Varying fuel pressure on Lycoming 0320

[stevemorris]

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

[asav8tor]

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.

[airguy]

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.

[LifeofReiley]

More info... Hours on the plane/engine combo. You very well could have a plugged vent line.

[Sam Buchanan]

Originally Posted by stevemorris

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

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/communit...+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 .

[Geico266]

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?

[Kyle Boatright]

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.

[Geico266]

Originally Posted by Kyle Boatright

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.

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.

[Sam Buchanan]

Originally Posted by Geico266

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.

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.

[jthocker]

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.