Low Vacuum Reading

[Bob]

I have a 80 M20K with a 3 yr/300hr since new vacuum pump and 15TT on the engine.  Yesterday vacuum was reading low.

Idle/700rpm:  Very low reading, suction gauge reads 2.5

Idle/700rpm with standby vacuum: Read proper, suction gauge reads 5.0

1000rpm, standby vacuum off:  Reading proper, suction 5.0

After my engine install, I did inspect hoses, clamps, etc. All looked good.  Hoses are about 3 years old and replaced when new pump was installed.

Is this a sign of a vacuum pump going bad?  Before I take a look tomorrow, does anyone have any thoughts?

[Loogie]

Have you made sure all the filters for the pump are ok? Did you change them when you put the new pump?  I think there is a main filter w time life expiration, and there is another filter behind he instruments that can be changed.

 

if you checked hoses, make sure filters are ok before anything else.

[kortopates]

You can't expect to see normal vacuum at 700 rpm. Maybe a little sooner than 1000 rpm, but having normal vacuum at 1000 rpm isn't bad at all. I'd be happy with that. 

[Bob]

Part of the problem is, I did not fly my plane for 6 months when it was down for an engine replacement, so I am not in my normal scan & routine.  In the past, I would check vacuum during the run-up, but don't remember if I ever checked it at idle.  But on my last flight, the low vac dummy light cam on while at idle.  This has not happened in the past, low voltage is on, but never low vac. 

The filters were replaced during the last annual, 30 hrs ago.  From a past experience years ago, when the suction line to the main filter collapsed, it simulated a fully blocked off filter and using the back-up vac pump did not help.  Yesterday the back-up pump had full suction.

 

[kortopates]

Just as likely then that your new engine installation is actually able to idle lower than your prior engine. For example, I can't idle at 700 - that is really low, mine is closer to 800 rpm but if I really try I can get it to 750-775.. That would make a difference.

[Loogie]

My vacuum is in the green at 700 rpm on the vacuum gauge and my dummy light stays off even at that low rpm.  For comparison purpose. I have a Rapco dry pump. Something definitely that needs to be looked at, filters are the first step, if hoses and pump are relatively new. If filters check ok with no debris, the regulator that controls the volume of air to instruments would be next thing to look at. 

 

W new hoses and a new pump, it sounds like an obstruction or an air regulator to instruments. 

 

Not a A&P but I did go through this last year.  My problem was with the last air filter, it had debris in it, and it had been changed at the annual, two months later it was blocked...

 

hope it helps!

Edited February 28, 2016 by Loogie

[Bob]

3 hours ago, Loogie said:

My vacuum is in the green at 700 rpm on the vacuum gauge and my dummy light stays off even at that low rpm.  For comparison purpose. I have a Rapco dry pump. Something definitely that needs to be looked at, filters are the first step, if hoses and pump are relatively new. If filters check ok with no debris, the regulator that controls the volume of air to instruments would be next thing to look at. 

 

W new hoses and a new pump, it sounds like an obstruction or an air regulator to instruments. 

 

Not a A&P but I did go through this last year.  My problem was with the last air filter, it had debris in it, and it had been changed at the annual, two months later it was blocked...

 

hope it helps!

Thanks everyone for the ideas!  New engine rpm is 720 and old engine was 700, so I am ruling out setup differences.  

Loogie, Good to know you are in the green at idle.  This confirms that something is not proper.  I did not think about the regulator before your post.  I do still have my old pump if needed for testing or to use if needed while parts are ordered.

i just had a thought, I wonder if vac lines were plugged or sealed off properly while it sat for the engine.  Bugs, spiders or other contamination.  Everyone has helped here, Thanks!  I will report back on the cause in a couple days.

 

[cnoe]

Bob, this was the topic of a very recent discussion here on MS under the title "Suction at Idle". Search for that and read for more input.

Lower vacuum at idle seems to be the norm from what I've read and experienced. The backup pump is typically "electric" and would be unaffected by idle speed, and if you're checking it while the engine is running you're not seeing its (the backup's) actual performance by itself. My unit's supplement says to check it with the engine off to confirm proper operation.

