Anybody up for a puzzler?

[0TreeLemur]

So, annual done on our M20C.  Went to pick it up.   Odd vacuum behavior noted.

At last year's annual done by MSC, I reported a squawk on the vacuum system running in the bottom of the green arc just barely above 4.5" Hg.  They tried adjusting the regulator, but couldn't get it any higher.

This year, with about 450 h on the vac. pump, I asked the A&P doing the annual to replace it, which he did.  When I arrived at his shop, he was scratching his head because he was unable to get it to reliably run in the green.   Here's what we saw during testing.

He connected my old vacuum pump directly to an a/c electric motor for testing.   With a 3500 rpm a/c motor, he was probably turning it a lot faster than normal.   Closed off and connected to a vacuum gauge the pulled almost the whole atmosphere.   Connecting that pump to the aircraft and adjusting the regulator to 5" Hg, seemed fine.  This suggests that the regulator is working, at least when connected to a very strong vacuum source.

Idling the aircraft, the A&P noted that the new vac. pump on the engine was pulling a vacuum on his finger.  Shut down and re-connected the system.

On runup, at 1700 rpm it only barely registers on the gauge, between 2 and 3" Hg at most.   At 2400 rpm where I normally cruise, it pulled  4.5" as before with the old pump.   With bad hail storms in the area causing me to want to get the aircraft back to my home drome in its covered nest, I agreed to not fly IFR in it until we resolve this. 

When I took off, at 2700 rpm this morning, it was up slightly above 6" Hg!

In cruise at 2400-ish rpm, it fell down to about 4" and varied a bit.   PC system still worked even at that low vacuum.

Whiskey Tango Foxtrot?  Brand new pump.   He checked all hoses and connections.   Removed vacuum gauge and tested it outside the aircraft.  

I suspect a faulty regulator.  If there was a major leak, it seems that the PC system wouldn't work.  Should I try a new regulator?

If you are tempted to suggest that I dump the vacuum system and all that awesome Brittain tech and instead spend 20 AMUs on a glass panel, don't waste your time.  I ain't doin' that. 

[carusoam]

Regulator should be keeping the vac constant...

Sounds like it is misbehaving... or plumbed funny...

How old, How many hours on it?

Standby for somebody with instrument regulator experience...

Best regards,

-a-

[0TreeLemur]

6 minutes ago, carusoam said:

Regulator should be keeping the vac constant...

Sounds like it is misbehaving... or plumbed funny...

How old, How many hours on it?

Standby for somebody with instrument regulator experience...

Best regards,

-a-

The reg is old as dirt as far as I can tell.  Might be orig.  Never mentioned in logs.  It does have a brand-spankin' new garter filter tho.

[cctsurf]

35 minutes ago, 0TreeLemur said:

If you are tempted to suggest that I dump the vacuum system and all that awesome Brittain tech and instead spend 20 AMUs on a glass panel, don't our time.  I ain't doin' that. 

^^^^^^^^^^^

This!

[cbarry]

I’m guessing there’s a small leak in the vacuum gauge hose.  Why?  Because you’re getting a lower than normal reading after trouble shooting everything else and then your getting an intermittently higher than normal reading after turning up the regulator.  I would bet there’s a little dry rot on the gauge line/hose.

[jamesm]

Curious to know How often do the Vacuum Regulator fail in a Mooney? I have heard of Vacuum pump fail and Instruments fail.

I was having problems with my Vacuum system many many years ago. I was temped to shot gun trouble shoot the problem by replacing  vacuum regulator. The ol' replace the easiest component first philosophy or in the electronic realm biggest chip first .  It wasn't until I had talk to the instrument shop and quit trying to treat the pneumatic circuit like electronic circuit  by me kept trying to add a return which was my problem. In my  case I had added the First G5  and still had the vacuum step and DG and Wing lever relying on vacuum.  I needed to cap off the vacuum line.  I have  since replaced all the pneumatic system after the wing lever finally failed.

Edited April 10, 2021 by jamesm

[markgrue]

One suggestion...  If you have the Eastman vacuum lines you might want to replace them.  They can look fine on the outside but the inside is probably falling apart and leaking like a sieve.  

 

Mark

[Guest]

Where is the vacuum gauge plumbed into the system. Sounds like a leak in the system or gauge line.  Do you have access to a Parker 343 kit?

Clarence

 

 

[EricJ]

Sounds like it's not regulating, so either there's a big enough leak that it can't keep up except at high rpm, or the regulator is bad.

 

[1964-M20E]

sounds like it could be a leak somewhere.

