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Fuel Pressure Fluctuations ‘67 C


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Prior to doing the annual on my 1967 M20C I had noticed some slight fluctuations on the fuel pressure gauge during climb out and didn’t think much about it until I started getting the same fluctuations in cruise (nothing wild but enough to warrant investigation).

The fuel pressure gauge had been removed for inspection a year earlier (previous owner) which was reinstalled after the completion of a good bench operational check without any defects noted. All the fuel lines were replaced with new around the same time.

I just completed its current annual and replaced the engine driven fuel pump thinking that was the source of the problem.  First flight out of annual it appeared the issue was fixed. Second flight, I noticed fluctuations (seen in the attached video) on climb out and once again they disappeared during cruise. During the fluctuations I turned on the boost pump and the pressure increased to a steady 6. In cruise, 

the gauge holds a steady 6 without the boost pump on as well.

Any pointers would be greatly appreciated as I continue to track down the issue.

-Lance

Edited by Lfreebird
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That looks like it could be an instrument set-up issue...

The fuelP gauge uses an air bubble and a tiny calibrated hole inside the line... this buffers the pressure fluctuations so the needle doesn’t bounce...

Your mechanic can probably read the maintenance instructions on how to restore the air bubble...

The hole in the line has a name... I used to ask @Bob_Belville what it was called, because I never remember this thing on my own...  :)

PP thoughts only, not a mechanic...

Best regards,

-a-

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