Vacuum System Woes
Robert Monsen
I've been experiencing some difficulties with my vacuum system, and
was hoping to again tap the net.experts on the subject. The problem is
low vacuum at lower RPMs (ie, < 2000 rpm). I have a Cessna 172, 1968 vintage.
My mechanic suggests that this is usually caused by an incipient
failure of the vacuum pump. However, my reading suggests that dry vacuum
pumps don't fail this way, but rather usually fail without warning.
This same mechanic has been gradually turning up the pressure on the relief
valve for months now. It doesn't fail, and my vacuum instruments are mushy,
and I don't want to replace a pump that is not really bad (it was replaced
at overhaul, about 350 tach hours ago, 7 years ago).
My guess is that it is the vacuum relief valve. In a 172, the valve's
job is to keep pressure constant over variations in RPM. It does this by
dumping vacuum when it gets above a certain value. I believe that its stuck
open, and always dumping vacuum, so that at low RPM, too little vacuum is
driving the gyros.
Who do you think is right? Are there tests I can perform to determine
whether the valve is really stuck? Valves are about 1/2 the cost of pumps,
so I can't just buy a new one to try it out...
Thanks,
Bob Monsen
Orion Network Systems
Berkeley, California
Chris Schuermann
> Folks:
> I've been experiencing some difficulties with my vacuum system, and
> was hoping to again tap the net.experts on the subject. The problem is
> low vacuum at lower RPMs (ie, < 2000 rpm). I have a Cessna 172, 1968 vintage.
>
> My mechanic suggests that this is usually caused by an incipient
> failure of the vacuum pump. However, my reading suggests that dry vacuum
> pumps don't fail this way, but rather usually fail without warning.
>
wouldn't suspect the pump. (they are quite expensive to replace, but
a rebuild kit has recently become available)
> My guess is that it is the vacuum relief valve. In a 172, the valve's
> job is to keep pressure constant over variations in RPM. It does this by
> dumping vacuum when it gets above a certain value. I believe that its stuck
> open, and always dumping vacuum, so that at low RPM, too little vacuum is
> driving the gyros.
>
Thats it! A Cessna relief valve is VERY simple. A little plastic button
is spring loaded against an aluminum hole. The adjustment simply presses
the button down harder. Vacumm lifts (sucks) the button inward allowing
bypass air to enter the system. These little guys seem to have quite a
problem with leaks. The button get worn and dirty and doesn't do its thing...
Get your local A&P (or yourself) to remove the regulator and inspect the
button. While you are at it, replace the vacume filter. Gyros are getting
quite expensive these days!
Hope this helps,
Chris Schuermann
Charlie Gibbs
> I've been experiencing some difficulties with my vacuum system, and
>was hoping to again tap the net.experts on the subject. The problem is
>low vacuum at lower RPMs (ie, < 2000 rpm). I have a Cessna 172, 1968 vintage.
> My guess is that it is the vacuum relief valve....
Probably a reasonable guess. When my gyros went mushy I didn't
suspect the vacuum pump because I don't have one; my 172 is a 1961
model and still has venturis. But one day while flying I managed
to reach under the panel and cover the intake port of the relief
valve with the palm of my hand, and the vacuum immediately jumped
to well above the normal range. Bingo.
My 172 isn't IFR certified, the valve isn't a structural part,
and it's a pretty simple gadget anyway. I pulled it out and took
it apart just for the hell of it. Everything looked OK aside from
the fact that it was quite dirty, especially at the seat. I cleaned
it up and reassembled it, and haven't had any trouble since.
Charli...@mindlink.bc.ca
"I'm cursed with hair from HELL!" -- Night Court
ed Hutton ext. 6488
>valve for months now. It doesn't fail, and my vacuum instruments are mushy,
>and I don't want to replace a pump that is not really bad (it was replaced
>at overhaul, about 350 tach hours ago, 7 years ago).
