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Varying fuel pressure on Lycoming 0320

S

stevemorris

Member
Hey Guys,
Maybe someone can advise if they know anything about the following problem.
I have the 0320 D2A installed in my RV8. I have suffered variable fuel pressure between 2 psi and 6 psi.
I have a brand new fuel pump installed and if low fuel pressure occours then switching the electric back up on always resolves the concern.
If in a continual climb for a long duration I will suffer low pressure, but often at low power settings it will drop right back without any explanation.

Any ideas or has anyone seen this before?

Steve
 
Bubbles are forming in the fuel.

Just like when you open a bottle of pop the pressure on the liquid is reduced to the atmosphere bubbles form. When your engine driven pump sucks the fuel the pressure on the fuel is reduced and bubbles come out of solution (fuel in this case). When you turn on the boost pump the increased pressure makes the bubbles go away.

The reason you are not seeing it at reduced power is because the reduced suction is not enough to form the bubbles.

1 solution: run the boost pump anytime you see the reduced pressure.
 
You're seeing mild vapor bubbles in the fuel, the beginnings of true vapor lock. Anything you can do to cool the mechanical fuel pump and the fuel line leading to it (think blast tube) or increase the pressure (think boost pump) will help.
 
stevemorris said:
Hey Guys,
Maybe someone can advise if they know anything about the following problem.
I have the 0320 D2A installed in my RV8. I have suffered variable fuel pressure between 2 psi and 6 psi.
I have a brand new fuel pump installed and if low fuel pressure occours then switching the electric back up on always resolves the concern.
If in a continual climb for a long duration I will suffer low pressure, but often at low power settings it will drop right back without any explanation.

Any ideas or has anyone seen this before?

Steve
Click to expand...

Very, very common with a carbureted engine and an issue that has been kicked around the RV community since the beginning of "RV Time". I don't want to minimize the issue, but don't be surprised if after you have investigated everything you know to check, the variable pressure indications remain.

This has been discussed at great length on this forum. Unfortunately, I can't seem to get the search feature of this forum to look for particular phrases or groups of words, so it takes a while to poke through the archives.

Here is one such thread about fluctuating fuel pressure:

http://www.vansairforce.com/community/showthread.php?t=19060&highlight=low+fuel+pressure

Good luck with the diagnostics. After flying my plane for nine years, sometimes with "0" indicated pressure and no engine glitches, I've decided I'm seeing errors created by the inaccuracy of the low pressure transducer. According to Lycoming's specs a carbed engine only needs 0.5 lbs fuel pressure to keep the bowl filled .
 
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Classic VP. Been there, done that.

Install a blast tube to the suction side of the mechanical FP. Make sure any metal bulkhead fittings are not getting blasted by the cabin heat exit air. Insulate all FF forward fuel lines & fittings. Make sure your fuel caps are not leaking air, and that the vent tubes are not plugged. If you have a gasolator run a blast tube to it also. If you drilled a small hole in the backside of the vent tube fix it, the tanks need the pressure.
Boost pump on anytime you are climbing out, less than 1,000' AGL, or when you see low FP.

Are you running mogas? Have you checked your fuel filter lately?
 
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A pressure transducer uses a reference pressure (static pressure) and a dynamic pressure. The it compares the two and displays the difference. If either the static or dynamic pressure are wrong, you get an inaccurate reading. Just like the ASI would if your airplane had a plugged static system or a mud dauber in the pitot tube.

Most of our (experimental) engine instruments and transducers are automotive in nature. On fuel pressure transducers, this causes a problem because the static orifices are not large enough to allow the reference pressure in the transducer keep up with the altitude of the airplane. It is akin to a plugged (or almost plugged) static system.

The result is that when are in an extended climb, you'll see an erroneous low pressure reading, and in an extended descent, you'll see an artificially high fuel pressure. In either case, flying at a steady altitude for a few minutes will result in the pressure on the reference side of the transducer equalizing with the outside air, and you'll get an accurate reading.

Check and see if your fuel pressure behaves as described above. If so, this is probably the answer to your question.
 
