Fuel Pressure Relative to Manifold pressure

[JoeRuscito]

Ive been trying to figure out why Im seeing a pressure drop on my fuel pressure sensor (912is) when I start the engine. I see a full 43-44 psi before start and when running consistently ~38 psi (either fuel pump). Im wondering if it could be due to the fuel pressure regulator trying to hold constant pressure relative to the manifold pressure. I.e. when I start the engine manifold pressure drops so the regulator also drops the pressure slightly and my gauge shows the drop. For you 912is guys does that make sense? Its just a hypothesis. What are yall seeing before engine start and while running? Im using the 70 psi UMA fuel pressure sensor right before the fine filter.

Thanks as always!

[DesertFox4]

Could it be as simple as when the engine is off, the injectors are all closed so pressure builds as the pumps push fuel into a closed system with nowhere for the fuel to go until the engine starts. I would imagine that is when you would see the highest pressures. As soon as the engine starts, fuel flows thus reducing indicated pressure and also may fluctuate slightly as injectors take turns opening and closing.
No expert on fuel systems so just a theory.

[JoeRuscito]

The logic totally make sense. However Im below the min fuel pressure for the engine. 46.5 psi is min. Im around 39-39 while running.

[Shadowrider]

Does it change when the rpms come up? Can you verify indication is correct? Any way to see the fuel pressure your 912IS is seeing like accessing the ECU?

[JoeRuscito]

Not sure if there is a way to see what the ECU is seeing. Yes it does come up when the rpm is increased. Gets into the 40s.

[JoeRuscito]

at 5000 rpm I see 44.9 psi.

[efwd]

I agree Joe. At the time I was looking for anything because I was finding fiberglass fragments in my fuel samples. Still am. I will check out the G3X. Thanks for the guidance.

[efwd]

Well.... I need to do some study. This is all new to me. I flew yesterday and found 44psi at 7500ft. I have done nothing to the G3X. I mentioned this long ago to the Rotax Mechanic. He told me my readings were normal. I aim to have this discussion again as well as looking into the G3X instructions since you have posted this thread. I removed, tore open and then replaced the fine filter after about 4 or 5 hours of flight because of these readings. I was looking for the reason for the low readings. Filter had nothing on it whatsoever. I now have 102.5 hours without trouble. Wish I had more for ya Joe.

[JoeRuscito]

I wonder what yours would show if you went into configuration > engine > fuel pressure > and switched to manifold compensation. I would guess if anything low values would indicate a restriction on the low pressure side. Whereas a clogged fine filter would increase the pressure I think.

[efwd]

Joe, Im seeing the same. I think mine drops to 40-41 until Im off idle. I believe Im seeing 43 at 4000rpm. Just did my 100hr and the ECU told us nothing. That's a good thing IMHO. Im fixing to go fly so I will verify.