>
Just a question, when was the last time you changed the main
vacuum filter? You shouldn't adjust the the relief valve if the filter
is out of date. Sure the relief valve brings the vacuum back, but with a
partially clogged filter the pump is straining.
--
edward...@ColumbiaSC.NCR.COM (Ed Hutton)
..!uunet!ncrlnk!ncrcae!hutton ...!gatech!hubcap!ncrcae!hutton
Aviation or flying? Take your pick. -- Richard Bach
Alan Peterman
> I've been experiencing some difficulties with my vacuum system, and
>was hoping to again tap the net.experts on the subject. The problem is
>low vacuum at lower RPMs (ie, < 2000 rpm). I have a Cessna 172, 1968 vintage.
>
> My mechanic suggests that this is usually caused by an incipient
>failure of the vacuum pump. However, my reading suggests that dry vacuum
>pumps don't fail this way, but rather usually fail without warning.
Pumps sometimes do give some warning by poor performance. This is
usually measured in hours - the fact you have been experiencing it
for a long time suggests it is not the pump.
>This same mechanic has been gradually turning up the pressure on the relief
>valve for months now. It doesn't fail, and my vacuum instruments are mushy,
>and I don't want to replace a pump that is not really bad (it was replaced
>at overhaul, about 350 tach hours ago, 7 years ago).
That's not much time for a vacuum pump (although not unheard of). They
typically last 1000 hours or so..sometimes much more.
> My guess is that it is the vacuum relief valve. In a 172, the valve's
>job is to keep pressure constant over variations in RPM. It does this by
>dumping vacuum when it gets above a certain value. I believe that its stuck
>open, and always dumping vacuum, so that at low RPM, too little vacuum is
>driving the gyros.
Not an impossible diagnosis, but unlikely that you would be able to
maintain steady pressure at higher RPM's without getting excessive
vacuum.
> Who do you think is right? Are there tests I can perform to determine
>whether the valve is really stuck? Valves are about 1/2 the cost of pumps,
>so I can't just buy a new one to try it out...
You can test the valve by removing the vacuum hose and blocking it.
See if you get more vacuum. I'm betting you won't..
My suspect would be a vacuum leak in the hoses, connectors or
instruments. Second thing could even be a clogged air filter.
You DO change the filter every 100-200 hours as recommended, I
hope. Most people don't..
--
Alan L. Peterman (503)-684-1984 hm
a...@qiclab.scn.rain.com
It's odd how as I get older, the days are longer, but the years are shorter!
David Papworth
I've seen two aircraft in which the Airborne 211CC carbon rotor pump
showed somewhat low vaccuum at low RPMs for some months before it completely
failed. I'm sure others will cite cases where the pump failed with
no such warning, and other cases where low vaccuum had nothing to
do with the pump. Your mechanic is right to the extent that the low
vaccuum could in fact be caused by a high time pump. Then again, it could be
the valve, or the filter, or the hoses, or the gyros.
Dave Papworth
Claude Goldsmith
} I've been experiencing some difficulties with my vacuum system, and
}was hoping to again tap the net.experts on the subject. The problem is
}low vacuum at lower RPMs (ie, < 2000 rpm). I have a Cessna 172, 1968 vintage.
}
} My mechanic suggests that this is usually caused by an incipient
}failure of the vacuum pump. However, my reading suggests that dry vacuum
}pumps don't fail this way, but rather usually fail without warning.
}This same mechanic has been gradually turning up the pressure on the relief
}valve for months now. It doesn't fail, and my vacuum instruments are mushy,
}and I don't want to replace a pump that is not really bad (it was replaced
}at overhaul, about 350 tach hours ago, 7 years ago).
}
} My guess is that it is the vacuum relief valve. In a 172, the valve's
}job is to keep pressure constant over variations in RPM. It does this by
}dumping vacuum when it gets above a certain value. I believe that its stuck
}open, and always dumping vacuum, so that at low RPM, too little vacuum is
}driving the gyros.
