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Low fuel pressure

N24YW

Well Known Member
I have a 0-360 and have the vans analog fuel pressure gauge and sender unit. I see around 5 psi when I start up with the electric fuel pump on and a slight decrease in pressure when I shut off the electric during taxi. But in flight I will see very low readings sometimes they will go to zero. I have 40 hours on the plain and at first it bothered me to see low readings but the plain seems to run fine. I am going to take the fuel line off at the carburator and put an adjustable flow control on it and run it into a catch can to see if I have a restriction any where. Also at the fuel sender I will install a mechanical test gauge with a 0-5 psi scale. Just wondering if anyone else has seen a similar problem.

Jim
 
You're Not Alone

Jim,

I have noticed the same behavior, and I am using a GRT Fuel Pressure sensor and EIS 4000 system. I have done flow tests, and have run the engine hard when the pressure was indicating low, so I know that whatever the reading might be, the fuel system is doing its thing. I have not had a chance to really tear into anything to figure out what is going on, but I am not worried that I have an engine problem.

Paul
 
This is an old and well documented problem with aircraft with fuel pressure gauges. In particular, I bet both of you are seeing low pressures in climb, or after you level off at a higher altitude. Later, you see unexpectedly high pressure as you descend or when you level off at a lower altitude. I've seen, or read about, this problem in RV's, Pipers, Beechcraft, etc. There are numerous theories as to why this happens, but it *probably* isn't cause for worry.

Kyle Boatright
RV-6 N46KB 350+ hours.
Ex Tomahawk owner...
 
Same here

My first and second flight was October 1st this year. On my second flight, I climbed harder and (stupidly) forgot to turn on the electric fuel pump at takeoff. I had a big grin on my face after seeing 2400+ FPM climb out only to look over at the fuel pressure gauge and see it reading close to ZERO! My heart skipped a beat, I leveled off immediately, turned on the electric fuel pump and did a 180 back over the airport. Of course the fuel pressure came up slowly and was soon in the green again (I have the Van's guages). I put out a post and found out that this was not uncommon. Since then I have noticed this behavior after climbs and at altitude but have had no problems whatsoever with the engine so I have relaxed a little as I don't believe there is cause for concern.

FWIW
 
I had this same worry as well in my 0-360 with Van's gage. I talked with Tom Green at Van's about it and it is common, but a non issue. As you climb the fuel has to be lifted higher to get to the pump and as you descend, the fuel is higher in the tank. These changes in height cause momentary pressure drops and increases. The carby lycosuarus only needs .5 psi (or actually less) to operate. Many older planes don't even have engine driven or electric boost pumps and operate only on gravity feed. If you get wary, like I sometimes do, just kick in the boost pump.

Roberta
 
Good question

About $35, and another $35 for the transducer. ;)

All kidding aside, that is a good question. I turn my fuel pump on for takeoff/landing and whenever the fuel pressure reads low(only seems to happen on climbout at night). So, it is primarily there to make me worry about fuel pressure.

Perhaps it would be handy if an abnormally low fuel pressure were detected before takeoff with the aux fuel pump on?
 
I've heard these various reports and I'm at the stage of figuring out what's going in the panel. Can't figure out whether to bother. Is this just with certain models of the gauge?

But it's an odd bug that you all describe, and I really need the instruments to tell me something I can -- sort of -- hang a hat on. But in this case it sounds like the instrument can't be trusted to any reasonable degree.
 
How many low-dollar pressure guages have you seen that can detect less than 2 psi? Your asking a lot from a low-dollar pressure guage. This is very common with pressure gauges when trying to measure such low readings.
Mel...DAR
 
Bob asked if this is aproblem with a particular gauge... Nope, it has been experienced by a wide variety of gauges in different aircraft applications.

My belief is that it has to do with an undersized static vent on the fuel pressure sender, which doesn't allow the reference pressure to correct to ambient as fast as the airplane changes altitude.

Kind of like what would happen to your altimeter if your static system was clogged or partially clogged.
 
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
 
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.
 
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?

Jim
 
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.
 
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.

George
 
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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.
 
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.
 
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