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?

There is an Andair filter immediately before the electric 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.

 

[Cactus Charlie]

have them inspect the pump, filters, etc. and go from there

This is very unlikely to be associated with filter contamination. Contaminated filters give no symptoms until they get contaminated so badly they reduce fuel flow.

The labor cost for someone stripping your fuel pump and rebuilding it would likely be prohibitive. I doubt that an A&P would sign off the work without fitting a complete overhaul kit (if one is available).

A competent owner could open the pump, clear any contamination, and re-assemble. That's what I did with my fuel servo after extreme rich mixture caused an engine stoppage. Problem free for the last approx 250 hours.

 

[Neal]

Okay, I'll consult with them but first, fly and monitor. I'm seeing this occur sporadically and then restabilizing. If I see this behavior consistent I'll see if the maint shop wants me to get a new pump and then bring it over. I thought I was done with "stuff" but I guess not Today's flight was another joy of everything being perfect including a centered ball, and then, this. It's getting hot now too, was around 70F out

 

[Cactus Charlie]

It's getting hot now too, was around 70F out

It's that cool here in the early morning too.

 

[Neal]

Anyone dealt with Lycoming for warranty claims? I'm 2.5 years in but wonder if they'd flex? About 130 tach hours, doesn't look good on Lycoming for parts to fail so soon.

On another note, could this just be some foreign object situation that could clear or is this indicative of a failure?

 

[Cactus Charlie]

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.

 

[kshunz]

I did open up the pump, and it appeared that the outlet valve had failed. It's dirt simple, just a solid elastomeric disk held to a perforated metal disk by a small spring. Yet the contact made was only on one side with a significant gap on the other side. There was no FOD or anything to explain it. I disassembled it and it all looked pristine. When I put it back together, it made contact perfectly.

Maybe not that surprising, since the pump would work just fine for around 20 seconds and then briefly stumble. I can't explain it. I don't know if it's possible to get a replacement valve; I just replaced the pump.

 

[Neal]

I really like on flysto.net how you can add additional params to a chart so I can compare RPM vs Fuel Pressure for example instead of having to switch back and forth.

 

[Neal]

I went back through many of the flights since delivery, not all, and this situation seems random. It occurs, then goes away.

 

[Cactus Charlie]

I did open up the pump, and it appeared that the outlet valve had failed. It's dirt simple, just a solid elastomeric disk held to a perforated metal disk by a small spring. Yet the contact made was only on one side with a significant gap on the other side.

Thanks for the report. Do you have any photos you can share?

 

[kshunz]

This photo shows the two valves, with the outlet valve on the right. The two appear to be identical, just facing opposite directions. Behind the brass-colored seat of the outlet valve is the valve flapper. There's a spring behind it, pushing it against the seat. You can't see it in this photo, but the flapper is only touching the seat on the lower right side, and there's a gap of around 3/32" on the upper left side. I pulled the valve out and looked at it carefully, but it wasn't apparent why the flapper was cocked like that.

There wasn't any FOD between the seat and flapper, but I'm not sure I can rule out something preventing the flapper from sliding properly on the shaft.

 

[ve6yeq]

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?

I just went back through my logs, and it seems like a pattern that I have never seen. I often see +/- 2 psi with some larger +/- 5 psi random events.

The biggest drop in my logs is to about 16 psi for about 20 seconds before it stabilizes upon applying full power at takeoff (e.g., from ~1000 rpm to ~2700 rpm) if I do not have the electric pump on. I have only been using the fuel pump on takeoff when I don't have much runway to put down on if something doesn't feel right, as I have seen overpressure warnings on approach to landing, especially on hot days.

I will try to be more consistent with my "yellow light, blue knobs, red knob" check before I start the takeoff roll. I will do a few last minute go arounds to gather data but I doubt that the electric pump will make much difference due to the load on the flap handle at full power - I add partial power, raise flaps to 1st notch, then add full power.

 

[Neal]

With my boost pump on I'll push into the yellow. I was told it's normal. My last FX-3 did not do this. I'm not concerned about it. I'd rather have the over pressure than under pressure.