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Very intermittent, temporary drop in FP

RV7A Flyer

Well Known Member
Patron
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.
 
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
 
...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.

:confused: CJ
 
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
 
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.
 
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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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...
 
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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
 
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.
 
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.
 
Thanks for the replies.

I will take a look at bleeding the fuel pressure transducer line and see if that is the problem.

Brian
 
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.

Air in the sensor line, barring a leak, won't cause these symptoms. Pressure is pressure regardless of the medium. In fact air in the line will reduce pressure fluctuations at the sensor. There are aftermarket devices that purposefully retain trapped air to do just that - reduce fuel pressure pulsation at the sensor. They don't result in pressure drops, just more even indications of fuel pressure overall.

My FI RV7A would do what you are describing at high altitude - > 12,000 ft and I never found the cause. Running the boost pump would always cause the pressure to return. I chalked it up to the fuel vaporizing in the lines prior to the mechanical fuel pump or in the low pressure side of the pump itself. Perhaps a blast tube on the pump could have cured it but I never got around to trying it.

Instrumenting the mechanical fuel pump inlet with a temperature sensor might be illustrative.
 
Air in the sensor line, barring a leak, won't cause these symptoms. Pressure is pressure regardless of the medium. In fact air in the line will reduce pressure fluctuations at the sensor. There are aftermarket devices that purposefully retain trapped air to do just that - reduce fuel pressure pulsation at the sensor. They don't result in pressure drops, just more even indications of fuel pressure overall.

My FI RV7A would do what you are describing at high altitude - > 12,000 ft and I never found the cause. Running the boost pump would always cause the pressure to return. I chalked it up to the fuel vaporizing in the lines prior to the mechanical fuel pump or in the low pressure side of the pump itself. Perhaps a blast tube on the pump could have cured it but I never got around to trying it.

Instrumenting the mechanical fuel pump inlet with a temperature sensor might be illustrative.

If I ever get really ambitious about it, I might just do that...feed it to the EMS for recording purposes, and then just...wait.

I have to wonder how many planes have similar incidents and never know (or knew) about them prior to digital data and EMS's recording data at 1 Hz or better. It's like how once I started flying glass, the temptation to put RPMs at *precisely* 2500 or whatever becomes a black hole of focus :). Not 2490, not 2510...2500. Have to force myself to just set it and ignore it LOL!
 
Another one.

Since first flight in Jan 2018, I have had my 7 do this pretty clearly three times. Once was hot & slow on downwind after the third T&G. Once at 5 gph 2300 rpm, super lean at 8000ft after about 45 min, and once at 19,000 immediately following hot rapid climb at full gross on warm day.

I am wondering if (really how much) heat is part of these events and tried to get some data, but really needed all the thermocouples to be synced in time. I was reading from a two channel meter. I have now acquired a 4 channel data logger and will instrument:

1. Fuel line as it enters the cabin
2. fuel line 4" before it enters the firewall
3. Fuel line just before it enters the fuel pump
4. Fuel line as it exits the fuel pump.

The first two are pretty easy just contact the aluminum tube and wrap tightly to ensure the tube is being measured. For the last two, I'll slit the firesleeve and insert the thermocouple under the insulation. The fuel pump body seems to conduct back through the fittings so using metal conduction there does not seem to provide correct (or close) readings. I am not dedicated enough to remove all the fittings and modify for perfect fluid contact measurements.

This data should address the suction side of the equation. For temperatures at least, the real test is to get pressure to match. Not going to happen anytime soon. In real life when certificated aircraft do this, it seems there is a note in the handbook - if this then, boost pump-ON.

In the rapid ascent case, the fuel tank/soak temp could be a major factor.
In the cruise case, messing around peak EGT, it could be exhaust pipe heating or from the cabin floor.
In the pattern case, high fuel pump temps, plus exhaust heating.

I am wondering about the single exhaust I have, and the fuel line running parallel to it in the tunnel. Maybe a lower skin temp there would be a good idea too. The air velocity coming from the cowl exit is very low and could prevent convection cooling of that lower skin.

So many possibilities. . . . it will likely be a hot spring day to collect the data. :rolleyes:

Meanwhile, I shall fly and enjoy.:D
 
My 7 IO 360 with the same set up has done the same thing since I started flying in 2015. I insulated all firewall forward fuel lines, took out an elbow going into the red cube, repositioned the red cube, insulated the red cube, pressure tested the tanks. Tested vent lines. It does it maybe 2-3 times a year. Boost pump always brings it back but usually its back before I can get the pump on. I never have to hit the Boost Pump a second time. Its a mystery and not uncommon as my research has found out.
 
RV7A Flyer,

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

Its good to know it isn?t just happening to me. I have the EXACT same issue and symptoms. Usually high altitude, happens during warm and cold conditions. Aux Pump fixes it right away so it isn?t a sensor issue, but eventually does come up on its own if I sweat it out. I have seen it down as low as 14 PSI.

Really would like to understand what is causing it, because it doesn?t seem like it happened in the first few years of Flight. I have about 500 hours on the plane now, and it has been happening very randomly for the last 150 hours or so.

Thx
 
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