Fuel pressure, sensor

H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

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designrs
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Re: Fuel pressure, sensor

Post by designrs »

Thank your for your insight and comments.

Electric Pump: doesn't solve my problem, although I still turn it on.

New Kavlico Senders: show a minimum of 1.2 psi. So if your pressure were zero it would still show 1.2

Sender is on firewall not motor. Less vibration.

Other Case: There is another SportCruiser owner with the same problem in Australia. His motor DID stumble. He found a loose clamp at the hose leading into the Gascolator. Stuff happens. I checked my Gascolator connections.

Dead Stick: NOT Comfortable. Far better pilots than I (some highly regarded life-long CFI's) have tragically died under deadstick. No thank you, if I can help it.

MECHANICAL GUAGE: I already have fuel lines in the cockpit, under the rudder pedals. Not crazy about running a pressure fuel guage into the cockpit, but at this point it looks like that is what it will take to troubleshoot this issue.

Vans RV-12: Very interesting case study because of the widespread problem and Vans having an official position on the matter. If I just brought a brand-new aircraft with that issue I'd be screaming mad. I respect Vans and look forward to hearing about better solution between them & Rotax.

At this point:
1) MECHANICAL FUEL PRESSURE GUAGE
which will then allow me to focus on Dynon or Mechanical factors.

Thank you all for following & suggestions.
I hope something good comes of this.

P.S. Anybody know if Vans did a mechanical guage check and what they found??
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bstrachan
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Re: Fuel pressure, sensor

Post by bstrachan »

Electric Pump: doesn't solve my problem, although I still turn it on.

Sorry to hear that. Obviously your situation is different than mine. My aux pump ALWAYS brings the fuel pressure back up.

New Kavlico Senders: show a minimum of 1.2 psi. So if your pressure were zero it would still show 1.2

I don't know if there is a calibration feature on your Dynon to address this. I would look into it.

Sender is on firewall not motor. Less vibration.

Mine too. My point is, the sender is piped to the octopus, NOT the carb inlet(s). If I were going to install a mechanical gauge, I would break into the fuel line with a tee right at the carb inlet.

Other Case: There is another SportCruiser owner with the same problem in Australia. His motor DID stumble. He found a loose clamp at the hose leading into the Gascolator. Stuff happens. I checked my Gascolator connections.

Did fixing the loose hose clamp on this Australian SportCruiser make the low fuel pressure indication go away?

Dead Stick: NOT Comfortable. Far better pilots than I (some highly regarded life-long CFI's) have tragically died under deadstick. No thank you, if I can help it.

Well, if you ever have to do a forced landing (God forbid) it's a good thing to have practiced. Doesn't your CFI make you do simulated engine out landings?

MECHANICAL GUAGE: I already have fuel lines in the cockpit, under the rudder pedals. Not crazy about running a pressure fuel guage into the cockpit, but at this point it looks like that is what it will take to troubleshoot this issue.

The existing fuel lines in your cockpit are not pressurized. If you run a line from the positive pressure side of the pump into the cockpit, and (again God forbid) something breaks or comes loose, you will have fuel spewing into the cockpit and no way to turn it off without killing the engine. I had a mechanical fuel pressure gauge in my wife's VW for years because it had a 28 PSI fuel injection setup that I needed to monitor, but looking back, it was not a good idea. I never had any problems with it, though. Be careful.

Vans RV-12: Very interesting case study because of the widespread problem and Vans having an official position on the matter. If I just brought a brand-new aircraft with that issue I'd be screaming mad. I respect Vans and look forward to hearing about better solution between them & Rotax.

It might be worthwhile to contact Vans and ask if they are working this problem. As I said, if you have a full-time aux pump and the system works like mine, you will not be aware there IS a problem.

Good luck and keep us posted.
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FastEddieB
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Re: Fuel pressure, sensor

Post by FastEddieB »

designrs wrote:
MECHANICAL GUAGE: I already have fuel lines in the cockpit, under the rudder pedals. Not crazy about running a pressure fuel guage into the cockpit, but at this point it looks like that is what it will take to troubleshoot this issue.
Could you not mount the mechanical gauge outside the cockpit, but still within view?

Disclaimer about the legality, but you might find a mechanic to sign off a temporary installation like that.
Fast Eddie B.
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designrs
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Re: Fuel pressure, sensor

Post by designrs »

Re: Mechanical Fuel Guage: Clarification:
Proposed diagnostics only.
NOT saying I'm going to fly like that, unless it were completely legal.
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designrs
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Re: Fuel pressure, sensor

Post by designrs »

Sorry to hear that. Obviously your situation is different than mine. My aux pump ALWAYS brings the fuel pressure back up.

Yes. That's the frustrating thing, so many variables.

New Kavlico Senders: show a minimum of 1.2 psi. So if your pressure were zero it would still show 1.2
I don't know if there is a calibration feature on your Dynon to address this. I would look into it.

It's not the Dynon minimum, it's the physical minimum of the sender.

Sender is on firewall not motor. Less vibration.
Mine too. My point is, the sender is piped to the octopus, NOT the carb inlet(s). If I were going to install a mechanical gauge, I would break into the fuel line with a tee right at the carb inlet.

Good point.

Did fixing the loose hose clamp on this Australian SportCruiser make the low fuel pressure indication go away?

He obviously found one problem on the Gascolator connection. Jury is still out if he has further issues. We are comparing notes.

Well, if you ever have to do a forced landing (God forbid) it's a good thing to have practiced. Doesn't your CFI make you do simulated engine out landings?

