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  #1  
Old 04-17-2021, 07:02 PM
Tony Petrone Tony Petrone is offline
 
Join Date: Nov 2019
Location: SagHarbor,New York and Naples Fl
Posts: 13
Default Manifold Pressure Gauge fluctuates

I have a IO 360 with a 3 Blade MT prop.The panel has dual G3Xs. In cruse at a constant altitude the manifold gauge will go from 24 inches to 22 inches to 25 inches.The fuel flow remains constant ,egt and cylinder head temp also remain constant.Both of the G3Xs manifold gauges fluctuate the same.
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  #2  
Old 04-17-2021, 07:04 PM
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wirejock wirejock is offline
 
Join Date: Oct 2005
Location: Estes Park, CO
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Default Restrictor

Check the restrictor fitting?
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  #3  
Old 04-17-2021, 07:22 PM
Tony Petrone Tony Petrone is offline
 
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Default

Ok thank you
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  #4  
Old 04-18-2021, 06:38 AM
BillL BillL is offline
 
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Location: Central IL
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Default

Quote:
Originally Posted by Tony Petrone View Post
I have a IO 360 with a 3 Blade MT prop.The panel has dual G3Xs. In cruse at a constant altitude the manifold gauge will go from 24 inches to 22 inches to 25 inches.The fuel flow remains constant ,egt and cylinder head temp also remain constant.Both of the G3Xs manifold gauges fluctuate the same.
Since you have the G3X, take some data and plot GPS altitude and attitude to ensure you are not in a rising/falling air mass.

If it is a data aliasing issue, where the pulse frequency is too close to the data capture frequency, then the orifice in the sensor hose could be the issue. You could have one at each end of the hose if needed. Are you sensing from #3 per plans?

Lastly, that hose has been known to capture fuel and leave syrup residue affecting the reading, but it usually affects start up MAP until it is warm.
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  #5  
Old 04-18-2021, 06:52 AM
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bjdecker bjdecker is online now
 
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Location: Georgetown, TX
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Default

From the Garmin G3X Installation manual, Rev. AM. Page 24-24

NOTE
A restrictor or snubber fitting (not available from Garmin) may be installed to dampen
manifold pressure fluctuations. One example is the PS-8G fitting from Omega
Engineering.


Apparently the Garmin G3X + Kavlico/Sensata transducers will pick up the "thumps" of the intake valve closing. This seems to happen around 2400 RPM and lower.

Good luck.
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  #6  
Old 04-18-2021, 06:53 AM
lr172 lr172 is online now
 
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That is a pretty big swing. I was getting oscillation of about an inch before installing a restrictor, but never 3" swings. If you cannot track it down with the other ideas mentioned, I would consider a compression/leak down test on the cylinder feed the gauge. An intake valve that is not closing fully could cause this.

If you haven't installed a restrictor, that is probably a good place to start.

Larry
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  #7  
Old 04-18-2021, 07:22 AM
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AlexPeterson AlexPeterson is offline
 
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Quote:
Originally Posted by BillL View Post
SNIP

If it is a data aliasing issue, where the pulse frequency is too close to the data capture frequency, then the orifice in the sensor hose could be the issue.

SNIP
This is exactly what I believe the problem was in my case with identical symptoms. I believe there is a beat frequency between the data sampling rate and the cyclic pulses physically present in the engine. I had the same thing with my G3X installation 4 years ago, same cyclic oscillation of MAP as the OP posted. I split the line into two lines, each with their own restrictor. One of for the MAP sensor with a small hole (don't recall what size and length), and the other less restricted line goes to the ignition systems. I did not want a delay in ignition MAP sensing when going from idle to WOT.
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  #8  
Old 04-18-2021, 08:36 AM
rick9mjn rick9mjn is offline
 
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Location: northern ill
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Default

another part number for the restrictor fitting is, WICKS part #ea22r-0 ,
,,,,it has 1/8 NPT threads.....
/ good day / rick
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  #9  
Old 04-18-2021, 08:54 AM
motodave motodave is offline
 
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Location: Littleton, CO
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I had planned to take the manifold pressure off the #3 cylinder with a restrictor then to both PMags and then finally to the Garmin sensor, any issues with that config?
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