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07-19-2010, 12:19 AM
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Join Date: Dec 2006
Location: Arroyo Grande, CA
Posts: 938
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Is your pressure gauge reading pressure relative to ambient or differential pressure of the fuel pressure to the manifold pressure. Since injector flow varies with the square-root of a pressure change, a 2% pressure change will cause fuel flow to change by 1%. Many systems control fuel pressure relative to the manifold pressure so that if MAP drops by 6", about 3 psi, the fuel pressure indicated on a differential gauge will show no pressure change, but a gauge vented to ambient will show the change!
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07-19-2010, 12:15 PM
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Join Date: Jan 2005
Location: Winslow, UK
Posts: 18
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For the first 140hrs or so fuel pressure was steady at 28psi then it started 'dropping' to 23psi for about a second then back to 28psi, this repeated every 5-10 seconds. With the boost pump on the pressure was steady at 30psi. After scratching my head for a while I replaced the pressure transducer (Dynon supplied) and the problem went away. Being the sort of person I am I took the transducer to bits and the tracks were worn, I'm guessing at the 28psi reading. It looks like when the boost pump was on it pushed the slider to an unworn part of the track and I got a steady reading.
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07-19-2010, 03:37 PM
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Join Date: Oct 2007
Location: Sacramento, CA
Posts: 3,351
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My fuel pressure runs between 24-26 PSI but mostly around 25 psi on idle or cruise and with the auxiliary fuel pump on, the pressure will go to around 30 PSI but then after a few second it will start coming down to the normal 25-26 psi.
Keep in mind that you have a pressure relive valve at your auxiliary pump setup that is suppose to open up if the pressure build up higher then normal. Some times the o-ring may get stock and keep the valve from operating as it should. The o-ring may be affected by the blend of the fuel which can vary from station to station.
__________________
Mehrdad
N825SM RV7A - IO360M1B - SOLD
N825MS RV14A - IO390 - Flying
Dues paid
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08-13-2010, 12:29 PM
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Join Date: Feb 2006
Posts: 42
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Airflow Performance pump pressure too high?
RV7 QB IO360 180 hp.
I am using the Airflow Performance 7/7A F.I. PUMP INSTAL kit on my RV7.
With the AFP pump on engine not running I get 50psi downstream (after) the mechanical pump and 25 psi upstream (before) the mechanical pump. I used two different new mechanical pressure gauges and get the same reading on both. The AFP manual says the pressure relieve valve is set at 26 psi so the AFP pressure relief valve is serviceable.
Can anyone tell me why I am getting a 25 psi increase going through the mechanical pump?
Is this normal?
__________________
Ian
RV7 slow QB
Last edited by gh6gh6 : 08-14-2010 at 01:26 PM.
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08-13-2010, 01:20 PM
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Join Date: Aug 2007
Location: Port Orange, Florida
Posts: 84
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It looks like enough people are having similar behavior with their fuel pressure to conclude that all are not having a problem.
I have the Silver Hawk injection servo with the pressure line coming off the ourflow from the mechanical engine pump. I also have the cooling shroud on the pump.
After starting the pressure can flutuate a lot during the first couple of minutes but typically does not go below 15 psi. With a cold or hot start I am putting the electric pump on after it starts for a couple of minutes. When I turn it off it settles at around 28 psi and doesn't vary much from there.
I have 65 hours on a new engine.
A few hours ago I decided to bleed the line going from the mechanical pump to the Dynon pressure transducer on the manifold supplied by Vans for fuel and oil pressure. I never did it before but should have. After doing this I found the pressure stabilized more rapidly. If there is air in this line it is the pressure of this air you are reading at the transducer and not fuel which if the line is free of air should give a more instantaneous reading.
However, I still remain suspicious of some kind of air leak in the plumbing that cold cause erratic readings at startup -- in other words, sufficient air leaking that allows the fuel to drain back to the tank from the lines to the fuel selector and then to the filter which in my case is vertical on the gear tower. This might add a fuel bubbles in the system that are eliminated when the electric pump is put on for a couple of minutes. Also supporting this is the very, very slight fuel smell I get and while there are no drips I have found a little staining on the lines below the fuel selector. Something to be tweaked at the first condition inspection.
