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  #1  
Old 05-20-2022, 04:45 PM
Ed_Wischmeyer's Avatar
Ed_Wischmeyer Ed_Wischmeyer is offline
 
Join Date: May 2005
Location: Savannah, GA
Posts: 1,654
Default Manifold pressure blues

In the five years I've had the RV-9A (third owner, O-320, carbureted), the manifold pressure readout (Garmin G3X Touch) has had an annoying tendency -- it lags the the throttle position. For example, reducing manifold pressure from cruise to 18", pull the throttle back to see 18", increase the friction so that the throttle doesn't creep. A few seconds later, it will read 16" or 16.5" or something.

We thought it was the restrictor at the manifold, or maybe the line between the manifold and the sensor was clogged, or... At the last annual, all of the old rubber engine tubes were replaced, so we did the manifold pressure line, too. When we took off the old line, we discovered there was NO restrictor. We put one in and put in the new line. No change.

So maybe it's the sensor. Stein had one, mercifully, and I swapped it in this morning. No change. (The old sensor has maybe 600 hours on it, apparently works as well as the new one, so if you need one, let me know.)

Another trait when I got the airplane, and I guess I just ignore it these days -- not sure if it's still there or not -- is the the manifold pressure wanders by +/- an inch in cruise. It's pretty obvious in the digital data.

The only other thing I can think of is that there is heavy filtering in the G3X Touch to reduce sensor noise...

Anybody else see this phenomenon? My old Cessna with 180 HP and analog gauge didn't do this...
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  #2  
Old 05-21-2022, 05:36 AM
Scott Hersha Scott Hersha is offline
 
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Location: Cincinnati, OH
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Do you know if there is a restrictor in the fitting feeding the MP line? It is supposed to be there to flatten out fluctuations in the indications.
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RV6/2001 built/sold 2005
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  #3  
Old 05-21-2022, 06:17 AM
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Walt Walt is offline
 
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You might try swapping to another cyl and see how it behaves.
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  #4  
Old 05-21-2022, 07:05 AM
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Bob Martin Bob Martin is offline
 
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Default

Ed,
No expert here.......
But MP is a measurement of intake suction or pressure.
So a fluctuation might be a leak. or intermittent leak.
Intake leaks affect engine performance/show up more at lower power settings.

You may have a cracked hose or fitting or even a loose fitting etc. that is hard to see or find. Could be anywhere in the MP system.
Best of luck in your search.
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  #5  
Old 05-21-2022, 08:03 AM
TS Flightlines TS Flightlines is offline
 
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There is now a restrictor in the #3 cylinder. Bobs was one of the last packages I personally helped with before my repsiratory disaster.
Tom
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  #6  
Old 05-21-2022, 08:03 AM
lr172 lr172 is offline
 
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In my experience, the G3X uses an excessive amount of sensor data "smoothing" in it's software. Many times a developer codes in smoothing algorithms to avoid presenting jerkiness in sensor based readings. IMHO Garmin over did it. I see this on my fuel flow readings, but never really noticed lagging in the MAP, as I don't look at it that much, but I'm sure they did the same.

This was very annoying when doing testing for injector sizing. It took forever for the EGT and GPH readouts to stabilize. I prefer to see the bouncing around, as it tells me something about what is going on. Unfortunately the G3X keeps all that in the background and presents a very stable reading that doesn't truly reflect what is happening at the sensor, at least not in real time.

Larry
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Last edited by lr172 : 05-21-2022 at 08:10 AM.
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  #7  
Old 05-21-2022, 01:12 PM
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Ed_Wischmeyer Ed_Wischmeyer is offline
 
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Quote:
Originally Posted by lr172 View Post
In my experience, the G3X uses an excessive amount of sensor data "smoothing" in it's software.
Larry, I agree that excessive filtering is the likely problem. Can't tell but I think that there may be a slew rate limiter in there as well as an inadequately damped second order filter. Hard to do parameter estimation with noisy data...

The whole rest of the system is new, the restrictor is there, etc. as I stated in the original post.

We shall see.

Garmin, any comments? I'm over $100 in new parts for what appears not to be a hardware problem...

Ed
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  #8  
Old 05-21-2022, 11:29 PM
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Daida Daida is offline
 
Join Date: May 2005
Location: Hannover, Germany
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Excessive system filtering sounds reasonable in the first place, but i think we would hear more about it as a root-cause for issues………..but it is not impossible to be.

Upon power reduction when you manifold blues happens, how does your RPM (i suppose a fixed pitch?) and FF behave?
Is it just the MP that changes or are there more values that change?

I would make a test run on the ground “in the chocks” , to eliminate airflow induced parameters whilst in flight, and write down what happens (or load down the file from the G3X) afterwards.

What MP do you see at full throttle? Is it ambient pressure (or almost) or are there a couple of inches missing as well?

If it is solely the MP i would vote for sensor issues, soft- or/and hardware related.
2” MP change should have impact on RPM (on a fixed pitch prop) and FF.

If more than one value changes it should/must be in the respiratory system of you engine……

Good luck and post results if you have.
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  #9  
Old 05-22-2022, 12:44 AM
Yen Yen is offline
 
Join Date: May 2010
Location: Benaraby Queensland. Australia
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Could you do a ground test using a mechanical gauge. It should be possible to tee into the existing line, or just use a mechanical gauge in place of the Garmin. If your figures are steady rather than changing with time, then the Garmin is the problem.
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  #10  
Old 05-22-2022, 10:47 AM
Finley Atherton Finley Atherton is offline
 
Join Date: Nov 2005
Location: AUSTRALIA
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Quote:
Originally Posted by Ed_Wischmeyer View Post
Another trait when I got the airplane, and I guess I just ignore it these days -- not sure if it's still there or not -- is the the manifold pressure wanders by +/- an inch in cruise. It's pretty obvious in the digital data.
I had this wandering MP issue with an MGL XTreme and solved it by adding more air volume in the MP line to dampen the fluctuations. Initial testing was temporarily installing an extra about 8 ft ?? of 1/4” ID plastic tube in the MP line aft of the firewall which stopped the fluctuations. Permanent solution was installing a pulsation damper from my junk bin in the MP line but anything that increases the air volume of the MP line would work.

Fin 9A.
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