How does the manifold pressure sensor work?

nickmatic

nickmatic

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Nickmatic
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
 
Makes sense. Would this prevent the engine from running or just make the built-in manifold pressure gauge not work properly?
 
nickmatic said:
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?
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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.
 
nickmatic said:
Makes sense. Would this prevent the engine from running or just make the built-in manifold pressure gauge not work properly?
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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
 
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
 
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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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