Kyle Boatright said:
A pressure transducer uses a reference pressure (static pressure) and a dynamic pressure. The it compares the two and displays the difference. If either the static or dynamic pressure are wrong, you get an inaccurate reading. Just like the ASI would if your airplane had a plugged static system or a mud dauber in the pitot tube.

Most of our (experimental) engine instruments and transducers are automotive in nature. On fuel pressure transducers, this causes a problem because the static orifices are not large enough to allow the reference pressure in the transducer keep up with the altitude of the airplane. It is akin to a plugged (or almost plugged) static system.

The result is that when are in an extended climb, you'll see an erroneous low pressure reading, and in an extended descent, you'll see an artificially high fuel pressure. In either case, flying at a steady altitude for a few minutes will result in the pressure on the reference side of the transducer equalizing with the outside air, and you'll get an accurate reading.

Check and see if your fuel pressure behaves as described above. If so, this is probably the answer to your question.
Click to expand...

I'm not sure if it was the static pressure erroneous readings or the dynamic pressure erroneous readings, but when the big fan up front started to quit until I hit the boost pump I call it vapor lock. The non-erroneous variety. ;)
 
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Geico266 said:
I'm not sure if it was the static pressure erroneous readings or the dynamic pressure erroneous readings, but when the big fan up front started to quit until I hit the boost pump I call it vapor lock. The non-erroneous variety. ;)
Click to expand...

Larry, nobody is doubting your experiences with vapor lock and I'm glad you got it solved. However, the symptoms described by Steve in the original post, while they may be due to vapor lock, also closely resemble the very-well documented observations of scads of RV pilots over the years who have flown bunches of hours with electric transducers indicating fuel pressure at or near zero. Kyle's explanation is the same as what several folks who know a whole lot more than me about pressure transducers have offered in response to this known "problem".

After wrestling with the fluctuating pressure thing for a very long time, even some very experienced RV pilots admit they don't have a positive explanation for it. It happens, it has been studied to death, diagnosed from one end to the other, and it still happens......and the engines keep running. :)
 
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Sam

I agree with you! 5 RV's (soon to be 6) with 4 carbs and I have seen the same fluctuations. The engine always keeps running and the low pressure always resolves itself eventually.
 
Sam Buchanan said:
Larry, nobody is doubting your experiences with vapor lock and I'm glad you got it solved. However, the symptoms described by Steve in the original post, while they may be due to vapor lock, also closely resemble the very-well documented observations of scads of RV pilots over the years who have flown bunches of hours with electric transducers indicating fuel pressure at or near zero. Kyle's explanation is the same as what several folks who know a whole lot more than me about pressure transducers have offered in response to this known "problem".

After wrestling with the fluctuating pressure thing for a very long time, even some very experienced RV pilots admit they don't have a positive explanation for it. It happens, it has been studied to death, diagnosed from one end to the other, and it still happens......and the engines keep running. :)
Click to expand...

Sam,

Just a question, should we then placard the fuel pressure indicator inop?
 
Sam Buchanan said:
After wrestling with the fluctuating pressure thing for a very long time, even some very experienced RV pilots admit they don't have a positive explanation for it. It happens, it has been studied to death, diagnosed from one end to the other, and it still happens......and the engines keep running. :)
Click to expand...
Until they stop. I attributed my fuel flow fluctuations in climbs and at altitude to the transducer 'problem'...they turned out to be an overheating gascolator. No harm done in my case - just be sure you (in general, not you Sam) understand the problem before discounting the worst possibilities.

Nauga,
hot and bothered
 
LifeofReiley said:
Sam,

Just a question, should we then placard the fuel pressure indicator inop?
Click to expand...

Not necessarily. I too went through all the usual diagnostics (new fuel pump, added blast tube, insulated fuel lines, etc) and after having zero engine problems over many hours and reading of the same behavior from many carbed engines decided my low pressure indications were transducer related.

I don't suggest anyone ignore low pressure readings. My contention is that some electric fuel pressure gages have this characteristic even though the fuel system is healthy. It is up to each builder to determine whether low indications are indicative of an actual pressure problem or gage error.

