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Fluctuating fuel pressure gauge


RobertE

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Before asking my A&P I figure I ought to see if anyone else has experienced my symptoms and knows root cause.

My fuel pressure gauge sometimes flutters above the red line.  It never does this when at full or cruise power, only at reduced power.  If I add throttle or richen the fuel mixture at low power - in other words, do anything that boosts fuel consumption- it returns to a normal, stable setting below the red line.  Two more clues.  When I turn on the master without the engine running the gauge is pegged above the red line.  Also, when I'm witnessing the flutter the right tank fuel level gauge begins to wander. 

How about that collection of symptoms!   Anyone got any ideas?  Mine is a faulty pressure sensor but I can't explain the wandering fuel level gauge that is coincident with the fluttering fuel pressure.  (By flutter I mean the needle rapidly dances in the region above the red line, rather than being steady).

Thanks.

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9 minutes ago, N201MKTurbo said:

Remove the hose from the sensor or gauge and the other end from the servo. Blow the line out with compressed air and reassemble. 

The sensor or gauge relies on the air in the line as a compliance to smooth out the pressure pulses from the mechanical fuel pump.  

Will do.  Thanks.

 

 

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On your J the sensor is essentially just a variable resistor with a spring. As the pressure changes the contact point changes and the reading on the gauge changes. Since the fuel pressure sits at about the same place most of the time the contact at that point starts to go bad. Then small changes can make it jump around. I just replaced the sensor on my 1981 J for the same reason. The gauge is now steady. 

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  • 10 months later...

I read the mooney space comments about my fuel pressure gauge hitting 80 psi after landing and I did unhook the lines to blow liquid out of the line. Where does the fuel come from  or how can I repair it permanently I have a 65 Super 21  fuel injected 200hp    Thanks    DaveB

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Dave, stay with me for a few seconds....

Fuel pressure in old mechanical gauges...?

Is a direct line of fuel coming from the place you want to measure pressure at...

Are you thinking that the line isn’t supposed to have fuel in it?

MP has vacuum from the closest cylinder...

OilP has oil in it...

RPM has a twisting cable attached to it... that is actually spinning geared of the engine....

To eliminate these archaic devices...

Look up JPI, EI, and insight...

Be prepared to spend, to avoid learning how this archaic stuff works... :)

The newer technologies, remote mount sensors under the cowl and transmit data through a wire into the cockpit...

In 1965 these old gauges were state of the art... the art has changed over the last half century....

PP thoughts only, not a mechanic.

Best regards,

-a-

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23 hours ago, DaveB said:

I read the mooney space comments about my fuel pressure gauge hitting 80 psi after landing and I did unhook the lines to blow liquid out of the line. Where does the fuel come from  or how can I repair it permanently I have a 65 Super 21  fuel injected 200hp    Thanks    DaveB

The fuel pressure pick up is taken from the finger screen bore on the fuel injection servo.  Originally the hose/ line was air filled, over time the air is slowly displaced by fuel.  The air acts to absorb/ dampen the pulses of the diaphragm fuel.

As Rich mentioned above, if you open the line at the gauge and servo and blow it clear and reconnect it you should restore smooth operation of the gauge.

Clarence

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2 hours ago, M20Doc said:

The fuel pressure pick up is taken from the finger screen bore on the fuel injection servo.  Originally the hose/ line was air filled, over time the air is slowly displaced by fuel.  The air acts to absorb/ dampen the pulses of the diaphragm fuel.

As Rich mentioned above, if you open the line at the gauge and servo and blow it clear and reconnect it you should restore smooth operation of the gauge.

Clarence

Neat Doc! Not may have ever explained it that well. Pressure is pressure no matter what is in the line. 

Thanks!

-Matt

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  • 7 months later...

Hi M20Doc,

I am a french pilot flying a Mooney for 20 years now. My friend and I are co-owners of a M20E and we had large fluctuations of the fuel pressure needle. 

we tried what you suggested and it is successful. Now, it seems to be solved after 2 flights.

Thank you very much.

Edited by Fabrice
Adding info
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