Fluid in Manifold Pressure Gauge

I have a UMA 7-100-20 Manifold Pressure gauge in my -6, installed circa 1996. Getting ready to start up last night, my co-pilot noticed the gauge appeared to be about a quarter filled with what looked like oil. Rut roh.



I pulled the cowl and nothing seems amiss. The manifold pressure line comes off a primer port to a T fitting, with one line going through the firewall to the gauge and the other line going to the MAP port on a P-Mag, mounted on the right. All of the lines seem clear of fluid.

I've pulled the gauge, and the fluid will drain out of the instrument through the vacuum port. It doesn't really smell or feel like oil. I'm not sure what it smells like. I've put it under the nose of anyone who will smell it; an A&P friend said it's not oil, he thinks it smells like glycerin (?).

Perhaps the instrument has a fluid damping mechanism that's sprung a leak? Other than that... could oil be getting into the P-Mag?

This one has vexed all of the airport pros and bums I've run it past. Anyone have any ideas?

-jon

Nobody?

Wow, I expected to wake up to some insight (VAF spoils you I guess). Perhaps I should rephrase the question...

If YOU had this happen in your plane, what would you do? Drain the fluid and go flying? Pull the P-Mag to inspect? Order a new engine?

Thanks!

-jon

Its not uncommon to see a 'small' amount of oil or even water sometimes in the vaccum line, so I would suspect that is what the fluid is. Check to make sure you have a restrictor fitting somewhere in the line prior to the instrument and Pmag, that will help to reduce it as well as smooth out the pulsations on the line.

I would also be curious as to what your oil consumption is, you may just need a new motor

Walt said:

Its not uncommon to see a 'small' amount of oil or even water sometimes in the vaccum line, so I would suspect that is what the fluid is. Check to make sure you have a restrictor fitting somewhere in the line prior to the instrument and Pmag, that will help to reduce it as well as smooth out the pulsations on the line.

I would also be curious as to what your oil consumption is, you may just need a new motor

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Thanks so much for taking the time to reply, Walt! Sounds like this is somewhat normal and no cause for alarm... although it does seem odd that after ~400hrs with zero fluid, 1/4 of the instrument gets filled up either overnight or during the previous flight. I do have restrictor fittings (2 before the instrument), as well as a loop in the line above the primer port... and my oil consumption is ok!

I thought the fluid was draining out freely, but it doesn't seem to be, after all. I will plug the line while I look for a new gauge and hope it doesn't happen again.

Thanks!

-jon

The only thing I can think of that would cause a sudden 'gush' of fluid like that would be that the fluid had been collecting in the line for some time and then for some reason it all got pushed up into the gauge. The vaccum port is behind the intake valve, if the valve stuck open slightly, it is conceivable that the area behind the valve got slightly pressured and forced the fluid up into the gauge. Just a WAG.

Not for sure on this but I had the same thing happen on my old -4.

I was told that the oil you see is actualy the dampening oil that supposed to be contained internal in the guage but sometimes (in your case) leaks out into the face.


I dont think it came from the engine.

Contact UMA and ask them about it.

Walt said:

Contact UMA and ask them about it.

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UMA got back to me (very quickly) with the following:

In installation instructions for this manifold we have: * Must use restricted type fittings to dampen needle fluctuations in gauge due to induced pressure impulses, and to reduce possibility of fuel flowing into gauge. I think you have this installed properly. So, this must be some mix of fuel, oil and water condensation generated in supplying lines. You should have some drain valve in the line in the lower portion of the line, to drain out this mix from time to time. It isn't a faulty gauge, it must be something else causing this. Since this is a gauge with 1 diaphragm, the housing of the gauge acting like second diaphragm and vacuum is pulling the fluid from the lines inside the gauge. Gauge should be still working after cleaning it out, but dial face and pointer will change the color and paint may be separating from the aluminum. New gauge and drain valve is preferable in this case. Walt is pretty much correct about everything on Vans AirForce Forums.

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Of course I would have preferred an internal fluid leak, but sounds like this isn't a huge deal. I'm a little bit concerned with what happens when/if the same goop makes its way to the sensor on the P-mag, but it does have more of an uphill run to get there. A low-point drain sounds like a good idea, I've never heard of it or seen one before.

There should always be a 'drop' in pitot-static and vacuum lines, using some kind of clear tube allows for easy inspection. A simple 'T' fitting allows for easy draining.

Had same prob....filled female NPT on cylinder side with solder, then drilled with smallest bit I had. No trouble since. In fact, still using same gauge 30 years on....J N95JF

MP gauge fluid

I heard of and saw a water/oil mix in the MP gauge in certified planes over the years. I was told Piper looped the line in an outside loop with a very small drain hole drilled in the line at the bottom of the loop where the fluid would collect. This loop was supposedly located under the panel behind the instrument so it obviously did not leak much fluid. I did loop my MP line FWF and if it does effect the accuracy of the readings it can't be much because I can't tell. I flew with the hole open and also closed with electrical tape during tests. I have 400 hrs on it with no problem.

Dick DeCramer
RV6
RV8 ready for finish kit
Las Cruces, NM

DickDe said:

I heard of and saw a water/oil mix in the MP gauge in certified planes over the years. I was told Piper looped the line in an outside loop with a very small drain hole drilled in the line at the bottom of the loop where the fluid would collect. This loop was supposedly located under the panel behind the instrument so it obviously did not leak much fluid. I did loop my MP line FWF and if it does effect the accuracy of the readings it can't be much because I can't tell. I flew with the hole open and also closed with electrical tape during tests. I have 400 hrs on it with no problem.

Dick DeCramer
RV6
RV8 ready for finish kit
Las Cruces, NM

Click to expand...

The only problem with a drain hole would be it would be a vacuum leak on the associated cylinder during engine operation. If someone wanted to install a vacuum operated valve that would be ok, but a bit of overkill in this situation. A simple drop loop with some clear or opaque line makes it easy to spot and drain during annual inspection.