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Fuel pressure plunge - is this normal?

JackinMichigan

Well Known Member
Fellows,

I currently have about 3 hours in my RV-10. The last time I was up I wanted to check the vertical speed when in a glide with the throttle set to idle.

I was gliding at idle for about a minute, then I pushed the throttle all the way in and re-set my 75% power parameters. I then watched the fuel pressure plunge from 28psi to below 5psi, where it stayed for a good 20-30 seconds before slowly climbing back to normal. The engine didn't sputter or miss a beat, but it briefly had me looking for a field to crash land into.

Is this normal? After I had time to think about it I reasoned that the mechanical fuel pump simply needed time to re-establish fuel flow after being at idle for a while, but I cannot recall ever noticing this on takeoff. I do have an electric boost pump (which I forgot about in my state of near-panic), but I want to make sure there isn't something else going on that I hadn't thought of.
 
What was the fuel pressure reading while you were at idle before you applied power? Also have you looked at all of the engine data? I'd like to see the other engine parameters for the time period. In going from idle to power I've never seen any fuel pressure behavior like you describe. Since your boost pump was off and the engine didn't sputter and I assume appeared to make full power, my initial thoughts are a fuel pressure sensor issue, but there's not enough info for me with my limited experience to really know. Hopefully someone with the answer will chime in shortly.
 
Someday, perhaps, someone will get to the bottom of the mystery of fluctuating fuel pressures. Pressure "drop-outs" have quite commonly been reported on this forum. I've had them myself for 16 years, with never a miss in the engine. A sudden throttle up will always cause the pressure to drop to, perhaps, 15-20 psi. That is not really much to worry about. More interesting are the seemingly random drops to 12-15 or so. On a recent flight, I pulled about 3 g's while rolling left, and noticed a drop to about 15 or so. I could not repeat it again on the same flight, with the same maneuver. OAT was perhaps 30F. I've seen dropouts on long climbs from very hot conditions on the ground, but this is not too surprising, as the fuel in the tanks is still hot, while the ambient pressure is dropping. Switching on the boost pump fixes this.

In this OP's case, it seems improbable that the pressure dropped all the way to 5psi, as I believe that is below the pressure needed for fuel delivery at 75%.
 
I've always wondered if the line going to the fuel pressure sensor needs to be purged of air and if there is air in that line if it would affect pressure reading?
 
I've always wondered if the line going to the fuel pressure sensor needs to be purged of air and if there is air in that line if it would affect pressure reading?

Yes, the air in the line needs to compress when the fuel pressure increases, but I suspect that is measured in milliseconds, not seconds.

Larry
 
I will add to those that have seen the mysterious fuel pressure plunge. I was in level cruise at 9K when my master caution went off. I watched the pressure drop from 24psi down to 12 psi and back up again. Afterwards it continued to fluctuate between 23.3 and 24. This occurred about 5 minutes after I switched tanks. Normally I switch tanks with the boost pump on, this time I did not.
 
The closer your fuel pressure transducer is to your carburetor or spider, the more likely this is. The carburetor is the easiest visual: when the needle unseates the fuel flows in to fill the bowl. Even if you have plenty of fuel flow, the pressure right at the entrance to the bowl is almost 0.
When you go to full throttle, the pump is moving fuel, not building pressure.
Also. High flow through an orraface (fitting) can cause negative pressure depending on the geometry.
My view is that you should only use fuel pressure trends or statick readings...
 
Mine would start to slide as I went up in altitude. IIRC, at around 13,000' it slowly went down to 14 psi in cruise. Made me a bit nervous though the engine never skipped a beat. The electric boost pump would bring the pressure back up, but would go back down after I switched it off. Since my engine was a used motor, I was going to replace the mechanical pump with a new one but sold the plane before I ever got around to it.
 
The closer your fuel pressure transducer is to your carburetor or spider, the more likely this is. The carburetor is the easiest visual: when the needle unseates the fuel flows in to fill the bowl. Even if you have plenty of fuel flow, the pressure right at the entrance to the bowl is almost 0.
When you go to full throttle, the pump is moving fuel, not building pressure.
Also. High flow through an orraface (fitting) can cause negative pressure depending on the geometry.
My view is that you should only use fuel pressure trends or statick readings...

While it is true that the fuel pump design has an inverse relationship between pressure and volume, the variability is relatively small (mostly non-existent, save transient changes, until the pressure falls down the curve until it is below that of the pump's pressure relief valve ratin - at lower flows, the pump produces in excess of the designed pressure and the excess is bled off) until you get closer to the pump's capacity around 40-45 GPH. Simple phsyics tells us that if your pressure at the carb opening was 0, there would be no meaningfull flow through that opening, unless gravity was doing the work and gavity won't help in a closed system.

Larry
 
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