Rapid erratic vacuum gauge bounce

Huckster79

Huckster79

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Huckster79
On my 2nd training flight we noticed this and headed home to get it checked out. Mooney M20F. The vacuum gauge needle was rocking back and forth rapidly- I couldn't post the video but you can see it in the screen shot… Any ideas? I’m going to have my mechanic take a look but I appreciate the community input to maybe head them in the right direction…. It is a wet pump…
6D3A49D8-F8BD-40C8-8711-0ED9405FF00B.jpeg
 
Last edited:
Bell206 said:
Did the needle oscillate at the same rate throughout the flight until shutdown? Any recent maintenance on the system?
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yes once we noticed it it stayed like that- regardless of power setting, No maintenance directly of vacuum system but did just come out of annual- first flight out.
 
I am going to make a (completely uneducated) guess that this is an indication error. Judging by the pic the needle is vibrating very quickly (~1/shutter speed of whatever camera you used) and it just does not seem like any component of the vacuum system could cause that rapid a suction change.
 
Huckster79 said:
but did just come out of annual- first flight out.
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First place I would look is for any loose connects, loose filter, cracked line or fitting, etc. Since it appeared to happen during your flight out of annual my 1st guess is theres a decent leak somewhere.
 
Another guess.

Dry or wet vacuum pump?
If dry check the pump vanes.
 
I'm curious - is that meter driven by voltage from a sensor? Could a loose wire also cause the flucuating needle?
 
IMHO The Dry Pump was one of the biggest fiascos ever foisted on the Aviation Community.

I have replaced hundreds of Dry Pumps.

One ( 1) Wet Pump.

2 of them still going locally.

Cessna 150’s - 1966 & 1967!
 
Huckster79 said:
On my 2nd training flight we noticed this and headed home to get it checked out. Mooney M20F. The vacuum gauge needle was rocking back and forth rapidly- I couldn't post the video but you can see it in the screen shot… Any ideas? I’m going to have my mechanic take a look but I appreciate the community input to maybe head them in the right direction…. It is a wet pump…
View attachment 117502
Click to expand...
I'm going to call this problem "Schrodinger's Vacuum Suction". Is it at the low end of the green arc, the high end of the green arc, or every linear combination in between? No one can say for sure.
Looks a lot like my piper fuel pressure gauge, tbh! Kind of a neat photo you got of it. Sorry I can't be of help in diagnosing it!
 
IIRC- The “F” Executive - used vacuum for the A/P servos and the retractable step.

Isolation and diagnosis can be more involved than most.
 
Magman said:
IIRC- The “F” Executive - used vacuum for the A/P servos and the retractable step.

Isolation and diagnosis can be more involved than most.
Click to expand...

You are correct.
 
GOOD NEWS

looks like it was the gauge itself- we swapped a spare one my IA had and reading was at steady. Going flying in the morning to confirm it stays that way….
 
Unfortunately the gauge is toast and needs replaced. Your fix was the correct solution.
 
Huckster79 said:
On my 2nd training flight we noticed this and headed home to get it checked out. Mooney M20F. The vacuum gauge needle was rocking back and forth rapidly- I couldn't post the video but you can see it in the screen shot… Any ideas? I’m going to have my mechanic take a look but I appreciate the community input to maybe head them in the right direction…. It is a wet pump…
View attachment 117502
Click to expand...

When our Mooney did this, we yanked the entire vacuum system - gauge, pump, electric standby pump, and all the hoses and filters and such, and replaced the AI with a GI 275. Gained several pounds of useful load too.

One of the better "fixes" I've ever done.

Now I'm just waiting for my ELEC TRIM switch to fail again so I can "justify" a new GFC500 autopilot. :rofl:
 
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