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life of fuel pump?


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Fuel pressure gauge may be a worthwhile investment. I have one and it shows pressure by electric and mechanical engine pump, good for verification.

Depends on your plumbing setup; switching on my boost pump makes no difference to the pressure after the initial fluctuation.

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Depends on your plumbing setup; switching on my boost pump makes no difference to the pressure after the initial fluctuation.

I have Aviasport gauge and distinctly shows an increase when electric pump in turned on.

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Today I took out the fuel pump and had a good look at it. As has been said, the diaphragm is a double thing with a drain in between.

 

The pic shows the 2 diaphragms with a vented plastic spacer in between. The biggest vent is facing the camera.

 

Bruce, is there a vent line attached to the fuel pump? Can't see if there is provision for that in the picture. I would be concerned about fuel escaping inside the cowl.

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IF the diaphragm perforates, fuel will exit through the vent provided in the design.. The push rod to the camshaft? probably has no return drain. Sometimes the small shaft the lever pivots on wears in the case and stroke is lost or reduced. If the electric pump has equal or slightly less pressure the gauge won't show any increase. All that happens then is the diaphragm is forced to the spring most compressed position and sits there..Nev

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Yes, the fuel will go inside the lower cowl for sure if there is an upper diaphragm leak. There is no catcher and no drain.

But the upper diaphragm is in good condition. I reckon zero jabiru's have succumbed to this problem. They obviously think the fuel pump is ok till TBO.

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I was wrong about the fuel going into the lower cowl. As Thruster88 thought, there is a clear plastic vent line which goes from that hole in the pic to an exit out the lower cowl. My memory of building the plane must be fading a bit.

Well today I flew ( after lots of ground checking, but the ground checks were not at full throttle) and the fuel pressure readings were puzzling, although the plane went fine.

There was about 2 psi after starting, with just the mechanical pump. ( This was the same as with just the electric pump, which I tried before starting as usual )

With the engine running, there was NO increase in fuel pressure! Old K has noticed this, and I can't explain it. The 2 pumps are in series on the same line, and the fuel-pressure take-off is between the mech pump and the carby. I reckon Blueadventures observation agrees with common sense, but there really was no increase in this Jabiru. Could I have done something wrong?

On climbout, with both pumps on, the fuel pressure dropped to as low as 1.2 psi, and it didn't improve when I stopped climbing, having reached circuit height, at which point I came home, after just flying a circuit. If I didn't have the fuel pressure gauge, I would have said that the engine went real good.

Roundsounds, there was JSB 040-2 which was due to a faulty pump where an inlet pipe fell off. This was checked out at the time. But you got me rechecking that bulletin and I notice how they now show 2 drains. One from the diaphragm, and one from a formed metal catcher (like a metal cup ) which would collect from a leak anywhere in the fuel pump. This is the only service bulletin I know of.

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IF both pumps are rated at the same pressure the pressure doesn't change. With the electric pump on the engine one does not stroke and plays no part in doing anything till you turn the electric one off then it works normally. If there's no sign of leakage the only other fault possibility is one of the (2) valves not working properly and a reduced flow and or pressure will show that.. Nev

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I agree Nev, the next thing will be to test the flow from the mechanical pump. This is not a standard thing to do, as I will have to connect the electric pump to the carby and arrange for the mech pump to have some fuel to suck from and deliver to. The electric pump flow test was fine, with the pump delivering well in excess of the min required rate.

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My Jabiru is so old it has the fuel tank behind the seats, from where it won't gravitate. It would be possible though to pressurize the fuel tank a bit and this would make flow happen.

Here's the test results I did yesterday, and I am perplexed to say the least. NOTHING was changed between the tests of 3 days ago and yesterday, but the pressures are significantly higher in the second lot of testing.

Here's what I did..

