Low fuel pressure

N24YW · #1 12-11-2005, 07:30 PM

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

Ironflight · #2 12-11-2005, 09:00 PM

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

Kyle Boatright · #3 12-11-2005, 09:06 PM

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...

hecilopter · #4 12-12-2005, 01:13 PM

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

robertahegy · #5 12-12-2005, 04:22 PM

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

LettersFromFlyoverCountry · #6 12-12-2005, 04:47 PM

So what's the value of a fuel pressure gauge?

redbeardmark · #7 12-12-2005, 04:56 PM

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?

LettersFromFlyoverCountry · #8 12-12-2005, 05:46 PM

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.

Mel · #9 12-12-2005, 06:19 PM

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

Kyle Boatright · #10 12-12-2005, 08:21 PM

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.