The bottom line is that I think pressures as low as 15psi at low power settings and then in the mid 20's are normal. Since I am at sea level I haven't paid a lot of attention to the effect of altitude and while I've flow up to 10,000 ft havent paid a lot of attention to the reading while there. I see further back in the posts Marc Ausman who lives is much higher up in New Mexico. Someone else noted that ambient pressure effects the reading so that might be a factor. On my other plane with a gravity feed system to an engine driven pump I loose a couple psi of fuel pressure above 5000ft on the guage.
__________________
Tom Agin
Port Orange, FL
QB RV-8 completed!!
XIO-360-M1B with LASAR
GNS-430, GI-106A, GTX-330, 9000EX, Becker 4201, Digiflight II VSGV, ADI, Dynon Dlight Dek 180, Vertical Power VP-200
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08-14-2010, 02:14 AM
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Join Date: Oct 2006
Location: Seattle, wa
Posts: 679
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Some instalations require a ground wire if the transducer is located at the end of the pressure line and tied up in some fashion. This set up, while providing the least fatigue on the transducer threaded connector presents a difficult grounding solution. Sometimes a tab clamped to the transducer. A marginal ground in this or any other mount senario can cause the pressure to indicate low.
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08-14-2010, 07:04 AM
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Join Date: Feb 2005
Location: Chesterfield, Missouri
Posts: 4,514
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I would suspect the electric transducer as being flaky.
Other than that, a friend had problems with pressure recently with a new system and found the big filter clogged with lots of junk, cleaned it and pressure returned to normal.
My system runs at 26-28 with the boost pump on and 21-22 on the mechanical engine pump. Pressures are rock steady. Altitude does not seem to be a factor. Two days ago I recorded some numbers at 9500' MSL (DA 12,220') and fuel pressure was 21 psi running on the mechanical pump.
The pressure regulator is a ball and spring deal, kind of bullet proof, and the pumps are generally very reliable, except ethanol could ruin the mechanical pump.
The weak link in the system is the pressure transducer and/or how your particular indicating system is dealing with the volt signal from the device. The system here is Grand Rapids EIS 4000 and it has been working quite well across the board.
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RV-12 Build Helper
RV-7A...Sold #70374
The RV-8...Sold #83261
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Last edited by David-aviator : 08-14-2010 at 07:16 AM.
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08-14-2010, 07:27 AM
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Join Date: Jan 2008
Location: Sheridan, IN
Posts: 94
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I have seen similar things in my plane. The pressure has dropped several times just after refueling, on hot days, at the top of climb to 7-9000ft. I have a separate sensor for the "idiot" light and it flashes on as well. This makes me think that it is not the transducer. I now keep the boost pump on until the top of climb plus a minute or two and leave the pump on after changing tanks for several minutes. I suspect it is vapor related. I plan on installing a fuel pump cooling shroud and more insulation on the fuel line from the firewall to the selector valve.
Van's fuel pressure gauge
Bendix Silver Hawk injection
Horizontal induction
Firesleeved lines
100LL
__________________
Drew Cook - K9QV
Sheridan, IN
RV-7 N673AC
Superior IO-360 Horizontal Induction
Hartzell C/S Blended Airfoil
IFR Steam gauges
TruTrak Digiflight IIVS
500 Hours on airframe
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08-31-2015, 06:27 PM
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Join Date: Sep 2014
Location: Falmouth MA
Posts: 15
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High Fuel Pressure
Came across this thread while looking for guidance on high fuel pressure I'm seeing on my RV-8, IO-360-A1B6D. I've had it about a year, and the GRT EFIS and EIS most often are reading way over the 45 psi limit in Lyc manuals, often 50-60. Engine runs fine, it seems, but does take a lot of leaning to bring FF in cruise to book reference fuel flows, and it seems to need a lot of leaning on ground. As I say, operation seems ok, but concerned. I'm wondering if pressure sensor is in wrong place in system? Any ideas would be appreciated.
__________________
Bill McClure
RV-8
Cape Cod, MA
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08-31-2015, 08:07 PM
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Join Date: Dec 2014
Location: Just Minutes from KBVI!
Posts: 1,039
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Quote:
Originally Posted by Jamie
I'm suspicious of vapor getting inside the fuel pressure transducer line. If vapor gets in the line, it will rise to the transducer. I'm not sure if the vapor would actually affect the transducer's output or not -- but I'm sure someone will chime in.
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Pressure's pressure. I could see where a vapor pocket might act as a "spring" in an oscillating pressure fluctuation, but it shouldn't affect the steady state number much.
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