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Some fluid behind glass in manifold pressure gauge

snopercod

Well Known Member
As I was doing my pre-flight runup on Thursday I noticed some brownish fluid gurgling up behind the glass of my mechanical MAP gauge. The level went from zero to 1/4" depending on engine RPM. The fluid looked like gas with some oil in it. This gauge is connected via 1/8" aluminum tubing from the panel to the firewall, and a -4 flexhose from there to the intake manifold of the #3 cylinder of my O-290-D2. I have 160 hours on my plane and have never seen this before. I'm guessing it's just condensed fuel vapors from the manifold, but then the last time I had the intake tubes off there was a lot of black gunky stuff in there (see below). I was told this gunk was "normal", but I'm not so sure. Anybody have any thoughts on this situation?

Ib7rWu.jpg
 
Anybody?

So have I stumped the RV community here, or am I the only one still using a mechanical MAP gauge?
 
Not right

You should not get any fluid behind your sight glass. Replace it. I'm not sure about residue in line. You "may" get some varnish residue in the tubing due to location of MAP pickup but not that much.. Any other pictures? Of sight window?:confused:
 
You're not the only one with black goo in the MP line. My IO-360 does that even after an overhaul. The MP line runs from #3 intake down low on the engine mount and back up 18" or so before it goes through the firewall. There's a removable cap at the low point and I drain it at each oil change. Occasionally, after flying a while at a constant MP setting, there is a slight stumble when the MP is reduced. My theory is that some of the liquid is sucked into #3 when the MP is reduced. Would like to know why some engines do this and some don't.
 
Thanks

There's a removable cap at the low point and I drain it at each oil change.
I think that's what I need to do. My MP line starts at the #3 cylinder, and drops down to a bulkhead fitting on the firewall maybe 16" below. From there it goes back up to the gauge in the panel. A "drip leg" on the engine side of the bulkhead fitting would probably solve the problem.

I suppose I could start the engine, let it idle, then crack the fitting at the gauge which would suck all the "gunk" back into the #3 intake. I'm still going to have to flush the gauge itself, though.
 
I think that's what I need to do. My MP line starts at the #3 cylinder, and drops down to a bulkhead fitting on the firewall maybe 16" below. From there it goes back up to the gauge in the panel. A "drip leg" on the engine side of the bulkhead fitting would probably solve the problem.

I suppose I could start the engine, let it idle, then crack the fitting at the gauge which would suck all the "gunk" back into the #3 intake. I'm still going to have to flush the gauge itself, though.

Certain Pipers actually had that setup---- a spring loaded push valve right underneath the instrument panel by the pilot's knee that you actuated after engine start to drain the manifold line.

I make sure I clean the line from the engine to the firewall at every condition inspection and mount the sensor in the cockpit higher than the cylinder.

Vic
 
Vic, I looked at Walts condition checklist and did not see the manifold pressure hose on it. I have my hose running uphill to the transducer, but does it collect goo on an injected engine?

I could see pulsing into the hose and fuel getting in there and leaving some heavy ends, but with the orifice it would not seem as likely. Any experienced comment?
 
I have an injected engine (RV-10) and the hose connected at cylinder 5 with the restrictor fitting. Yes, it still does get blue avgas and other stuff in it. I take it off, run some Brakleen through it until it is clear, air blow it dry and put it back on.

I wouldn't think carberated or injected would make a difference.

Vic
 
I never knew

I make sure I clean the line from the engine to the firewall at every condition inspection and mount the sensor in the cockpit higher than the cylinder.
This is one of those many things that I never heard about when building my airplane. Thanks for confirming that this is normal. I'll add it to my condition inspection spreadsheet.
 
A standard after start up checklist item on T 6 Harvard's and almost any thing with wings in that era that had an MP gauge was to purge the MP line to let the engine suction the line clean. Sometimes a push button, sometimes a 90 degree turn valve... but almost always some type of valve was easily within reach for the job.
 
I'm learning a lot

A standard after start up checklist item on T 6 Harvard's and almost any thing with wings in that era that had an MP gauge was to purge the MP line to let the engine suction the line clean...
I'm learning a lot, and it's kind of embarrassing. After building and flying for many, many years, I never knew about this. Thanks for helping me out.

Due to the price of these gauges, I plan to just flush it out along with the connecting tubing. Anybody have any recommendations on a solvent to use so I don't melt the case?
 
As I was doing my pre-flight runup on Thursday I noticed some brownish fluid gurgling up behind the glass of my mechanical MAP gauge. The level went from zero to 1/4" depending on engine RPM. The fluid looked like gas with some oil in it. This gauge is connected via 1/8" aluminum tubing from the panel to the firewall, and a -4 flexhose from there to the intake manifold of the #3 cylinder of my O-290-D2.

No flow restricter on the line? Pretty sure I have one of these to keep stuff out of the line. Screw it into the engine and run the line from this fitting. My line runs uphil to the firewall bulkhead fitting. 750 hours and my line clean as a whistle.
 
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no flow restrictor

No flow restricter on the line?
No. I have one in the oil pressure line, but I never thought of putting on on the manifold pressure tap. Thanks for the suggestion. My flexhose runs downhill to the firewall and I intend to put a drip leg on the bulkhead fitting there next time the cowling is off.
 
It has been a long time, but my recollection is that I called UMA and they exchanged my fluid filled unit for the dual diaphragm unit for the difference in price between the two.

I didn't know that. I'm having trouble finding one in 2-1/4", though. It should be UMA P/N 7-200-35.
 
I have an automotive fuel filter in my MP line to help smooth out pulsations, it helps with the goo too.
 
MP line goo

It's amazing the things I learn on this website!

I'll keep an eye on this. My MP line rises up after leaving the cylinder, but it's not a long distance to the two PMAGs that have an MP input - I suspect goo getting in there would not be good. I like the idea of putting a little filter in the line.
 
When that oil and goo finally works its way up into the gage the needle will start "sticking" too. Best to keep it out as Vic suggests. I like the filter suggestion as well but have never tried it.
 
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