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  #1  
Old 01-08-2021, 11:27 PM
charrois charrois is offline
 
Join Date: Jan 2019
Location: Legal, AB
Posts: 14
Default Manifold pressure sensor problem

Hi everyone. I have a strange manifold pressure sensor indication that's just started in the last couple of flights.

After takeoff, I reduce my manifold pressure to 25", which I then maintain as long as possible during climb by adding throttle (if I don't, of course, the pressure drops as I gain altitude). However, lately, I see my manifold pressure *rising* as I climb, even without adding throttle, which makes no sense if things were working as they should. On my latest flight, right after reaching 10000', wide open throttle was showing around 26" initially, though in my experience, it should be closer to around 20".

Over the course of 10 or so minutes, the indicated pressure slowly reduces to what it should be reading.

When I change throttle, the indicated manifold pressure changes quickly, and as expected, so my first thought isn't necessarily blockage in the tube running from the engine to the sensor. I've just seen (so far) that during a climb (and presumably during a descent, though I haven't tested this), the indication is unreliable until levelled off for several minutes. Is there a vent on the sensor to outside air that may be partially blocked? Or equivalently, is there a fixed pressure chamber in the sensor that should be holding a constant pressure but is leaking to ambient?

Obviously, I can just replace the sensor and I expect the problem would go away. But I'm trying to understand what might cause this (and if it's just possibly a partially blocked vent hole, it may be easy enough to just clean out).

Thanks for any suggestions!

Dan
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  #2  
Old 01-09-2021, 07:40 AM
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bjdecker bjdecker is offline
 
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Location: Georgetown, TX
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On the increasing/decreasing MAP indication, I would inspect the plumbing from the primer port, hose/tube, AN fittings, sensor orifice for blockage or other contamination.

Is there a reason you reduce MAP on climb out? Noise abatement perhaps? If not, do yourself a favor and leave the throttle in 'til top of climb, then use the Propeller and Mixture to adjust for desired power & fuel flow.
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2020 RV-14 QB -- Under construction - Tailcone & Empennage Complete.
2018 RV-7 QB -- Built, Flying
2007 RV-7 QB -- Built, Drowned, Resurrected and flying
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1980 Mooney 231

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Last edited by bjdecker : 01-09-2021 at 07:49 AM.
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  #3  
Old 01-09-2021, 07:57 AM
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Bill.Peyton Bill.Peyton is offline
 
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Location: St. Louis, MO
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I could possibly be that the o ring on the connector are sealing the vent hole. If you look inside the connector shell on the sensor, you will see the pressure vent hole. I had a similar issue with the fuel pressure sensor. Remove the o ring and see if that corrects the issue. Normally the sensor is able to "breath" through the wires.
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  #4  
Old 01-09-2021, 09:29 AM
vic syracuse vic syracuse is offline
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Most likely you have a blocked hose to the sensor. They tend to fill with fuel and oil, especially if they are not routed "UP" after leaving the cylinder.
disconnect the hose at the cylinder and to the firewall, and clean it. See if any junk comes out when using a solvent like Brakleen.

Vic
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  #5  
Old 01-09-2021, 11:44 AM
lr172 lr172 is online now
 
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No vent on a map sensor. Blockage with a liquid (you can get thick carbon based liquid from the gas in the intake manifold) is my best guess. That would explain why it eventually equalizes.
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  #6  
Old 01-09-2021, 08:33 PM
charrois charrois is offline
 
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Thanks for the responses, everyone! I'll pull apart the cowl in the next couple of days to see about cleaning out the hose going to the engine and generally looking for crud anywhere, and will let you know how it goes.
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  #7  
Old 01-16-2021, 10:30 PM
charrois charrois is offline
 
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Unfortunately, still no obvious culprits as to the cause of the problem. I removed the MAP sensor, and inspected the hose leading to it and it seemed clear and dry. I tried blowing into the hose (towards the engine) and there didn't seem to be any restrictions at all. The sensor itself seems clean, with no obvious contamination. I'm including a MAP and altitude plot from the EFIS logs during a climb out to 10000'. It started out at 25" and then dropped a bit (I can't recall if I may have brought the throttle back trying to see its reaction), but then climbed to a high of 26.7" by the time I reached 10,000' (and no, I don't have a turbo, so that should be impossible). Obviously, during a climb with no throttle changes, it should steadily decline, not increase.. and that's what it used to do until a couple of flights ago. Then, with no power setting changes, over the course of 8-10 minutes, it settled down to around 20.6", which is where it should have been in the first place.

Another reason I don't suspect a blockage between the engine and sensor is that it does react instantly to throttle changes - for example, when I advanced the throttle for takeoff, it jumped right away from 13" to 25", which it shouldn't have been able to do if the hose was blocked.

The exponential decay on the MAP after reaching altitude still leads me to suspect a blockage or leak somewhere, somehow. But I'd don't know anything about the internals of a MAP sensor to develop a theory on exactly what is causing this.

The MAP sensor I'm using was, if I recall, provided by Advanced Flight Systems for my EFIS 5600. I see no markings on mine at all, other than a Part number (12247561) and Lot number. I'd like to contact the manufacturer for their input and suggestions, but I'll also contact Advanced Flight Systems to see if they have any input.

Though I can just replace the sensor and see if that fixes the problem, I'd like to understand what could be causing it first.
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Last edited by charrois : 01-16-2021 at 11:00 PM.
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  #8  
Old 01-17-2021, 11:45 AM
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bjdecker bjdecker is offline
 
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Delphi MAP Sensor -- used by AFS for a looooooong time.

You could also check the wiring and signal levels (see attached schematic)
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Brian Decker
Retired Firmware Guy and Airplane Builder

2020 RV-14 QB -- Under construction - Tailcone & Empennage Complete.
2018 RV-7 QB -- Built, Flying
2007 RV-7 QB -- Built, Drowned, Resurrected and flying
1998 RV-8 QB -- Started, Sold
1986 Mooney 205SE
1980 Mooney 231

Aided and abetted building a number of other RV's.
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  #9  
Old 01-17-2021, 08:27 PM
charrois charrois is offline
 
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Location: Legal, AB
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Quote:
Originally Posted by bjdecker View Post
Delphi MAP Sensor -- used by AFS for a looooooong time.
Thanks - that gives me something to go off of when looking for a replacement!
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  #10  
Old 01-18-2021, 10:29 AM
lr172 lr172 is online now
 
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Quote:
Originally Posted by charrois View Post
for example, when I advanced the throttle for takeoff, it jumped right away from 13" to 25", which it shouldn't have been able to do if the hose was blocked.
I think that is your clue. WOT at 1000 MSL should result in 29"+ of MAP. I would guess 3500 should result in 26 or 27". I would consider replacing the sender. Are you sure you don't have a snubber at the cylinder port. These can be as small as .004" and will easily block with carbon.

Larry
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Last edited by lr172 : 01-18-2021 at 10:32 AM.
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