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Thread: 912iS fuel pressure read out to G3X and MIN MAX RPMS

  1. #1
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    Default 912iS fuel pressure read out to G3X and MIN MAX RPMS

    As I've been getting ready for first flight I've been going over all my engine data and just recently did my first run up in summer temperatures. I noticed that my fuel pressure reading was a bit low (39.1psi) vs the minimum in the manual being 40.5psi. With both pumps on it was 40.1 psi. After some reading I learned that the 912is uses manifold pressure to determine the correct pressures and so I made the adjustment in the G3x settings to use manifold pressure in the calculation. In doing so my fuel pressure then only showed a max of 32.7 psi. Previous run-ups during inspection in March show fuel pressures from 40.8-42.6 which are acceptable (without the manifold setting). Am I missing something here or should I be running through the fuel system to look for issues? I'm going to have to get an in line pressure gauge to manually test I guess. Or are summer temperatures (26C) going to make a difference?

    Additionally, during run up the throttle allows the engine to go below the minimum (1400 rpm idle). When testing idle it got down to 800 rpms and cut the engine. I didn't want to test max. Is this a situation of adjusting the cable?
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  2. #2
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    Default Re: 912iS fuel pressure read out to G3X and MIN MAX RPMS

    Well, no final solutions here yet - a few more questions and theories. Idle was easy to change. Fuel pressure is still problematic. I get in the green with pumps on before engine start. Once running, it's consistently in the 39 psi range. After some discussions with a few other pilots and mechanics one theory is the header tank is full of foam, causing the pumps to cavitate. Possibly moving the plane with fuel in the header tank partially caused it? Anyway, I'll have to return to the hanger this weekend. Plan is to completely fill the right tank and see if we can see foam going back up the return line. Anyway, that's the current theory.
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  3. #3

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    Default Re: 912iS fuel pressure read out to G3X and MIN MAX RPMS

    I suggest you go on the Rotax owners forum and search 912iS fuel pressure. There are in-depth discussions concerning 912iS fuel pressure and the ins and outs of setting various EFIS displays to accommodate the 912iS fuel pressure system.

  4. #4
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    Default Re: 912iS fuel pressure read out to G3X and MIN MAX RPMS

    Thanks. I spoke directly with the Canadian rep and he's fairly confident that it's all about the location of the sensor. To be accurate to the 912is requirements, it needs to be placed at the end of the fuel rail. He says best thing to do without moving it is run a manual test gauge at the end of the rail and calibrate the sensor. Problem now is hunting down/machining a special fitting to do so.

    I will check out that forum to see what others have done too.
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  5. #5
    Senior Member JoeRuscito's Avatar
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    Default Re: 912iS fuel pressure read out to G3X and MIN MAX RPMS

    Hopefully this isn’t too late but my fuel pressures on my 912is read low from the start. Right about where you are at with the same effect when adjusting for manifold pressure. I’ve got 670 hours on mine now with no issues. I think it comes down to where our fuel pressure sensor is like you said. I don’t think you will have any problem. Feel free to pm me and I’ll give you my cell if you want more info.


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    Default Re: 912iS fuel pressure read out to G3X and MIN MAX RPMS

    Quote Originally Posted by JoeRuscito View Post
    Hopefully this isn’t too late but my fuel pressures on my 912is read low from the start. Right about where you are at with the same effect when adjusting for manifold pressure. I’ve got 670 hours on mine now with no issues. I think it comes down to where our fuel pressure sensor is like you said. I don’t think you will have any problem. Feel free to pm me and I’ll give you my cell if you want more info.


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    Thanks Joe. After some extensive research over in the rotax forums there was some great information about calibrating the sensor (and leave it where it is). It's probably best to verify manually that the below procedure correctly calibrates the sensor, but here's what I did.

    - with sensor setup as normal in the g3x config page I turned on lane A&B on the 912is and then switched on a fuel pump
    - obtain the current sensor value. At this point the engine manifold pressure is the atmospheric pressure and there shouldn't be any calculations required
    - switch the sensor to "use manifold pressure" and then adjust with calibration until the readout matches what you got above.

    I did this a few times. So far it seems to be working. But I would still like to verify manually at the end of the fuel rail.

    One thing to keep in mind is that apparently the G3X calculation will not take into account density altitude. I've read that for every 1000ft you will loose about .25 psi in the fuel pressure readout. As a result I was told to calibrate it based on your home location and normal density alt.
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