Vibration in fuel pressure gauge

[Chris]

My M20E seems to have a a fast vibration of the fuel pressure gauge needle during normal operation.  The needle's staying well within the green arc, but it's definitely shaking up a storm.  What would cause this?  I am wondering if this is something to be concerned about.

 

Thanks.

[carusoam]

Thinking panel vibration may have changed?  Be looking to possibly change vibration adsorbing hardware, or it might need to be tightened?

Are you familiar with these in your plane?

....thinking out loud.

-a-

[tony]

My fuel pressure gauge does the same thing when I'm just using the engine driven pump, smooths out when I turn on the boost pump.

[Joe Larussa]

Did you guys ever figure this out?

[N201MKTurbo]

It seems to me that if there is fuel all the way to the gauge, the gauge will see the full impact of the fluctuations of the fuel pressure. After all the mechanical pump makes pulses of fuel. I believe if you were to disconnect the pressure sense line at the gauge and the servo and blow out all the fuel so the line is full of air and reattach it. The air in the line would form a compliance in the system that would filter out the pulses.

[PilotPierce]

It seems to me that if there is fuel all the way to the gauge, the gauge will see the full impact of the fluctuations of the fuel pressure. After all the mechanical pump makes pulses of fuel. I believe if you were to disconnect the pressure sense line at the gauge and the servo and blow out all the fuel so the line is full of air and reattach it. The air in the line would form a compliance in the system that would filter out the pulses.

has anyone tried this? I also have vibrating fuel pressure needle in my 74f. 

[carusoam]

Welcome aboard Pierce.

Best regards,

-a-

[Joe Larussa]

I called Tempest about this and they said to bleed all the air out of the line. Did not help!

[N201MKTurbo]

has anyone tried this? I also have vibrating fuel pressure needle in my 74f. 

give it a try and report back. No parts involved, nothing to lose.

[Guest]

I called Tempest about this and they said to bleed all the air out of the line. Did not help!

they are wrong.  In a diaphragm pump as installed on most Lycoming engine you need an air cushion to dampen the pulses from the pump.  Open the fuel pressure line at the gauge and at the servo finger screen and gently blow all of the fuel out toward the engine.  Re connect the lines and try it again.

Fuel pumps used in Continental engine use a rotary vane pump and generally have the fuel line filled with fuel.  Many Cessna twins even have a bleeder port to drain the air out.

Clarence

[PilotPierce]

Wanted to update everyone on this. After looking at the fuel line that runs to my pressure gauge I noticed it was old and somewhat corroded, it looked like it had been weeping on the bottom. My a&p and I pulled it off and replaced it and sure enough it was full of fuel, and guess what no more vibration! Anyone who is having this problem should definitely take a look at that fuel line running from the firewall to the gauge as I believe mine was weeping just enough to allow it to fill with fuel and cause the vibration condition. Kind of frightening to think about what would happen if it totally ruptured while in flight!

[carusoam]

Great update Pierce,

I believe being full of fuel is normal for the mechanical Pgauges...

Possibility of leaking is a good driving factor for upgrades to electric sensors on the other side of the firewall.

My old C had the MP line break in half.  Vacuum leaked all over the cabin.

The OilP and fuelP are actual lines that are capable of delivering a small flow to the cabin while registering pressure to the panel mounted gauge.

Best regards,

-a-

[Guest]

10 minutes ago, carusoam said:

Great update Pierce,

I believe being full of fuel is normal for the mechanical Pgauges...

Possibility of leaking is a good driving factor for upgrades to electric sensors on the other side of the firewall.

My old C had the MP line break in half.  Vacuum leaked all over the cabin.

The OilP and fuelP are actual lines that are capable of delivering a small flow to the cabin while registering pressure to the panel mounted gauge.

Best regards,

-a-

Air in the fuel pressure line is required to dampen the pulses from the mechanical engine fuel pump.

Clarence

[carusoam]

Thanks Clarence,

I get it, now...

The air headspace is a requirement for damping the pressure waves.

I missed the forest because of all the trees.

-a-