Oscillations in Fuel Flow

[hnorber]

I've been experiencing a steadily fluctuating fuel flow (generally +/- .5 to 1.0 gph, or about 5-10%) for a very long time.   The frequency is about every 7 or 10 seconds.  It's pretty regular, although once in a while (maybe every 5th flight) it doesn't happen.   Fuel pressure remains constant, and use of the aux fuel pump has no impact.   Also, neither altitude nor switching tanks seems to have any impact on it (it even occurs during high-power runups on the ground).   Any thoughts on what could be causing this?   Could a diaphram in the fuel servo be sticking?  (BTW - it's definitely NOT a transducer or probe issue, since my EGTs seem to fluctuate consistent with the fuel flow).   Any thoughts would be appreciated!
I'm pasting below a few links to my JPI data from random flights where this can be clearly seen:

https://apps.savvyaviation.com/flights/shared/flight/5164625/f923df3b-6882-473b-981e-155d64281972

https://apps.savvyaviation.com/flights/shared/flight/9155179/a046b712-1326-4f7c-b303-be4bb92369d3

https://apps.savvyaviation.com/flights/shared/flight/9155181/30073e47-3804-4572-8f26-00599b630286

 

Thanks in advance for any responses!

Howard
N7LU

[PT20J]

Does the engine run smoothly? I would think that much variation in fuel flow would create some roughness.

[TheAv8r]

Do you have an EGT sensor? If the EGTs aren't fluctuating much, a 1gph oscillation would cause them to fluctuate, therefore, it's the sensor.

[hnorber]

Engine is running smoothly, however the EGTs are fluctuating as well (so seems that it's not the sensor).  You can see it in the linked data.

 

[mike_elliott]

When was the last time the mechanical pump and the fuel servo/spider were overhauled? I had a fuel pump diaphram issue back in 2000 that caused the FP to drop with DA so a leak/pinhole can cause some weirdness like your describing. The fact that if it is a pressure issue and gets past the Servo causing EGT variations would have me look at those parts also just to be safe.

Additionally, make sure your intake tubes are tight and not leaking any. They like to work loose as the gasgets age and take a set.

[PT20J]

The RSA troubleshooting manual suggests that it might be an air leak in the fuel line somewhere.

15-810_B.pdf

[hnorber]

Thanks.   A mechanic also thought it might be an air leak (maybe at the fuel selector valve) - but I don't understand how that would cause such a regular oscillation

.

 

[PT20J]

1 hour ago, hnorber said:

Thanks.   A mechanic also thought it might be an air leak (maybe at the fuel selector valve) - but I don't understand how that would cause such a regular oscillation

.

 

Bubbles, I think. There is a video on Precision Airmotive’s website. 

[mhrivnak]

Have you looked for an induction leak up front, where it would affect all 4 cylinders? How's the seal on your alternate air door for example, and is its spring tension correct? Do you have the correct air filter, and it's installed correctly?

The fuel servo's metering responds in part to changes in air flow. I don't know if it would be enough to cause this, but it's worth looking for any potential leaks, cracks, loose connections, etc.

[hnorber]

Thanks.  I haven't checked  - but will do so.   I'm still having a little trouble understanding how an induction leak or air in the fuel line could create such a regular oscillation - but I can certainly try to check.