The output of the vacuum pump is proportional to the
engine rpm. Therefore as the engine rpm decreases, the output of the
vacuum pump will decrease. This is masked from you by the regulator
until the output of the pump falls out of the regulators range. At
that point you will see the vacuum indication drop off as the engine
rpm is decreased.
What isn't clear from your posting is whether your mechanic is adjusting the
regulator to keep the reading on the gauge constant i.e. reading has
been decreasing over time, or, has the mechanic been INCREASING the
vacuum to compensate for slugish instruments.
If it's the latter (increasing to compensate) then you probably need
to get the gyros overhauled. If it's the former (decreasing readings)
then it could be the regulator as you suspect or it could be the
gauge. I recently replaced the vacuum gauge in my Mooney after
discovering that the decreased vacuum was a symptom of the gauge going
bad.
As usual, "your mileage may vary". :-) Good luck.
Claude Goldsmith, CFI-IA <claude%sunfse.es...@eagle.is.lmsc.lockheed.com>
twisted-pair (408) 742-7514 | Opinions expressed are not
on the air - WB6UOO | necessarily those of my
through the air - Mooney N6416U | employer.
Reece R. Pollack
In article <1992May1.1...@ichips.intel.com>, papworth@pdx022 (David Papworth) writes:
|>In article <24...@goofy.Apple.COM> r...@apple.com (Robert Monsen) writes:
|>>Folks:
|>> I've been experiencing some difficulties with my vacuum system, and
|>>was hoping to again tap the net.experts on the subject. The problem is
|>>low vacuum at lower RPMs (ie, < 2000 rpm). I have a Cessna 172, 1968 vintage.
|>
|>showed somewhat low vaccuum at low RPMs for some months before it completely
|>failed. I'm sure others will cite cases where the pump failed with
|>no such warning, and other cases where low vaccuum had nothing to
|>do with the pump. Your mechanic is right to the extent that the low
|>vaccuum could in fact be caused by a high time pump. Then again, it could be
|>the valve, or the filter, or the hoses, or the gyros.
We had this debate in our club for months. With the vacuum regulator
cranked all the way up, the vacuum in out Piper Warrior wouldn't go
above 4.75" at 2000rpm, and the "Low Vac" light came on at idle.
We got our answer at about 8:30pm one night when the AI fell over
while I was on a student night flight. Fortunately, I was flying
with my instructor at the time, and it was clear with a well-defined
horizon. We turned the failure into an opportunity to get some actual
partial-panel experience (how many Private students get actual partial
under the hood?) and finish up my unusual-attitudes training.
I checked with a couple of A&Ps, and they tell me it's not at all
unusual for the vacuum to drop off before the pump fails. I've also
heard that failure of the pump vanes can result in the debris being
sucked into the gyros requiring gyro overhauls in 1 out of 5 cases.
--
Reece R. Pollack
PP-ASEL -- Octopus Flying Club
But I don't *want* to wear a hood anymore...
Robert Charles Monsen
>
>I checked with a couple of A&Ps, and they tell me it's not at all
>unusual for the vacuum to drop off before the pump fails. I've also
>heard that failure of the pump vanes can result in the debris being
>sucked into the gyros requiring gyro overhauls in 1 out of 5 cases.
>
>--
>Reece R. Pollack
>PP-ASEL -- Octopus Flying Club
>But I don't *want* to wear a hood anymore...
With regards to the gyros being destroyed by debris, this won't happen
on cessna 172s anyway, because the vacuum goes the wrong way. The inlet
for air is before the instruments, then the instruments, then the regulator,
then the pump. If the pump blows its little $400 mind, at least it won't
take my gyros with it. Unless I crash, that is...
This is good, since I just had the gyros rebuilt. So did the prior owners,
probably for the same reason, ie, sluggish response, wandering DG, out of
wack AI...
Bob Monsen
Orion Network Systems
Berkeley, Ca.