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  #1  
Old 06-02-2016, 10:27 PM
RV7A Flyer's Avatar
RV7A Flyer RV7A Flyer is offline
 
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Default Very intermittent, temporary drop in FP

The bare facts:

IO-360 stock Lycoming with an Andair boost pump, about 400 hours on the engine
Very, very, very intermittently, I see the fuel pressure drop from a normal 25 or so PSI down to 12-14 PSI, then back up again to normal, over about 2-3 seconds; by intermittent, I mean maybe once every few *months*. This has happened perhaps 5-6 times since the plane first started flying.
I cannot correlate this with boost pump use...it's happened after turning on the pump to switch tanks, then turning it off, BUT it's also happened in straight and level flight, as well as in maneuvering flight.
I cannot correlate this to MAP or RPM, nor to a particular tank (although I'm now collecting data to try to do this).
Vents are rocket-style, coiled lines in the wing root, and appear to be clear (i.e., they're venting just fine, and occasionally belch a little fuel when the tanks are very full and get hot sitting on a ramp, etc.).

I have no working hypothesis to account for this. One which was suggested was some sort of an issue with the bypass valve on the Andair pump, but I don't know how to test this.

Needless to say, the first time this happened, 3 years ago, it got my attention (and the EFIS alarmed); 2nd time, maybe 6 months after that, I was a bit more calm. The last couple of times, over the last 6 months (or thereabouts), I just shrugged and watched it go back to normal, but *obviously* I'd like to find the source of this and fix it.

Open to any and all ideas.
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  #2  
Old 06-03-2016, 02:09 AM
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Captain_John Captain_John is offline
 
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I had the same problem, but pressure only dropped to 18 pounds.

I discovered a fair amount of debris in the Andair filter/screen at 350 hours.

My guess is that the debris was causing the fuel to cavitate and hence the odd pressures with the mechanical pump.

My pressures were fine with the andair pump on.

Hope this helps.

CJ
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  #3  
Old 06-03-2016, 02:11 AM
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Captain_John Captain_John is offline
 
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...btw, the debris was dog hair and insects from the build process.

I thought I cleaned the lines out well enough before startup but it seems thatI didn't.

CJ
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Old 06-03-2016, 05:02 AM
TS Flightlines TS Flightlines is offline
 
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Joe---same kind of process that Gregg has undertaken--is it instrumentation, or real pressure loss? In Gregg's case, he uncovered alot more than anticipated, but has been thorough in the troubleshooting process.
Tom
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  #5  
Old 06-03-2016, 06:07 AM
n816kc n816kc is offline
 
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Default fittings and fuel selector

you may also want to check for any air being pulled into the fuel system. it only takes a very small amount to create anomalies.
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  #6  
Old 06-03-2016, 06:36 AM
BillL BillL is offline
 
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Recently did some fuel system flight readiness testing on a friends 10.

Ground test, low tank levels, boost pump on, 50F ambient, 100LL, servo bypassed, 50gph @ 28psi, and fuel restricted at the hose to the spider with a ball valve. Had a clear piece of tubing to see the post valve flow. It always had some bubbles showing just due to the sudden pressure drop across the ball valve and turbulence generated. The clear section was moved upstream of the valve to verify no suction leaks. It remained clear.

Lesson - even cold fuel at near sea level pressures can cavitate and out gas. This gas will take some (unknown, but not short, more than a second) time to disappear.

So - if -if -it is always following a tank change, it is possible a bubble(s) have been formed in the flow and briefly captured in the diaphragm cavity.

My friend and I have both checked our fuel systems for leaks using air pressure at 30 psi. Blocked at the wing root, and after the mechanical pump. Pressurized with air to 30 psi and shut off the valve. We both found, and fixed, small leaks in our plumbing. You could still do this, but if you do, be sure to purge the fuel out first. Wing root - to servo. This would rule out a suction leak. But not a restricted fuel filter.

Smart (my opinion) to get to the bottom of this - happy hunting.
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Last edited by BillL : 06-03-2016 at 06:41 AM.
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  #7  
Old 06-03-2016, 10:54 PM
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RV7A Flyer RV7A Flyer is offline
 
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Thanks for the ideas...I might try the pressurized line test next time I'm decowled.

