914 Fuel Pressure Sensor

For the 914 you want to measure the fuel pressure above the airbox pressure, NOT absolute fuel pressure.

The airbox pressure varies with boost, and you want a constant value over that.

For this reason, many install a differential fuel pressure sender that measures both airbox pressure and absolute fuel pressure, and then sends the instrument the value that we actually care about.

This is all discussed in the Rotax 914 Installation Manual.

OK, I understand. I believe that's what I have (need to confirm), so what does that imply about what I observed?

You said "Before that, it seemed to be reading just fine: About 4.7 psi during taxiing and about 9+ during takeoff."

If you normally see a fluctuation in fuel pressure from taxi to takeoff, then you do not have a differential fuel pressure sensor. With a differential fuel pressure sender you would see a constant readout. It would not go up and down with throttle input.

I cannot tell you if your fuel pressure sender is faulty or if your actual fuel pressure is incorrect, but I can tell you that the number you are interested in is fuel pressure above airbox pressure, and that pressure reading should be constant.

Possible dangers from incorrect fuel pressure are engine stoppage due to fuel starvation (low pressure) or flooding (high pressure).

OK, interesting.  I guess I don't have the differential sensor then.

As I said, the engine seems to run just fine, so maybe whatever sensor I have is flaky (or wiring I suppose).

Hi Craig

The supply scope of the Rotax 914 has no sender for fuel pressure.  Whoever supplied yours would be the right person to ask.  (Sling?) As others have noted the correct way to measure pressure on the turbo engines (similar to the injected) is to read the fuel pressure differential to air box or boost pressure.  A direct reading has little value as it changes every time you move the throttle.  

Cheers