Of course! I'm as prepared as I can be, not overly fearful, would hopefully "do the right thing" under pressure, but that doesn't mean I have to like it. LOL

The existing fuel lines in your cockpit are not pressurized.

Good point.

It might be worthwhile to contact Vans and ask if they are working this problem.

Will do.

As I said, if you have a full-time aux pump and the system works like mine, you will not be aware there IS a problem.

True in many cases. However not in my case... low warning will exist with electrical fuel pump on. Classic scenario... normal flight, pump on for landing, low fuel psi on final or go-ariund. Electric pump on the whole time.

Good luck and keep us posted.

Will do. Thank you!
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bstrachan
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Re: Fuel pressure, sensor

Post by bstrachan »

With the usual disclaimers about not being an expert of any sort ... I've flown for 10 minutes with the fuel pressure showing "0". I can assure you, there was some pressure there, because that big fan looking thing on the front of my airplane, kept turning.

Good one :D

Another thought: fuel is 6 lbs/gal, water is 8 lbs/gal. I know water pressure goes as 0.5 PSI per foot of depth from my submarine riding days. It follows that fuel pressure goes as 6/8 X 0.5 = .38 PSI per foot. The gas tank on my Triumph is about 10 inches deep, so even with a FULL TANK the pressure at the inlet of the carbs would be about 10/12 X .38 = 0.32 PSI. That isn't very much, and it would be correspondingly less as the fuel level goes down. I know for a fact that the bike will run (until the carb bowls empty out anyway) with ZERO pressure at the carb inlets, because I've forgotten to open the fuel valve a time or two. My point is, maybe Bing carbs don't need much inlet pressure to work. 0.2 PSI may be too little to register on our EIS monitors, but it's plenty to keep my 790 cc bike runnning. For what it's worth.

Your comment that the big fan looking thing kept turning with ZERO fuel pressure indicated supports this wild and crazy theory. As long as there is fuel flow, the engine will run, and apparently it doesn't take much pressure to keep a Bing happy.
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designrs
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Re: Fuel pressure, sensor

Post by designrs »

Thank you for the above explanation and calculations. I was wondering about the effect of running the test guage down about 1.5 feet and then up about 1 foot. Technically it would be about +0.18 psi. Realistically maybe less from friction and bending.

Either way, the test guage should be a realistic baseline.
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designrs
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Re: Fuel pressure, sensor

Post by designrs »

Alright guys. After reading some RV-12 info I can conclude that their fuel pressure issues are different from mine. For some reason their fuel system experiences low pressure for which the electric pumps compensate. Vans recommends leaving the electric pump on at all times. Owners often like to switch test though to check both pumps for operation. Electric pumps are rated for about 500 hours.

My fuel system is usually just fine without the electric pump.
Further, switching on the electric pump does not correct my problem.

Lots of talk about possibly faulty pumps, so I'm glad I replaced mine to rule that out.
(Same issue both pumps.)

Also, reading all about how little fuel pressure is required.
Even when I get the low pressure warning, pressure is fluctuating, so the carb bowls stay filled.
This is a nuisance but probably not critical at all... unless I were to be sucking in lots of air.
BrianL99
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Re: Fuel pressure, sensor

Post by BrianL99 »

bstrachan wrote:With the usual disclaimers about not being an expert of any sort ... I've flown for 10 minutes with the fuel pressure showing "0". I can assure you, there was some pressure there, because that big fan looking thing on the front of my airplane, kept turning.

Good one :D

but it's plenty to keep my 790 cc bike runnning. For what it's worth.

Your comment that the big fan looking thing kept turning with ZERO fuel pressure indicated supports this wild and crazy theory. As long as there is fuel flow, the engine will run, and apparently it doesn't take much pressure to keep a Bing happy.

I assume you're riding a Hinckley Bonneville, like this?
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Nomore767
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Re: Fuel pressure, sensor

Post by Nomore767 »

As far as the RV-12 SLSA (not sure about ELSA/EAB) the electric fuel pump is wired on all the time. Don't even have a switch, just a fuse.
roger lee
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Re: Fuel pressure, sensor

Post by roger lee »

My money says they will eventually find a heat source. There are a couple areas I would change. Heat is the #1 issue for this problem and it seems to be wide spread in the RV12 group.
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designrs
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Re: Fuel pressure, sensor

Post by designrs »

One good thing came of this ordeal thus far...
I loooove my exhaust wrap.
Thanks Roger!

Perhaps the RV-12 could benefit from the exhaust wrap.

Yes, the S-LSA RV-12 has an always on electric fuel pump setup.
I guess you can still flip on the master and instruments before start and verify psi from the electric pump, then see the psi go up when the mechanical starts up.
SportPilot
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Re: Fuel pressure, sensor

Post by SportPilot »

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bstrachan
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Re: Fuel pressure, sensor

Post by bstrachan »

It depends on the restriction due to the smallest hole or tube the fuel has to flow through. Think of a bucket with a pinhole in the bottom... not much flow. A hole the size of a quarter allows much greater flow rate. I'm guessing the governing restriction in the fuel systems we're talking about is the size of the opening in the float valve (needle valve) in the Bing carb. Apparently this opening is big enough that adequate fuel flow (5+ GPH in WOT climb?) is maintained with relatively little pressure ( < 0.2 PSI) at the carb inlet. If the float valve had a tiny hole you would need much more pressure to obtain a 5 GPH flow rate through the needle valve.
3Dreaming
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Re: Fuel pressure, sensor

Post by 3Dreaming »

The fuel flow though the needle is less, 5GPH/2=2.5 per carb.
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