indicator inconsistencies.

I know all this flies in the face of your well-known reputation for looking down on our experimental panels, but this flow issue is just the way it has been for a long time with inexpensive, low pressure transducers.

I have never recommended anyone, especially the original poster, to ignore instrument instruments, and I suspect the same could be said for Roberta. But if every RV with a carb and fluctuating fuel flow indications was grounded, then a huge percentage of the fleet would be taken out of action. And we are talking about planes that have and will continue to safely rack up thousands of hours. This is one of those cases where we learn the idiosyncrasies of the system and work within them.

Good luck with your project, and if you use fuel injection you will never see what we are discussing. :)
Click to expand...

Sam,
Thanks for the good wishes on my project. I too have a carbureted Lycoming. If I find my remote mounted fuel flow sensor to be unreliable, I will immediately install a direct-reading gauge in it's place. Disregarding instrument indications is a very slippery slope, because one never knows where to stop. While I agree with your analysis of YOUR low pressure indications, you are infact ignoring low fuel pressure numbers by substituting those of a gravity system. How will you know when your engine driven pump is malfunctioning if your guage is always near zero?
 
robertahegy said:
And a lean engine will hesitate just off idle until the accellerator pump kicks in.

Roberta
Click to expand...

He is complaining about problems well above the idle circuit, not "just off idle"
 
While I agree with your analysis of YOUR low pressure indications, you are in fact ignoring low fuel pressure numbers by substituting those of a gravity system. How will you know when your engine driven pump is malfunctioning if your guage is always near zero?
Click to expand...

John, either I haven't made my position clear...or you are trolling, and I suspect the latter.

1) I never said the fuel pressure indicator always read near zero. I said it is inconsistent. More often than not it indicates 3-6 psi, but I don't panic when it drops to a very low value because I understand the fuel system in my plane.

2) I will know the engine pump has failed if the gauge reads zero, the engine quits, and it won't restart until the electric pump is turned on. At that point I will land when practical and install a new pump. However, a change of clothes may be in order...

3) I am not ignoring any gauge indications. I check them often and know what I should expect to see.

4) There ain't no way I'm bringing a fuel line into the cabin to run a mechanical gauge. The electronic indicator is just fine for me.

5) All a carb needs is enough fuel to keep the bowl full, the carb doesn't care whether it is gravity or a pump, the minimum fuel flow requirements are the same in either case. The carb on my plane ran fine in the C172 it found itself in when it left the factory (gravity feed) and it runs fine in my RV-6 (pump feed).

I don't dispute that the engine pump is designed to deliver 3-6 psi, and I suspect the pump on my engine is performing as it should. I don't know positively if low readings are accurate or a gauge anomoly, but since the engine has never shown any signs of fuel starvation, I am lead to believe the fuel system is working fine.

There.......that pretty much depletes my capacity to explain this situation.
 
Last edited:
Sam,

You've made your position abundantly clear. I respectfully disagree. Trolling.....I'm not really sure what that means. Just trying to help, that's all.
 
low fuel press and rough engine

check your fuel vent lines. When the tanks are full it is possible to get fuel in the P-trap set up in your vent lines. This will cause a drop in fuel press. /what you describe sounds like vapor build up to me. Thats why it is better with boost pump on. There was an AD on some mech fuel pumps that concerned a problem like you are having too.
 
Yes I did in fact report two problems. The fuel pressure symptoms was one and the discussion here have satisfied my concerns about that.

I am still left with the rough running engine "sometimes not always" at the upper end of the rpm scale.
I think I can narrow down some symptoms.
(1) I will run at max rpm "2700 rpm" for five or more min and upon reducing power to 2500 rpm it feels like a fouled plug. If I reduce the power more to around 2400 and below it smooths up.
(2) Also of more concern. If I were in the pattern on final approach and happen to reduce power to around 1200rpm and need more power then the engine will stumble for a moment. That most likely is accelerator pump.
(3) I have the #3 cylinder running 100deg hotter EGT all the time. On my engine the #3 baffle is shared with the oil cooler so It could mean that some of the cooling is going through there but that would result in a much hotter CHT than a hotter EGT.
It has been suggested that I might have an intake leak on the hotter cylinder.
My CHT's never exceed 400 and EGT's run around 1200to1300 with #3 always 100deg hotter.
(4) It takes a long time for the engine to shut down after pulling the mixture but I do observe a 50 rpm increase just before it quits.
Thats the best I can describe for now.

