Vacuum Gauge

[isaacpr7]

Since my last MX had me remove the entire PC system I have noticed the vacuum gauge reading on the low side (between the bottom of the gauge and the bottom of the green arc). I do not know if this is due to removing the PC but that is when I first noticed it. I do not get any HI or LO light indications and my vacuum instruments work well as I do not have to constantly set my heading indicator to the mag compass. Does anyone have an idea of what might be the problem? The only thing I could think of is a faulty gauge since everything else seems to be working fine.

[N33GG]

It could be the gauge, but that would be quite a coincidence with all the recent changes that could affect the vacuum system. I would get it checked out.

Good luck!

[Marauder]

Isaac -- as you know, the PC system uses the engine vacuum pump. You may have a small leak from one of the tubes or connections. Possibly when the mechanic was putting all of the remaining vacuum tubes back together again.

[orionflt]

removal of the PC system should have reduced the load on your vacuum system and if anything increased your reading depending on where the relief valve is set and the load the old PC system put on the system. if you have a leak in the system after the removal of the PC system it will give you a low indication and make your vacuum pump work harder to maintain proper vacuum pressure. I would get it checked out soon, also if you have an opening in the system you are drawing unfiltered air into the system and have the possibility of contaminating the system.  

[Bob_Belville]

If you have a retracting step you might want to check the servo. Several MSers have commented on failures recently. The rubber on our '60s era planes is rotting. Repairs/exchanges come from Brittain Industries, Tulsa OK.

 

http://mooneyspace.com/gallery/image/33683-step-retraction-servo/

[JaredDavis]

On my plane the vacuum gauge on the panel was way off.  I'd check the vacuum gauge against another gauge or two as one of the first steps.

[isaacpr7]

Isaac -- as you know, the PC system uses the engine vacuum pump. You may have a small leak from one of the tubes or connections. Possibly when the mechanic was putting all of the remaining vacuum tubes back together again.

Chis,

 

The thing that does not add up is that the turn coordinator does not drift during the flight and I do not get a low or high vacuum light. This tells me that the vacuum should be okay; however, the needle jumps up and down rapidly on the low side.

[isaacpr7]

If you have a retracting step you might want to check the servo. Several MSers have commented on failures recently. The rubber on our '60s era planes is rotting. Repairs/exchanges come from Brittain Industries, Tulsa OK.

 

http://mooneyspace.com/gallery/image/33683-step-retraction-servo/

Bob, no retracting step on my 68C.

[isaacpr7]

On my plane the vacuum gauge on the panel was way off.  I'd check the vacuum gauge against another gauge or two as one of the first steps.

Jared,

 

That was my first though and I compared it mainly to the heading indicator and it was rock solid. How did you end up resolving the issue?

[jamesm]

What type of vacuum gauge do you have ? I bought an UMA 1" and out of the box it was low ,

So I had an instrument shop calibrate  the gauge for me, they  this was common problem UMA 1" gauge.

This doesn't say much  PMA  part(s) process.

James

'67M20C

[isaacpr7]

What type of vacuum gauge do you have ? I bought an UMA 1" and out of the box it was low ,

So I had an instrument shop calibrate  the gauge for me, they  this was common problem UMA 1" gauge.

This doesn't say much  PMA  part(s) process.

James

'67M20C

James,

 

I am not sure about the type of gauge I have but you can see what it looks like from the picture attached.

[mooniac15u]

Have you checked the connection at the back of the guage?  If the MX was moving tubing around the connection may have come loose or the tubing may have cracked if it is old.

[jamesm]

Looks to me it's probably an UMA  1" panel hole diameter  from your photo. they are  usually pretty easy to take out. I happen to have 2.25" diameter  vacuum gauge  that  I  used to compare the two  vacuum gauge readings so I had a good idea that the 1" diameter  gauge was low.

James

'67M20C

[JaredDavis]

Here is what I did (with an IA over the shoulder)

 

  Take a hand vacuum pump with a built in gauge and ensure it is accurate. I checked mine against two shop calibrated gauges and it was spot on.