[0TreeLemur]

3 hours ago, M20Doc said:

Where is the vacuum gauge plumbed into the system. Sounds like a leak in the system or gauge line.  Do you have access to a Parker 343 kit?

Clarence

 

 

Hi Doc- the gauge suction port is connected to the spare plenum port on the artificial horizon, and the other gauge port is vented to atmosphere through a plastic filter ala the type Brittain used.  The hoses are old.

[0TreeLemur]

2 hours ago, EricJ said:

Sounds like it's not regulating, so either there's a big enough leak that it can't keep up except at high rpm, or the regulator is bad.

 

I've reached a similar point in the fault logic tree.  Hard to believe that it could be a leak that big and have the PC system still operate.   It was quite turbulent when I flew today, and the PC system was very responsive.  So now I'm leaning big leak in hose going to gauge from AH, or bad reg.

Regs are surprisingly expensive it seems, new.

[RLCarter]

It could be an Instrument case leak as well 

[Andy95W]

20 minutes ago, 0TreeLemur said:

Regs are surprisingly expensive it seems, new.

You can have mine, working fine when removed, pay me back the shipping.

[0TreeLemur]

15 minutes ago, RLCarter said:

It could be an Instrument case leak as well 

Ok.  It would have to be the AH.   A plan of attack has emerged based on input above, I'll replace the vacuum line between AH and gauge tomorrow.  If that doesn't do it, I'll then swap regulator using the one that @Andy95W has graciously offered to the cause.   If that doesn't do it, then by deduction my AH must have a case leak.   Nothing else is in that branch of the circuit between the pump and gauge.

[Guest]

7 hours ago, 0TreeLemur said:

Ok.  It would have to be the AH.   A plan of attack has emerged based on input above, I'll replace the vacuum line between AH and gauge tomorrow.  If that doesn't do it, I'll then swap regulator using the one that @Andy95W has graciously offered to the cause.   If that doesn't do it, then by deduction my AH must have a case leak.   Nothing else is in that branch of the circuit between the pump and gauge.

Some vacuum gyros had the from and back ends of the instrument sealed with 3/4” vinyl tape, might check that they’re good.

The 343 test kit has a vacuum gauge with a hose, the end of the hose has a hypodermic needle allowing you to pierce rubber hose to test vacuum.  
 
Also most Cessna vacuum systems connect the vacuum gauge to a gyro and the second port to the line to the central gyro filter to give a more accurate indication.

Clarence

 

[Mcstealth]

Could any particular instrument be leaking?

[0TreeLemur]

42 minutes ago, Mcstealth said:

Could any particular instrument be leaking?

Seems like the AH is most likely if so- the gauge is connected to it, and it is connected directly to the regulator.

Second vaccine shot from yesterday has me feeling crappy today, so my plans to go to the hangar to test thwarted.

[0TreeLemur]

To recap, replaced vacuum pump at annual a couple of weeks ago, and the new vacuum pump failed to get the gauge in the green.  Seemed odd because old vacuum pump ran right at 4.5" in cruise.

Update:  Comparing my vacuum gauge against another showed that one of them was wrong.   One of my sons sent me a quality vacuum gauge.  Comparing against it showed that my gauge was all fouled up.   Luckily @Andy95W sent me a spare regulator an gauge from his aircraft.  His gauge is good.  Thanks Andy!

Replaced the gauge, swapped Andy's newer looking regulator for my old-style (probably, maybe original) regulator and spent hours going over the vacuum system.  Found a major leak.  The hose connecting the regulator to the TC100 was off at the TC, explaining why my system was struggling to make 5".

On the ground, the new vacuum gauge shows:

RPM    Vacuum (in Hg)

1160    3

1360   4

2400  5

I've not paid much attention to vacuum other than at higher rpm.  Is this normal?

Here's another puzzle...    In flight at 2400 rpm, the vacuum gauge vibrates between 4" and 4.5".    Numbers above verified after flight.

 

[Hank]

@0TreeLemur, check your gage at runup RPM. Mine shows 5"+ at 1700.

[0TreeLemur]

2 minutes ago, Hank said:

@0TreeLemur, check your gage at runup RPM. Mine shows 5"+ at 1700.

At runup today it showed in the green at 1700.  Didn't note it exactly but between 4.5 and 5".

[Hank]

Sounds like you fixed it. 

[0TreeLemur]

Yes, it does, but- In cruise the indicator vibrates at high frequency between 4-4.5", which is not in the green arc.  Might just need a new gauge.   My inner CB wanted to use an orphaned one.