Could I spray ether or carb cleaner around the intake tubes while it is running to check for a leak just like I have done on my boat racing engines?
Or is a check of the plugs going to show something?
 
Last edited:
Sam Buchanan said:
1) I never said the fuel pressure indicator always read near zero. I said it is inconsistent. More often than not it indicates 3-6 psi, but I don't panic when it drops to a very low value because I understand the fuel system in my plane.
Click to expand...
Sam - is there any pattern as to when the fuel pressure indication goes from normal to near zero?

Many of our experimental engine gauges use senders designed for automotive use, rather than the more expensive ones designed for aviation use. But, I've read speculation that senders that were designed for automotive use may have problems if the altitude changes, due to inadequate venting of the sender's diaphragm. The sender measures gauge pressure (not absolute pressure), so it needs to have fuel pressure on one side of a diaphragm, and atmospheric pressure on the other side. If the atmospheric pressure side of the diaphragm is not properly vented, then the pressure there would not be the same as the atmospheric pressure during a climb or descent. This could cause fuel pressure indications that were too low during a climb, and too high during a descent. The indications would slowly return to normal after level off.

Note that if this problem exists (I've never seen it proven), this is a sender selection problem, not an installation problem. There is probably nothing the user could do other than find a better sender.
 
Sam - is there any pattern as to when the fuel pressure indication goes from normal to near zero?

Many of our experimental engine gauges use senders designed for automotive use, rather than the more expensive ones designed for aviation use. But, I've read speculation that senders that were designed for automotive use may have problems if the altitude changes, due to inadequate venting of the sender's diaphragm. The sender measures gauge pressure (not absolute pressure), so it needs to have fuel pressure on one side of a diaphragm, and atmospheric pressure on the other side. If the atmospheric pressure side of the diaphragm is not properly vented, then the pressure there would not be the same as the atmospheric pressure during a climb or descent. This could cause fuel pressure indications that were too low during a climb, and too high during a descent. The indications would slowly return to normal after level off.

Note that if this problem exists (I've never seen it proven), this is a sender selection problem, not an installation problem. There is probably nothing the user could do other than find a better sender.
Click to expand...

Thanks for the reply, Kevin. You are thinking along the same lines that several others have proposed over the years. In the nearly ten years I have been aware of this issue I don't recall anyone ever telling of positively nailing this thing down, it just seems to be something that most of us see and learn to live with.

I haven't paid enough attention to the indications for a long time to be able to recall any sort of pattern, but it may vary as you suggested. I recall the vent pressure question being raised in the past and that may indeed be a reason for the inconsistent readings. I'll start watching it more closely in the future with the goal of determining patterns but I agree that this most likely is just a function of using inexpensive senders.

I don't see how the fuel pressure could actually vary as much as the gauge indicates. It's very unlikely everyone with inconsistent pressure readings has a defective fuel system. :)
 
Last edited:
Vapor Lock Beginnings

Sam,

I've been giving this alot of thought, and maybe we are going down the wrong road. What if the guage is telling the truth.........? If the electric fuel pump always restores normal pressures, then we must consider the possibility that the engine driven diaphram pump is experiencing the beginnings of vapor lock. Vapor in the pump, bubbles in the line, caused by the heat of the accessory case, 1000 degree exhaust pipes and the heater valve constantly dumping 200 degree air right behind the fuel pump. There have been enough unexplained engine outages in RV's that this might help explain some. Not everybody remembers to turn on the electric pump, or has time.

