Question Fuel pressure fluctuations in flight (fuel pump off)

[Neal]

2023 Carbon Cub FX-3 / Lycoming cc363i (IO-360)

Flew an hour this morning, all was great. I did notice my fuel pressure was cycling between 22 and 29 psi. Seems like a bit of an excessive range. What are others seeing? Any ideas of the cause or is this normal?

 

[Cactus Charlie]

Any ideas of the cause or is this normal?

Probably abnormal and likely an early sign of a failing mechanical fuel pump.

Similar data was posted on the CC forum. Pressure for that aircraft became stable after a fuel pump change.

Fuel pressure variation on FX-3 - CubCrafters Forums

CubCrafters Builder Assist program aircraft

forum.cubcrafters.com

You can confirm whether this is abnormal for your aircraft by comparing to data from a year or more ago.

Here is a plot I created some time last year for my FX-3 -

and some data from June 2024 -

 

[kshunz]

That looks similar to the issue I had, and I posted data on that same CC thread. It started out as small variations, and each flight got worse. Replacing the engine-driven pump fixed it.

 

[Neal]

That looks similar to the issue I had, and I posted data on that same CC thread. It started out as small variations, and each flight got worse. Replacing the engine-driven pump fixed it.

In the event I need to replace the pump (local maint. shop, not me) where did you source the pump and what's the cost? Prefer new.

 

[Cactus Charlie]

Replacing the engine-driven pump fixed it.

Did you ever open the removed pump? I suspected contamination intermittently causing the intake valve to leak.

Given that these fuel pumps are over $500 I'd remove, open, and inspect before replacing. On the other hand "A man's got to know his limitations".

 

[Neal]

@kshunz you mentioned in the CC forum inspecting the fuel filter. The only filter I'm aware of and to inspect during a condition inspection is the gascolator screen. Is there another filter that should be getting inspected I'm not aware of in the fuel system?

If you are not using flysto.net that site is gold. I uploaded my data logger files and I can easily view the charts and toggle between RPM and fuel pressure to see if there is a trend.

Today I flew at 22/22. While most of my flights do not appear to have this issue I am seeing it randomly and at the same RPM. I'm still analyzing flights but just wanted to say that web site is pure gold for analysis.

 

[kshunz]

I bought a Tempest AF-15473 pump from McFarlane for $475. Had to form a "finger" from some sheet aluminum to hold the pushrod up while I got the pump arm installed under it.

 

[Cactus Charlie]

where did you source the pump and what's the cost?

ref - CC363i/CC363i-FP Illustrated Parts Catalog
62B26931 FUEL PUMP, High pressure

62B26931 Lycoming New Fuel Pump | Aircraft Spruce ®

62B26931 Lycoming New Fuel Pump Factory New Lycoming Fuel Pumps.

www.aircraftspruce.com

Other sources may be less expensive but be sure to order the correct high pressure pump not the low pressure pump used on carburetted engines.

 

[Cactus Charlie]

The only filter I'm aware of and to inspect during a condition inspection is the gascolator screen. Is there another filter that should be getting inspected I'm not aware of in the fuel system?

The is an Andair filter immediately before the elecric fuel pump and a screen on the input to the fuel servo.

None of these filters protect against contamination that was introduced during parts manufacture or during assembly of the parts in the aircraft.

 

[Neal]

My last flight on the 19th was also 22/22 - fuel pressure was normal. I'll keep an eye on it. If it continues to do this then I'll take it in to the maintenance shop and see what they say and have them inspect the pump, filters, etc. and go from there before I spend 1K for parts I may not need. 2 year old plane, 150 airframe hours.