How does the manifold pressure sensor work?

Discussion in 'Maintenance Bay' started by nickmatic, Aug 23, 2018.

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  1. nickmatic

    nickmatic Pre-Flight

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    I just removed an Insight G3 engine monitor to send back for repair. Aside from the wire harnesses, I disconnected the manifold pressure sensor tube. My mechanic will be back from vacay in a couple weeks to reinstall and certify everything, but in the meantime I'm wondering can I start the engine with that hose disconnected? I have some avionics testing I want to do on the ground.

    Where does the manifold pressure tubing go and how does the system work? I can't seem to find very complete info online. Realized I know nothing about this part of the avionics!

    Mooney M20K Encore
     
  2. Tom-D

    Tom-D Taxi to Parking

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    I'd plug the tube, it will be a manifold vacuum leak other wise.
     
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  3. nickmatic

    nickmatic Pre-Flight

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    Makes sense. Would this prevent the engine from running or just make the built-in manifold pressure gauge not work properly?
     
  4. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser! PoA Supporter

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    It will prevent your engine from running properly - your air/fuel ratio will be too lean.
     
  5. nickmatic

    nickmatic Pre-Flight

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    Wow, really?

    Anyone know where I can find a diagram of how this works? Where does the other end of the hose actually connect? Right to the intake manifold?
     
  6. Kenny Phillips

    Kenny Phillips En-Route

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    Yeah, it's just a hole, no fancy sender. We used to put them on cars back in the day. And your air/fuel ratio may or may not be lean, as most engines are set up quite rich with the mixture firewalled, but plug it, as the A&P has mentioned.
     
  7. Tom-D

    Tom-D Taxi to Parking

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    Get real, it is a orifice fitting .010" but still should be capped.
     
  8. Tom-D

    Tom-D Taxi to Parking

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    the engine will run, but we don't like leaks no matter how small.

    The gauge won't work without being hooked up
     
  9. nickmatic

    nickmatic Pre-Flight

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    I think my guy can get it capped, but is there danger running the engine if he can't (uncapped)?
     
  10. wilkersk

    wilkersk Pattern Altitude

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    Operation is pretty simple. The MP sensor in the EMU sees the vacuum, measured in in/hg, created by the intake stroke of each piston. The internal mechanism in the EMU converts this to a digital signal for use by the G3. I believe that on most engines the MP tubing is just simply connected to a fitting with a small orifice that is threaded into the intake manifold on one side. Old round guages are just a simple aneroid wafer with a needle movement.

    You should be able to find a small plug that will thread into the "B-nut" compression fitting that was disconnected from the EMU. I think its and AN806-2D
     
    Last edited: Aug 23, 2018
  11. nickmatic

    nickmatic Pre-Flight

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    Thanks, makes sense!
     
  12. Tom-D

    Tom-D Taxi to Parking

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    MAP gauge, is nothing more than a absolute pressure gauge. disconnected it should read atmospheric or barometric pressure.
     
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