'77 M20J Fuel Pressure gauge stuck at 23 psi after Engine shut down

[Maxx]

The fuel pressure gauge normally goes back to zero when the engine is shut down .  Now it remains at 23 psi after shut down.

With the engine running, if I turn the fuel boost pump on, it goes up to 24-25 psi.  Off, it goes back to 23 psi.

Can anyone shed some light on this? The engine runs normally thru' all phases of ground/flight ops. 

Thanks!

[N201MKTurbo]

A burr on its sector gear.

[PT20J]

Probably the gauge, but just to be sure, pushing the mixture to full rich after shutdown will relieve any residual pressure in the lines.

[Maxx]

Thanks guys, I will surely check into it and update here.  Much appreciated.

 

[CG Roger]

  On 3/5/2023 at 8:36 PM, N201MKTurbo said:

A burr on its sector gear.

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I have a similar problem. My fuel pressure gauge just got stuck at 8 psi. It now sits there in the following conditions:

- Engine off, Electric Pump on

- Engine on, idle or full power

- Engine off, Electric Pump off

I suspect it is the gauge also. The questions is - what can be done? If it's a burr - can the gauge be serviced? I cannot find a replacement gauge online anywhere.

If I can't service or replace - what are my options?

[PT20J]

  On 3/6/2023 at 1:07 AM, CG Roger said:

I have a similar problem. My fuel pressure gauge just got stuck at 8 psi. It now sits there in the following conditions:

- Engine off, Electric Pump on

- Engine on, idle or full power

- Engine off, Electric Pump off

I suspect it is the gauge also. The questions is - what can be done? If it's a burr - can the gauge be serviced? I cannot find a replacement gauge online anywhere.

If I can't service or replace - what are my options?

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There are instrument shops that repair gauges. I would check with my A&P or avionics shop and see who they use.

Skip

[65MooneyPilot]

My E model has always had block line pressure anytime I run the fuel pump or shut the engine down after flight. It usually takes an hour to hour and a half to go back to zero from 25 psi. I use that pressure to prime the engine instead of running thee fuel pump after a short stop. 

[CG Roger]

  On 3/6/2023 at 5:11 AM, 65MooneyPilot said:

My E model has always had block line pressure anytime I run the fuel pump or shut the engine down after flight. It usually takes an hour to hour and a half to go back to zero from 25 psi. I use that pressure to prime the engine instead of running thee fuel pump after a short stop. 

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If it was just an issue of holing the pressure after shutdown, that wouldn't be as concerning. But the pressure never gets into the green with the electric pump or when the engine is running.

[65MooneyPilot]

I am talking about fuel injected engines only. You may absolutely have a gauge or transmitter problem, or a wiring issue in between. There are companies that overhaul those gauges or sell you a new sender if you determine it is malfunctioning. Check the fuel pump and see if it is providing 8psi, check the filter for contamination. If all those items are satisfactory then troubleshoot the indicator system.

[Maxx]

  On 3/5/2023 at 10:06 PM, PT20J said:

Probably the gauge, but just to be sure, pushing the mixture to full rich after shutdown will relieve any residual pressure in the lines.

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Sure enough, it was just trapped pressure in the line.  Pushing the mixture forward relieved it.  Much appreciate the insight!

[A64Pilot]

  On 4/11/2023 at 5:51 AM, Maxx said:

Sure enough, it was just trapped pressure in the line.  Pushing the mixture forward relieved it.  Much appreciate the insight!

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Residual engine heat will drive mine into the red if I just let it sit, so I now have the habit of relieving the pressure after shutdown, then going to idle cutoff.

I won’t leave the aircraft with mixture in

[carusoam]

  On 4/11/2023 at 5:51 AM, Maxx said:

Sure enough, it was just trapped pressure in the line.  Pushing the mixture forward relieved it.  Much appreciate the insight!

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Nice follow-up Maxx!

Thanks for sharing it…

Best regards,

-a-

[PT20J]

  On 4/11/2023 at 12:08 PM, A64Pilot said:

Residual engine heat will drive mine into the red if I just let it sit, so I now have the habit of relieving the pressure after shutdown, then going to idle cutoff.

I won’t leave the aircraft with mixture in

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On mine, if I do that it will immediately climb back up again. It may vary by servo. The AvStars seem to have very good sealing on the mixture valve in idle cutoff. Someone even reported having it build enough pressure to unseat the mixture shaft enough to disconnect the mixture control from the valve (the valve assembly is held together by pressure from a coil spring around the mixture shaft inside the servo).