Fuel Pressure Drops into Low Red Range when above 7500 ft

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rickvb92
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Fuel Pressure Drops into Low Red Range when above 7500 ft

Post by rickvb92 »

Hi fellow pilots and mechanics!

Here is the story: it was a sunny and extremely hot day in the Florida Keys (+92 F). While climbing to 7500, the fuel pressure indicator suddenly dropped into the low red range. I turned the electrical fuel pump immediately, and the fuel pressure returned to the green range; after that, I leaned the mixture, turned off the electrical fuel pump, and the fuel pressure stayed in the green range regardless of altitude. This has never happened to me in the past, even while flying at higher altitudes.

I suspect this could be due to the engine-driven fuel pump not adjusting for altitude. I also read in this forum that sometimes the fuel could turn into gas or vapor at high altitudes and cause a similar issue, so Diamond recommends using the electrical fuel pump at high altitudes always?

Has this ever happened to you, if so, how did you fix it? (if there is something to be fixed at all)
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Steve
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Re: Fuel Pressure Drops into Low Red Range when above 7500 ft

Post by Steve »

Ricardo - Welcome to the Forum. To answer your specific question - no, in 23 years this has never happened to my airplane. I live in the Southwest and routinely fly above 7500 feet in hot temperatures. I just concluded a trip from San Antonio to Phoenix and back at 9500 - 10500 feet. My fuel pressure normally runs 28-29 PSI, and I've never seen it below 27 at altitude. I don't routinely use the electric fuel pump, except for takeoff and landing. On the rare occasion I have turned it on at altitude I either see no change in fuel pressure, or a 1 PSI increase.

The engine-driven fuel pump does not compensate for altitude, although the fuel injector does have altitude compensation for the mixture, but this should not affect fuel pressure. I would check the fuel strainers at the gascolater and fuel injector to make sure they are clear.
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Re: Fuel Pressure Drops into Low Red Range when above 7500 ft

Post by Chris »

It's not all that unusual to need to briefly run the fuel pump at higher altitudes in the Lycoming DA40s, though I only remember needing to do it a handful of times over several years of flying them.

From the DA40 POH:
Operation at high altitudes with the electrical fuel pump OFF may cause vapor bubbles, resulting in intermittent low fuel pressure indications, sometimes followed by high fuel flow indications
The POH climb checklist states that it should be "ON at high altitudes". But AFAICT most people don't do that unless they see pressure fluctuations. I don't think there is a "fix" other than turning on the pump to clear out the bubbles.

See this thread as well.
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Re: Fuel Pressure Drops into Low Red Range when above 7500 ft

Post by Paul »

The only time I have ever experienced this has been after a quick turn where the engine is hot prior to starting.
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Re: Fuel Pressure Drops into Low Red Range when above 7500 ft

Post by nickname »

500 hours in the DA40 Lycoming, happened probably a dozen times. It was always brief, as in it just barely exceeded the threshold to set off a CAS message.

Like others have mentioned, it could be a combination of altitude, OAT, pitch angle in the climb, etc. If it stayed in the red for an extended duration without turning the electric pump on, that’s concerning. When it happened to me, it was always a brief exceeding of the bottom end of the range, then return to normal.
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Re: Fuel Pressure Drops into Low Red Range when above 7500 ft

Post by dant »

I got this today at around 7500 feet during a climb. Usually I get it a bit higher. Above maybe 10kish I usually have to have the pump on during cruise, depending on conditions.
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Re: Fuel Pressure Drops into Low Red Range when above 7500 ft

Post by jwx96 »

I saw this several years ago in a rental 2007 XL. The engine ran fine and the fuel flow didn’t change. The pressure came up a little when I turned on the electric pump. I reported it to the FBO and they ultimately traced it to a faulty transducer (by swapping the fuel and oil pressure transducers).
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