Question Fuel pressure fluctuations in flight (fuel pump off)

[Neal]

Found it. Attached.

 

[Cactus Charlie]

Has anyone ever found any significant contamination in the Andair filter?

I have never found any and don't see how it is possible if the gascolator is clean.

 

[Neal]

I was wondering that, didn't want to ask. Risk of messing with the fuel system vs. findings in the filter. Probably not something I'm going to do as anything that can make the engine stop is not something I touch so it will be my local A&P to do.

 

[ve6yeq]

Has anyone ever found any significant contamination in the Andair filter?

I have never found any and don't see how it is possible if the gascolator is clean.

From what I have heard, the gascolator screen catches the vast majority of contamination, and if the gascolator is clean during the 100 hr inspection, you can skip checking the Andair filter. However, as the Andair filter is finer than the gascolator, if you find anything in the gascolator, you should also check the Andair filter.

Personally, I have never found anything in the gascolator other than a couple of small drops of water, which I think are from condensation because I do not always refuel to full after every flight.

 

[Cubonaut875]

Interestingly, right before my ignition coil failed, I had a lot of spurious fuel pressure readings, tested pressure all normal. Then the coil failed, replaced the coil and then all the fuel pressure fluctuations normalized. Maybe a coincidence - just throwing it out there.

 

[Neal]

I have exchanged several emails with Brandon Dildine at

Technicalsupportn (Lycoming)<[email protected]>

He has always been helpful and supportive. I do no know what discretion he may have on warranty claims.

The parts in the fuel pump that could fail are the diaphragm, the inlet valve, and the output valve. I doubt any of these would have failed at such a low time. I would expect contamination intermittently preventing sealing of the input valve.

Perhaps Lycoming will take an interest in this early failure and offer an exchange part. Send your data to Lycoming and ask for support.

Well, I emailed this address when you provided it. Nothing heard to date, sadly.

 

[Neal]

Last night's flight, 22/22 and then trying 23/22 and still getting the back and forth oscillation between 25-29. Drop to 20/20 and it appears stable as is at 24/24.

Any opinions? Anyone else care to see what there's does at 22/22? Is this a safety of flight issue or something I can continue to monitor? Never heard back from Lycoming from my March 23rd email inquiry.

Click image for full size.

 

[Cactus Charlie]

The pressure variation shows a distinct difference with changes in rpm. I would be inclined to see if the variation depended on fuel flow.

You could run a bit rich a 2100 and leaner at 2200 so the fuel flows are identical for the two rpm conditions. A similar test could be run with changes in MP to give identical fuel flow at 2100 and 2200 rpm.

If the test results show the pressure variation depends only on rpm, and not on fuel flow or power, then I would suspect a data sampling artifact. The nature of the mechanical pump means that fuel pressure must vary as the pump cycles. How the GEA 24 sees those pressure variations will depend on the relationship between the pressure sampling time and the actual pressure variation.

 

[Neal]

I did go full rich enroute back to the airport at 24/24 and it was stable. I'll experiment with that in the zone where I'm seeing this issue on the next flight.

 

[Neal]

I figured I'd give chatGPT a try and see what it says. I haven't really used AI for airplane research like this. Interesting answer:

I have a Lycoming IO-360 in my Carbon Cub FX-3. Fuel pressure is fluctuating around 22/22 mag pressure/rpm. Pressure is fluctuating between 25 and 29 psi. What could be the cause and is there cause for concern?

Short answer: a small oscillation like 25–29 psi at a steady power setting on a Lycoming IO-360 is often normal, but it depends on how fast and how consistently it’s fluctuating—and whether anything else (engine smoothness, fuel flow, EGTs) is moving with it.

Let’s break it down in practical terms.

---

What’s likely normal​

On a fuel-injected Lycoming, especially in an experimental like the Carbon Cub FX-3:

• Fuel pressure gauges (particularly electric senders) can hunt or oscillate slightly
• The engine-driven fuel pump pulses a bit by nature
• Some installations (especially with lightweight plumbing) show 2–5 psi variation

A swing between 25–29 psi:

• Is within a typical operating range
• Isn’t automatically a problem if the engine is running smoothly

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Common causes of fluctuation​

1. Sensor or gauge behavior (most common)​

• Electric fuel pressure transducers can:
  • Drift
  • Pick up electrical noise
  • React to vibration
• If the engine is smooth but the number moves → this is your leading suspect

2. Engine-driven fuel pump pulsation​

• Mechanical pumps don’t deliver perfectly steady pressure
• Some installations lack enough damping → visible fluctuation on the gauge

3. Fuel system damping / plumbing​

• Lack of a snubber/restrictor fitting can make fluctuations more visible
• Light experimental aircraft are more prone to this

4. Slight vapor formation (less likely, but important)​

• Especially if:
  • Fuel is warm
  • You’re at lower pressure / higher altitude
• Usually comes with other symptoms (roughness, flow variation)

5. Electric boost pump interaction​

• If boost pump is on:
  • It can “fight” the engine-driven pump slightly
  • Cause pressure oscillation depending on system design

---

When to be concerned​

This becomes worth digging into if you see any of the following:

• Pressure swings more than ~5–7 psi
• Fluctuations are rapid and erratic, not smooth
• You notice:
  • Engine roughness
  • EGT/CHT fluctuations
  • Fuel flow instability
• Pressure drops below minimum spec for the IO-360
• Problem is new or getting worse

---

What I would do (practical next steps)​

1. Correlate with engine behavior
   • Is it running perfectly smooth at 22"/2200?
   • If yes → less urgent
2. Check fuel flow and EGTs
   • If those are steady → likely instrumentation
3. Turn boost pump on/off (in a safe phase of flight)
   • Does the fluctuation change?
   • That tells you a lot about pump interaction
4. Inspect the sender and wiring
   • Common issue in experimentals
   • Loose ground or aging transducer = fluctuating readings
5. Ask yourself: has it always done this?
   • If yes → likely normal for your installation
   • If new → investigate
6. At next maintenance
   • Consider:
     • Replacing the pressure sender
     • Adding a restrictor/snubber if not present

---

Bottom line​

• 25–29 psi fluctuation alone is not inherently dangerous
• If the engine is smooth and fuel flow is stable, this is very likely instrumentation or normal pump pulsation
• Still worth verifying—fuel system issues are not something to ignore—but this doesn’t immediately sound like an emergency

 

[Cactus Charlie]

Andrew at CubCrafters reported on this a few years ago and used a high speed data logger to record actual fuel pressure. I think that post was linked before but here it is again -

Fuel pressure variation on FX-3 - CubCrafters Forums

CubCrafters Builder Assist program aircraft

forum.cubcrafters.com

 

[Neal]

Good info. I'd hate to pay a lot of money for parts and labor and end up with no change. I'll continue to experiment but need to worry less about this. Just hate seeing a gauge fluctuate like this. Didn't have these issues with turbofans