Test1- pressure from electric pump alone ( engine not running) = 2.4 psi

 

Test 2. Flow from electric pump ( engine not running ) measured by taking the line off the carby, so the impediment to flow of the mech pump was there. As was the fuel filter. Flow = 56.5 l/hr and the spec is for between 50 and 60 l/hr.

 

Test 3... The motor was run directly off the electric pump and the flow from the mechanical pump was measured. This was 1.2 l/min at 1200 rpm, or 74.2 l/hr at 2800 rpm.

( the max the engine uses in practice is 18 l/hr, so there is plenty of spare capacity there.)( and yes the plane was tied down for this test with the engine running)

 

Test 4. Mech pump ONLY ( ie elec pump turned off ) pressure=3.68 psi .This was at the smooth idle of 1200 rpm, but the pressure is not changed by rpm, only the flow. ( the pressure is determined by the spring. The flow by the number of volumes per second)

 

Test 5. Mech pump PLUS electric pump at 1200 rpm idle = 3.72psi.

These are the sort of figures I expected and I am unable to explain why I got less pressure a few days ago. The manual says fuel pressure is ok between 0.75 and 3 psi, so the flight the other day had the fuel pressure (down to 1.2psi ) within specs, even though I didn't like it at all.

 

One caveat: the flow readings were taken without the flow impediment of the float-valve. If this were included, it would reduce the flows , I don't know by how much. Of course you could only measure the electric pump flow as you could not run the engine with the carby bowl removed.

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Looks pretty good to me especially after 20 years. I presume your electric pump is a Facet 2-4 psi pump or similar. These pumps are super reliable & almost never fail. I wouldn't stress about the variable values. There was a proverb from my time as a member of DECUS in the 80s which went a bit like "When all conditions of temperature, pressure, flow, energy etc (there are several more) are perfect, the organism will do as it damn well pleases".

 

I have the same pump but also have gravity feed from the main tank in the fuselage to the engine. My wing tanks are below the engine though so I have a 4-6 psi Facet that pumps from the wings into the main tank via a left/right valve. It is interesting when in flight watching the fuel gauge go up when transferring fuel. The main tank max is 100 litres with 35 litres in each wing tank so I have pretty good range.

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...( the max the engine uses in practice is 18 l/hr, so there is plenty of spare capacity there...

Bruce thanks for a very relevant report.

Just a minor point: for the couple of minutes of TO and climb, my J2.2 guzzles at least 24l/hr.

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Bruce thanks for a very relevant report.

Just a minor point: for the couple of minutes of TO and climb, my J2.2 guzzles at least 24l/hr.

From the Gen 4 manual, FWIW ....

1599171306120.png.3bc21b37029e1f7f70f0078d79f9289b.png

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Although, it does seem to matter what aircraft it is in : see the numbers below for CRUISE FUEL CONSUMPTION for J120, J170, v - book , and that they vary from the above numbers, is this the effect of different props and different airspeeds at the high end ?

 

1599172981226.png.312edf385458bd192a74318f3a867147.png1599173038926.png.f5f174d3d1f0d955af305a8ee38a891f.png1599173079613.png.cac742b7df7649cb28906cc2cc54bf83.png

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Interesting variations... I have never seen more than 17 l/hr actual consumption but this would include some fast cruising. I flight plan on 15 l/hour and generally do better than this with an actual use 13 to 14 l/hour. This is except when I am worried about the fuel remaining and then the consumption can be higher, and this is when you see 17 l/hour. So my experience is in line with the first table but I can see the sense of the "full power" figure of 28-30.

 

It is better to be a pessimist for fuel consumption I reckon. There was a story from a guy who had an embarasing landing in his Jabiru from it using more fuel than normal just when he was pushing the boundary a bit. My theory is that when faced with a nasty headwind, you fly at higher rpm and of course you take longer to get there.

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My Jabiru has a Navman fuel flow gauge which shows a higher flow rate for about 10 seconds if I turn on the facet pump. I think the facet puts pulses through the fuel which tricks the flow gauge to indicate a higher rate.

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