I don't know how to fix the fluctuating indications on my plane, maybe a different engine monitor with a different type of transducer would do it. Fluctuating pressure indications isn't my preference, but I have learned the characteristics of my engine installation, know the normal gage behavior, and am comfortable with it. The pressure gage on my plane behaves correctly 90% of the time, but I have learned that during climbs and descents the reading can get a little weird.

I am not saying every instance of flakey pressure indications is merely gage problems. But what I have described is real-world experience for my plane and many, many other carbed RV's.
 
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Kyle Boatright said:
Most of our (experimental) engine instruments and transducers are automotive in nature. On fuel pressure transducers, this causes a problem because the static orifices are not large enough to allow the reference pressure in the transducer keep up with the altitude of the airplane.
Click to expand...

Sounds right, but then why does turning on the electric fuel pump raise the pressure to near normal? :confused:
 
Sounds like my story

Sam Buchanan said:
Not necessarily. I too went through all the usual diagnostics (new fuel pump, added blast tube, insulated fuel lines, etc) and after having zero engine problems over many hours and reading of the same behavior from many carbed engines decided my low pressure indications were transducer related.

I don't suggest anyone ignore low pressure readings. My contention is that some electric fuel pressure gages have this characteristic even though the fuel system is healthy. It is up to each builder to determine whether low indications are indicative of an actual pressure problem or gage error.

I don't know how to fix the fluctuating indications on my plane, maybe a different engine monitor with a different type of transducer would do it. Fluctuating pressure indications isn't my preference, but I have learned the characteristics of my engine installation, know the normal gage behavior, and am comfortable with it. The pressure gage on my plane behaves correctly 90% of the time, but I have learned that during climbs and descents the reading can get a little weird.

I am not saying every instance of flakey pressure indications is merely gage problems. But what I have described is real-world experience for my plane and many, many other carbed RV's.
Click to expand...

My -6 was built with a JPI Slimline Fuel Pressure Gauge and transducer-2 wire type. I too had the dreaded low pressure readings. Like Sam, I was convinced it was a gauge or transducer problem. After replacing the wiring assembly between the transducer and gauge, I looked into replacing the transducer. While trying to buy a new transducer from JPI, I realized they now sell a 4-wire transducer, a newer more modern design, that goes with there EDM 930. JPI has made a updated Slimline gauge that mates with the newer transducer and I purchased one after reading it was a more sensitive transducer, also I'd installed a EI oil pressure gauge that used the same looking transducer with good results. Results are fanstastic! I now have a properly working fuel pressure gauge!!! It is wonderful !
The newer JPI fuel pressure gauge/transducer and wire harness was 350.
Plus JPI doesn't quite have there stuff together in the documentation dept, but that is another story, for another thread. But I finally do have proper fuel pressure readings and I didn't before.......I am very happy!!!
 
Mine does it to

At first I was concerned. It will drop below the green, down to maybe 2 pounds, never below that I can recall. engine runs fine anyway. Electric boost brings it back up, but I just ignore it instead.

It seems altitude related, higher it happens. Sometimes one tank or the other. I've checked vent lines, etc. They're fine.

When I queried this site about it, I think one pilot pointed out that Lycoming says it will run down to 1/2 psi.
 
Pipers and Lancairs too

We have a former Piper rep. here at work. After seeing similar behavior in my Lancair (O360), I asked him about it. He tells me Piper observed the same behavior many years ago and spent untold hours instrumenting engines and planes trying to get to the bottom of it. In the end they could not find an explanation.

One should certainly rule out other potential causes, but in the end you may be left scratching your head
 
I just raised the multiplier on the fuel pressure display in the GRT monitor.

It now reads a higher pressure and all is well.

On a carb engine, as long as u has some fuel pressure you are fine.
 
As always guys thanks for the immediate response. From what I am reading I need to look at the fuel caps, vent pipes etc and then probably not bother losing much more sleep over the subject.
If it happens to be "one of those glitches" but is still quite safe then I will let it be.

Thanks again for your help.

Steve
:)
 
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