 

  Connect the hand pump to the back of the gauge in the panel (make sure you note which is the vacuum and which is the vent)

 

  Slowly pump a vacuum and watch the gauges. The one in the panel read 3" at 5" on mine.

[N201MKTurbo]

FWIW

nothing you do to an aircraft vacuum system will make it work harder. It is a positive displacement pump. The flow through it only depends on RPM. The stress on the pump is only determined by the suction level. The air either flows through the instruments's turbines or the vacuum regulator. The regulator is not really a regulator but an adjustable leak to the atmosphere.

You could readjust the regulator to bring the vacuum back into spec, but it sounds like you may have a leak, which will allow unfiltered air into the pump possibly shortening its life.

[isaacpr7]

Here is what I did (with an IA over the shoulder)

 

  Take a hand vacuum pump with a built in gauge and ensure it is accurate. I checked mine against two shop calibrated gauges and it was spot on.

 

  Connect the hand pump to the back of the gauge in the panel (make sure you note which is the vacuum and which is the vent)

 

  Slowly pump a vacuum and watch the gauges. The one in the panel read 3" at 5" on mine.

 

Thanks Jared. This sounds like something I could easily do to troubleshoot the system. I will let you guys know the result when I complete the test.

[isaacpr7]

Here is what I did (with an IA over the shoulder)

 

  Take a hand vacuum pump with a built in gauge and ensure it is accurate. I checked mine against two shop calibrated gauges and it was spot on.

 

  Connect the hand pump to the back of the gauge in the panel (make sure you note which is the vacuum and which is the vent)

 

  Slowly pump a vacuum and watch the gauges. The one in the panel read 3" at 5" on mine.

Jared,

 

I tried what you suggested and you were right. The gauge on my instrument panel was way off. I pumped the hand pump to 5" and the gauge on the instrument panel read 6.6".  The green arc on my gauge is between 4.5" and 5".

[JaredDavis]

Please get an IA/AP involved to fix it.

 

Please remember people on this forum want to help.  However it is your butt in the plane and your responsibility to ensure the plane is airworthy.

 

On my plane, sometimes a simple fix is all that is needed. Sometimes it is the start of a long journey.

 

Regards,

 

Jared

[isaacpr7]

Please get an IA/AP involved to fix it.

 

Please remember people on this forum want to help.  However it is your butt in the plane and your responsibility to ensure the plane is airworthy.

 

On my plane, sometimes a simple fix is all that is needed. Sometimes it is the start of a long journey.

 

Regards,

 

Jared

No worries Jared. I apreciate your concern for me but when it comes to stuff like that I am very OCD about it. I want to make sure everything is by the book as written in black and white      The purpose of my queries here in mooneyspace is so that I can educate myself for two reasons: I want to know everything there is to know about the systems on my aircraft. I believe this will make me a better pilot and understanding the systems will take some distractions out of the way in the event of an emergency. The other reason is that I have had a couple of bad experiences with mechanics and I would like a better understanding as to what it is that needs to be happening when they work on my aircraft.  As soon as my MX gets back I will work with him side by side to get all the kinks out and ensure that everything is properly documented 

[BTR]

Since the subject is vacuum gauges, is it common for some planes not to have a true ( round) gauge? My '75 F only has the warning light for low vacuum. It seems that I should install a true gauge so that I would be able to note trends and maybe see an upcoming vacuum pump failure.

[Marauder]

My 75F had both the low vacuum light which was connected to either the AI or DG and a 2.5 inch round gauge. I think it was added at the factory but was optional. In my case, the little silver vacuum sensor that triggered the annunciation panel broke and a replacement was not available. I added a Precise Flight low vacuum indicator and replaced the 2.5" gauge with a 1" version. Unfortunately both of my vacuum pump failures in 23 years of ownership were fairly sudden. One squealed for a bit before it died. No real trend to see.