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Old 12-14-2005, 03:58 PM
Bob Martin's Avatar
Bob Martin Bob Martin is offline
Join Date: Jan 2005
Location: Charlottesville, Virginia
Posts: 1,313
Default Low Fuel pressure

Why does it happen to some and not others???
I have an RV-6 that I bought and it has the low fuel pressure happening and has for the last 300 hours I've flown it.....but I sure would like to fix it!!
Another -6 has arrived my airport and it doesn't have the problem, so I hope to compare the two setups to see what is different. Maybe where the pressure is messured is different or something like that. I hope I can find it, but sure seems like plenty of people ahead of me have looked to no avail.
Bob Martin
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Old 12-15-2005, 09:43 AM
TRAllen TRAllen is offline
Join Date: Dec 2005
Posts: 13
Default Same Problem in my RV-6

I have a RV-6 with about 200 hrs on it and I have the same problem. The engine runs fine and fuel flow is steady but the pressure reads "0" at high power settings on climb out. I have tried many fixes including pump changs, pressure sender change and the problem is the same. It always comes back to normal and reads fine in cruise,on the ground, and in the pattern.
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Old 12-16-2005, 06:36 PM
N24YW N24YW is offline
Join Date: Dec 2005
Location: Burlington Iowq
Posts: 111
Default Update test

I went ahead and hooked up a adjustable flow control at the hose next to carburator. I teed in a 0-5 psi pressure gauge next to the vans sender unit. When I turned on the electric fuel pump I was able to get over 5 psi and the Vans gauge showed about 3.5. I then opened up the flow control and let the psi drop to 1.25 psi indicated on my test gauge and the vans showed 0. At the 1.25 psi I calculated a flow of 27 galons per hour by catching the fuel in a measureing can. Conclusion is the gauge sender is not very accurate on the low end. I wonder if I could put a 10k resistor across the gauge to offset the low end?

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Old 12-18-2005, 09:20 AM
Rick_Luck Rick_Luck is offline
Join Date: Sep 2005
Posts: 44
Default How Low is Low?

Boy am I glas to see this thread! I've been pulling my hair out. A month ago on a flight with my wife in our RV9A (O-320, carburated), the Low Fuel Pressure alarm went off on my EIS 4000. It was showing 1.7 PSI, when it usually shows between 3.5 and 5. I immediately switched on the electric pump and talked to our local tower, indicating I thought I'd lost my mechanical pump. I headed back to the airport, staying over roads as I went. I tried turning off the electric booster pump several times and each time the pressure would drop immediately and keep dropping until I got too nervous and turned it back on. I switched tanks and tied again. I don't remember the response but eventually I was able to switch off the electric pump and the fuel pressure stayed up in the 4 psi range.

Hanger talk between mechanics and non-mechanics attributed it to everything from a few ice dropplets in the fuel system restricting flow via the mechanical pump only, to not having the fuel selector valve at the proper "demark".

The plane has flow fine in 2 or 3 flights between then and yesterday when it happened again. This time I also immediately switched the electric booster pump on and noted that it didn't make any difference which tank the gas was being fed from. I tried several different times to turn the pump off but each time the pressure dropped to below 2.5 psi before I turned it back on.

Reading this thread has been very enlightening. First of all, I didn't realize I could let the pressure drop ot almost or even zero and not have the engine stop. Next time it happens, I might be a little more adventurous before I turn the booster back on.

One thing that might be different with my experience was that the loss of fuel pressure occured after a very very gradual climb or even what you'd probably call level flight and both times I lost pressure, it must have been 3, 4 or 5 minutes (maybe it wasn't that long but it seemed like an eternity) before the fuel pressure was up in the normal range again using the engine driven pump only. Also, when the plane was back in the hanger, I used the electric pump only to check fuel pressure readings from the transducer. They seemed very credible. I took a plug out at the pressure tranducer mount and the fuel pressue immedately showed Zero. re-installing the plug, I could cycle the pump and achieve pressues between zero and 5 psi depending on the amount of cycling and the pressures seemed to corrollate with the noise the pump was making as it labored at the higher pressures.

I'm somewhat relieved and will probably hold off on ordering a new mechanical fuel pump, or fuel pressure transducer, or gerry-rigging a second fuel pressure gauge into the cockpit...... for now anyhow.

Thanks guys and any further input would be appreciated.
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Old 12-18-2005, 09:55 AM
gmcjetpilot's Avatar
gmcjetpilot gmcjetpilot is offline
Join Date: Jan 2005
Posts: 4,725
Default Spec for fuel pressure

I looked up the type spec for the Lycoming fuel pressure.

0.5-8psi for O-360 Lycoming.

0.5 psi ain't much.

It is not like fuel injection, as long as the fuel is flowing pressure is not an issue. No doubt the 0.5psi is with in the reading tolerance (probably) more. You should see something on the FP gauge when you put the boost pump on?

Concur I have heard of this for years. My last RV had a mechanical gauge that read down to 0.5psi. My new RV has a GRT4000 and not flying but hope the resolution of the FP sender (transducer) will pick up low pressure.


Last edited by gmcjetpilot : 12-18-2005 at 09:57 AM.
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Old 12-19-2005, 09:30 AM
Rick_Luck Rick_Luck is offline
Join Date: Sep 2005
Posts: 44
Default Low Fuel Pressure

Two more items of interest regarding the mysterious loss of fuel pressure some of us have experienced.

#1. I checked the data from when my engine was run after final assembly at Lycoming (when it was born). The mechanical fuel pump was delivering between 4.3 and 5 psi, depending on rpm and loading. Interesting enough, the highest pressure, 5.0 was at idle. The numbers seem to reflect an inverse relationship between rpm and pressure. I suppose that's because the fuel consumption is greater at higher rpms and even though the pump is also pumping at a higher rate, it doesn't keep up with the increased consumption of the carburator.

#2. I'm presuming the rest of you guys are familiar with the Airworthiness Information Bulletin dated 9/30/05 about Lycoming fuel pumps, essentially recalling all pumps manufactured between the 7th week of 2000 and the 4th week of 2001, inclusive. When I first got the Manditory Service Bulletin, I checked the date code on my fuel pump and found it didn't apply. Based on recent developments though in my fuel pressure, I might recheck the date code on the pump. The pump problem would definately cause decreased fuel pressure but I think it would be permanent instead of intermittent.
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Old 06-30-2006, 08:45 PM
hawker hawker is offline
Join Date: Jun 2006
Location: Hartford CT
Posts: 43
Default Fuel pressure

Usually showing between 1-5 psi on my Lyc O-360 with vision micro inst, it now reads O all day long. with or without boost on, no leaks, fuel flow and egt normal, I assume the pressure transducer is gone. Is it the same part # for carb or inj engines? VM 3010017 Answer, no 17 is for inj and 16 for carb.

Last edited by hawker : 07-06-2006 at 06:33 PM.
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