In Piper and Grumman installations, the engine driven fuel pump has a cooling jacket with a blast tube coming from baffles to keep the pump cool. They must have this for a reason, and the reason might just be what we are talking about........unstable engine pump pressures at the top of an extended climb. Think about it.
 
fuel sender location

I had problems with my fuel pressure jumping around. I had my sender mounter to the tee fitting coming out of the mechincal fuel pump. I noticed that certain rpm would cause it to flucuate. I mounted it to the fire wall and prolbem went away.

Jim Knight
Burlington Iowa
RV6 210 hours 0-360
 
I had problems with my fuel pressure jumping around. I had my sender mounter to the tee fitting coming out of the mechincal fuel pump. I noticed that certain rpm would cause it to flucuate. I mounted it to the fire wall and prolbem went away.
Click to expand...


Jim, I'm glad you got your pressure indication problem resolved.

I'm REALLY glad you removed the sender from the hard mount on the fuel pump. Mounting senders directly to the engine is not good practice because it can promote failure of the fitting due to engine vibration.
 
What if the guage is telling the truth.........? If the electric fuel pump always restores normal pressures, then we must consider the possibility that the engine driven diaphram pump is experiencing the beginnings of vapor lock. Vapor in the pump, bubbles in the line, caused by the heat of the accessory case, 1000 degree exhaust pipes and the heater valve constantly dumping 200 degree air right behind the fuel pump. There have been enough unexplained engine outages in RV's that this might help explain some. Not everybody remembers to turn on the electric pump, or has time.

In Piper and Grumman installations, the engine driven fuel pump has a cooling jacket with a blast tube coming from baffles to keep the pump cool. They must have this for a reason, and the reason might just be what we are talking about........unstable engine pump pressures at the top of an extended climb. Think about it.
Click to expand...

John, you're reading too much into these observations again.

1)Turning on the electric pump will not always restore flow to normal indication, it merely causes the flow to increase a couple psi over whatever is indicated.

2) The inconsistent flow indications do not appear just at the top of an extended climb but in other phases of flight as well. I've often seen 1 psi in level cruise.
 
Last edited:
Vapor Lock

No Sam, I'm not "reading too much" into your observations, I am responding to the original poster (you know, the reason for this thread). He reports little or no fuel pressure at the top of climb, but instantly remedied with the electric boost pump (4-5 psi). Read his post if you get a chance.

To conclude for sure this is a sender issue goes against logic and everything I have ever learned as an A&P. What would it hurt to install a blast tube on the fuel pump? Sounds like cheap insurance to me, especially for the guys running car gas.
 
Last edited:
John, I think you are really onto something. Please report back with your findings after testing a blast tube as soon as possible. This could be the answer.

Thanks,

Roberta
 
No Sam, I'm not "reading too much" into your observations, I am responding to the original poster (you know, the reason for this thread). He reports little or no fuel pressure at the top of climb, but instantly remedied with the electric boost pump (4-5 psi). Read his post if you get a chance.

To conclude for sure this is a sender issue goes against logic and everything I have ever learned as an A&P. What would it hurt to install a blast tube on the fuel pump? Sounds like cheap insurance to me, especially for the guys running car gas.
Click to expand...



I read the post, I believe you have this figured out. Thanks for your input, John.

I have a blast tube on the fuel pump on my plane.
 
Last edited:
Hummm...

I think, and don't know why no one has suggested this, that Jeff should call Precision Airmotive and let them diagnose his stumbling problem. Use to be a fellow there that was extremely RV friendly, but can't remember his name. Could be the accelerator pump as suggested, but could also be the carb he has may be one of those that needs some modes to run well on an RV. They are the experts, they should be the ones to say there is, or isn't, a problem with the carb.
Low or no fuel pressure readings have always been a problem in RV's, as has been pointed out. It can be argued all day long, but the fact is you're gonna get low or no pressure readings on electric fuel pressure gauges in some phases of flight. Not sure if the same would happen with a mechanical gauge, someone got one on a carbed RV? Another fact is they keep on running!
Just to add fuel to the discussion, my theory is the lines supplying the mechanical pump are just adequate for full throttle applications (see me ducking here!), feeding all the fuel rapidly to the carb, not allowing time for the fuel pressure to build enough for our gauges to register. Reduce throttle, or demand, or increase fuel flow slightly by turning on the electric fuel pump..you now have a reading. The less the demand, or the more the extra flow, the higher the reading. That's what I've seen anyway.
Interesting thread guys, glad you're not in the same room!!!
 
