Lionudakis

Fuel pressure C model

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37 minutes ago, Lionudakis said:

What kind of fuel pressure are you C drivers seeing in flight , mainly around  8-10k. 

which engine?  180 hp carb I see about 4psi in cruise...during climb out above 6k it can be near bottom of the green until leveled out and throttle is back....

 

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Mine is usually near the middle of the green stripe (2-3 psi?), but will occasionally dip to under 1, in the narrow part of the green stripe. Nothing changes when that happens, and I can't tell unless I'm looking at the gage when it happens. 

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2 hours ago, Raptor05121 said:

4psi. I've seen it as low as 2psi, as high as 6.

This is pretty much what I see as well. 

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The C's fuel pressure green band is large.  While mostly around the center band, I've  seen it higher and lower, but never understood exactly why/what conditions change it.

20160822_094419.jpg

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See if this logic helps....

The range on the gauge is physically large...

In real life, a few psi is hardly noticeable.

The gauge needs to be very sensitive.

A change of head pressure, with attitude, will probably cause a noticeable change on the gauge.  Any change of a fuel valve, like the float valve in the carb, probably generates the most change...  high power setting, the float valve opens to allow more fuel into the carb... less restriction of the valve, produces lower pressure, to allow more FF?

PP thoughts only. Not a mechanic...

Best regards,

-a-

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I overhauled my fuel selector, thought that would stop it from fluctuating (sucking air, maybe?).

Nada. Still does it.

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Thanks for input. Since installing my JPi I hadn’t gone very high ( shorty trips), until last year. The fuel pressure drops slowly as I climb, by around 6k ish it’s at zero and my Jpi starts flickering between zero and and X’ing out. As I descend will slowly come back by the same altitude and be back to normal. I just don’t remember it being that low back when I had the old gauge. Electric pump changes nothing. 

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26 minutes ago, Lionudakis said:

... The fuel pressure drops slowly as I climb, by around 6k ish it’s at zero and my Jpi starts flickering between zero and and X’ing out. ...

There's something not right.

I've never seen my fuel pressure go out of the green.

I'm wondering if it's something simple like air in the line?

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+1 On something being amiss.

even if air is stagnant in the line, pressure is still pressure.

Jpi also has an inline orifice to help with getting the right average reading.

Check the Mooney parts manual to see if there was one with the original gauge.

JPI must be an expert by now in electronic MP and FP readings... neither are very stable.

Might be a simple calibration exercise.  Actually getting a zero pressure FP reading would be really BAD.

PP thoughts only, not an instrument guy...

 

Best regards,

-a-

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My FP fluctuates from 2.3 to 6.0. Sometimes it goes above 6. I turn on the electrical pump and off it goes up and then down to 5.2


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1 hour ago, carusoam said:

......even if air is stagnant in the line, pressure is still pressure.

Best left to those who understand how the fuel pressure gauge is plumbed, but a pocket of trapped air would expand with an increase in altitude whereas a liquid would read a (mostly) constant pressure.

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5 hours ago, Lionudakis said:

Thanks for input. Since installing my JPi I hadn’t gone very high ( shorty trips), until last year. The fuel pressure drops slowly as I climb, by around 6k ish it’s at zero and my Jpi starts flickering between zero and and X’ing out. As I descend will slowly come back by the same altitude and be back to normal. I just don’t remember it being that low back when I had the old gauge. Electric pump changes nothing. 

Mine behaves similar. It will sometimes go as low as zero in climb (maybe 1/10 flights, more common in winter maybe), though normally settles around 2psi. When it drops near zero, boost pump brings it back to normal range. No issues in cruise.  This behavior terrified me when I first got my plane 3 years ago.  Changing the line, transducer, and display (factory gauge to JPI EDM900) made zero difference. I've let the reading stay at zero during a couple of climbs to higher altitudes with no passengers and not turned the boost pump, on just to see what would happen.  Fuel flow never drops, so pressure can't actually be going to zero, and I seriously doubt there is any dysfunction in the engine-driven pump.  The intermittent nature of the issue suggests that vapor lock develops in the line to the transducer under certain conditions.  I've heard of folks with similar issues changing fuel pumps with no benefit.  I am not pursuing the issue further unless someone offers new plausible insight.

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I agree with the 'leave it to the experts' idea.  Especially if there is an actual pressure challenge... and not a gauge issue...

 

Continuing the thought process...

If the system was closed, the expanding air bubble could not expand?  This is an open system.  Open to the fuel line end.

Often an air bubble entrained in the system has an additional vibration absorbing effect. Getting the right sized air bubble would probably be in the calibration and set-up instructions.  Normal for the old mechanical type gauges...

 

looking at atmospheric pressure being applied... and the effect of altitude...

MP is measured in inches of mercury. 1atm is 29.92”hg...

Going to 10k’, the atmospheric pressure is about 10” less...

head pressure, weight of fuel pushing down from above can be calculated knowing the density and plumbing line dimensions...

All these pressures get added to what the pump is building, as they apply to the system as far back as the tanks, or any air inlet open to the atmosphere...

 

essentialy the gauge is measuring the pressure difference from in the fuel line to the outside. Where a compressible diaphragm or piezo electric sensor gets mechanical and minutely squished...

The size of the bubble is a tiny amount of head pressure being lost or swapped for fuel...to get 2psi of fuel pressure, I would need 4” of hg... according to Siri.  That would be a lot more inches of 100LL.... 13+g/cc for hg.    <1g/cc for fuel... a 40” tall bubble...

 

Back to JPI for methods of improving such a delicate measurement....

questions one might ask JPI.

  • What is the effect of air in the FP line?
  • If the answer is none allowed... how does one eliminate the air that inevitably will collect over time?
  • Is there a method of mounting the sensor defined by JPI? Vertical or horizontal... a hint that a bubble may make a difference.
  • What is the effect of vapor pressure under really hot conditions....

I was expecting the answer to be no differences for such a technical gauge.  But a few people are reporting that climbing matters...

With that there are two things that stand out...

  • the uphill attitude may change the height of the carb over the fuel pump (and pressure gauge).
  • change in atmospheric pressure as the plane climbs.

Nothing that really stands out as a fix this or change that to get it to work properly...

See if JPI or one of the more user friendly gauge companies has an answer.

Lots of circular reasoning of a PP. No real answers in this post.  :)

The installation instructions might be interesting...?

Best regards,

-a-

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