Also, as you recently changed the filter I would expect to (possibly) see a slightly lower vacuum as a really dirty filter may cause the vacuum to be higher, particularly if both the instrument filter and garter (regulator) filter were replaced.

It doesn't sound like you have a significant (if any) problem to me. But then as many have said before me... "I'm just a lowly PP, not an A&P".

 

[Bob]

Cnoe- Thanks I missed that post last month. 

http://mooneyspace.com/topic/17243-suction-at-idle/#comment-247604

[BrianW]

I downloaded the Mooney M20J manual and it indicates for satisfactory operation of the Attitude Indicator and Directional Gyro that the satisfactory vacuum operating range is 4.00 inHg to 5.70 inHg.

However the HI/LO VAC malfunction indicator light on the annunciator illuminates when outside the range of 4.25 to 5.50 inHg.  The light blinks-on when below 4.25 inHg and is solid-on when above 5.50 inHg.  The manual says that when the light flashes or is on solid, the gyros (for AI and DG) should not be considered reliable.

My M20J is in for its annual and some work was done on my instruments.  My mechanic says the annunciator HI/LO VAC light wont turn off and recommends an actual suction gauge for improved reliability and better diagnostics. I don't mind the better information provided by an actual gauge. I still like the idea of the dummy light flashing when there is a malfunction to help get my attention and alert me.

I'm just writing this info down as a response to provide empirical numbers and make it easier for myself and others to figure out what is normal in the future. 

 

Edited June 4, 2019 by BrianW

[kortopates]

I downloaded the Mooney M20J manual and it indicates for satisfactory operation of the Attitude Indicator and Directional Gyro that the satisfactory vacuum operating range is 4.00 inHg to 5.70 inHg.
However the HI/LO VAC malfunction indicator light on the annunciator illuminates when outside the range of 4.25 to 5.50 inHg.  The light blinks-on when below 4.25 inHg and is solid-on when above 5.50 inHg.  The manual says that when the light flashes or is on solid, the gyros (for AI and DG) should not be considered reliable.
My M20J is in for its annual and some work was done on the VORs.  My mechanic says the annunciator HI/LO VAC light wont turn off and recommends an actual suction gauge for improved reliability and better diagnostics. I don't mind the better information provided by an actual gauge. I still like the idea of the dummy light flashing when there is a malfunction to help get my attention and alert me.
I'm just writing this info down as a response to provide empirical numbers and make it easier for myself and others to figure out what is normal in the future. 

Hate to say it but your mechanic sounds like he is more eager to modify your aircraft rather than maintain it to it's type certificate. It doesn't sound like he has trouble shooted the problem. Absolutely, you want to keep your annunciator working properly. The suction gauge, which is actually standard for example on my 252, is a great supplement to the annunciator but not a replacement for it. You'll likely never notice the gauge indicating an issue while instrument flying but you are much more likely to notice the red light in the annunciator.
Anyway I know the company that makes the Annunciator is still in business and servicing them - at very reasonable cost. But issue could be wiring or from my recollection an issue with a vacuum sensor mounted on the back of the attitude indicator. Obviously some troubleshooting is required to track it down but it should be quite fixable. But a little suction gauge is a good addition. But alone would be a downgrade IMO.


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[cbarry]

You may have a little “dry rot” with the neoprene style hoses that come off of the instrument filter and run to the suction gauge etc. that could cause a low gauge reading as well.

[carusoam]

For a pic of a dry rot hose... earlier this week...

 

For sharing a similar experience with the vac warning lights... and no vac gauge, also earlier this week...

 

For crummy business practices in aviation... see any of my old Narco posts... I sent a radio in for broken plastic gears... they offered to OH the thing at some unaffordable price.... they had the Gaul to go out of business before I could get my radio back... 

Spend more time to focus on moving forwards...

You don’t have enough time to spend on things that have already failed in the past...

Best regards,

-a-