I have insulated most of the fuel line fittings and the gascolator and installed a blast tube to the mech fuel pump in an attempt to solve the sometimes low pressure readings. It is a bit too early to say for sure yet, but since doing this I have not had the extreme ?zero? readings I had sometimes experienced before. Another thing I have noticed is that the fuel flow transducer located between the mech pump and the carb is now giving stable flow rates compared to the fluctuations sometimes seen before the insulation. I will be installing a second blast tube so there is a blast tube on each side of the pump. I need more flying to test this so don?t read too much into this yet.

Fin 9A
 
Finley,

Have you seen the aluminum jackets they make for production planes that go over the fuel pump to assure good cooling from the blast tube air? Lot's more effective than just the blast tube. I have also installed a cabin heat muff on both exhaust pipes, not only for better cabin heat, but to reduce the radiated heat from the pipes to the fuel system.

Also, take a look at the cabin heat bypass air. Blows right toward the fuel pump anytime cabin heat is off. I'm using the square version instead of the triangle in the hopes that the hot air will blow downwards away from the fuel
system.
 
John, do you think you could produce a "jacket" and test this for us. You might have a market if it works.

Roberta
 
Just so happens that I have an O-320 out in my shop that came out of a 150 hp C-150 with a cooling shroud on the fuel pump. I'll try to take pictures.
 
That would be great, Mel. I remember my Cherokee had one as well. Right now all I have is the blast tube coming from the Vac pump as shown in the Van's plans. If John could fabricate one and test it. That would be great.

Roberta
 
Roberta,
Just sent you pictures. Please post them if you think they are worthy.
 
Ray was my husband's dad's 1st cousin. I have a prop he made on our mantel. Great guy. He passed on a few years back.

Here is a shot of Rich and me next to his plane "Chuparosa" at EAA last year.

Roberta

chuparosacv9.jpg
 
Here are some pics of the fuel pump shroud Mel graciously sent me. See if any of you handy guys could fashion one and give it a test.

Roberta

img1638mf6.jpg


img1640qu3.jpg


img1641xi7.jpg
 
I'll never forget the last time Ray attended the Southwest Regional Fly-In. He was introduced and told, "It's good to see you here." Ray's response was, "At my age, it's good to be anywhere." Ray was a great guy to be around.
 
Thanks for the pictures Mel/Roberta. That one looks homemade. The ones I've seen are thinner aluminum and riveted if I remember right. I'm going to call a few salvage yards tomorrow for production versions, and I'll let you know what I find.
 
Don't know the origin of this shroud. This is the way it came out of the C-150.
 
Looks like the problem solution may be at hand. Probably a good idea to install right away during construction, since the pump is a little hard to get to on a completed plane. Anyone willing to give it a go?

Roberta
 
Yukon is right on. Had a similar problem with an RV9 with O-320:

* Fuel pressure slowly went to <1 PSI in climb.
* Always jumps back up to 5 PSI with electric pump on.
* Comes back up to 2-3 PSI in cruise.
* Never sputtered or missed a beat even when the Rocky Mtn said 0 PSI.

Tried a lot of different things to try to solve it including a new wobble pump. Then I noticed the same pressure problem in long ground ops/taxi. The problem was HEAT. I insulated the gascolator, which didn't do much, then put a shroud on the wobble pump with a blast tube and... Problem got a LOT better, climb pressure back up to 2 PSI min.

Would like to hear if anyone else solved their low pressure in climb problem this way?
 
You must log in or register to reply here.
Back
Top