FWIW, I've cleaned the Andair filter at each annual, and never found a thing in it after the first cleaning at 10 hours into Phase I (just a tiny fleck of blue paper towel material in that case).

Cavitation could be the cause, but as I said, it's happened *in the absence* of a switch in fuel tanks...just flying along fat, dumb and happy in level cruise when it dropped dooooown, then back uuuuuup over just a few seconds.

Anyway...I'll keep searching for it. I don't think it's an EMS/EFIS glitch, the data is smooth and continuous over that period of time on the plots I've looked at.

Thanks, everyone...and if anyone has any additional ideas to try, let me know! I'll report back if I find the cause.

ETA: Oh, and I think my *biggest* concern is, and perhaps someone can answer this, is this is ANY WAY an indication of a problem with the engine-driven fuel pump? I'm having a hard time thinking of a way for it to have a fault that is so extremely intermittent...

Last edited by RV7A Flyer : 06-03-2016 at 10:58 PM.
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  #8  
Old 11-05-2018, 12:24 PM
jasman45 jasman45 is offline
 
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RV7A Flyer,

Were you ever able to find the source of your fuel pressure fluctuations? I just completed a long cross country where I had similar symptoms. The pressure would drop (and the Dynon would alert me) and then it would slowly come back up. I was LOP and very high 15,500 and my fuel flows were around 4.5 to 5.0 gph. When I turned on the fuel pump, the pressure immediately returned to normal (29 psi with the pump on). When I did not turn on the pump, it would return slowly on it's own. It would drop down to about 14 to 16 psi before it went back to the normal 24 psi over the course of several seconds. It occurred on either tank (though it seemed to happen more on the left tank) and the engine never stumbled or seemed to get rough (but I was at a very low flow rate). I never had any issues down low.

Thanks
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  #9  
Old 11-05-2018, 12:49 PM
alpinelakespilot2000 alpinelakespilot2000 is offline
 
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Quote:
Originally Posted by jasman45 View Post
RV7A Flyer,

Were you ever able to find the source of your fuel pressure fluctuations? I just completed a long cross country where I had similar symptoms. The pressure would drop (and the Dynon would alert me) and then it would slowly come back up. I was LOP and very high 15,500 and my fuel flows were around 4.5 to 5.0 gph. When I turned on the fuel pump, the pressure immediately returned to normal (29 psi with the pump on). When I did not turn on the pump, it would return slowly on it's own. It would drop down to about 14 to 16 psi before it went back to the normal 24 psi over the course of several seconds. It occurred on either tank (though it seemed to happen more on the left tank) and the engine never stumbled or seemed to get rough (but I was at a very low flow rate). I never had any issues down low.

Thanks
You might try bleeding your fuel line to the fp transducer. All the problems above in this thread could be a result of an extra bit of air in the sensor line. This would also explain the engine not noticing the apparent drops. I had a similar situation on my carbed engine early on. Now I have "bleed fuel pressure transducer line" on my annual condition inspection checklist. Hope this helps.
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  #10  
Old 11-05-2018, 01:00 PM
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RV7A Flyer RV7A Flyer is offline
 
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Quote:
Originally Posted by jasman45 View Post
RV7A Flyer,

Were you ever able to find the source of your fuel pressure fluctuations? I just completed a long cross country where I had similar symptoms. The pressure would drop (and the Dynon would alert me) and then it would slowly come back up. I was LOP and very high 15,500 and my fuel flows were around 4.5 to 5.0 gph. When I turned on the fuel pump, the pressure immediately returned to normal (29 psi with the pump on). When I did not turn on the pump, it would return slowly on it's own. It would drop down to about 14 to 16 psi before it went back to the normal 24 psi over the course of several seconds. It occurred on either tank (though it seemed to happen more on the left tank) and the engine never stumbled or seemed to get rough (but I was at a very low flow rate). I never had any issues down low.

Thanks
I've never found it, and it happens *so* rarely it's almost impossible to get a handle on it (once maybe every 6-12 months). Same symptoms as you...never a hitch in the engine, only lasts a few seconds, but it's definitely there if you look at the data.
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