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]

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.

[aviator79]

Hey Eddie, Do you have your G3X set up to give you differential fuel pressure relative to manifold pressure, or normal "gauge" pressure relative to ambient? I don't have a 912iS or a G3X, but I believe Joe may have have come across the solution to this, and since you have the same engine/EFIS, you can probably help each other out.

[JoeRuscito]

I did find that setting and I’ve been exploring it. The next question is do you set a compensation value in the g3x or leave the manifold and gauge pressure unsynced. The garmin manual doesn’t explain this.

To clarify I mean the offset in the g3x. With no fuel pressure I see 15 manifold (sea level as
Expected) and 0 fuel pressure. If I then turn on the pumps I see 30 psi (without manifold compensation I see 44). It seems too coincidental that I’m off by exactly manifold pressure which makes me think I need to add in 15 psi to get back to 45. Likely overthinking it! Thanks for the help.

[neville]

I have 260 hrs. on my 912Is installed in an SS7, and using a Advanced 5600 EFIS. Fuel pressure will show +/- 47 psi when start power is applied and drops to 44 psi when engine starts. (only one pump on for start). It stays within specs until I exceed 10,000 ft when it drops to 40psi. The Advanced 5600 uses the formula;
Fuel pressure= analog fuel pressure + ambient pressure- MAP pressure. The off-set shows up on the 5600 admin page for fuel pressure settings.

[aviator79]

I'm pretty sure Neville's setup is correct. The Pressure sender is probably gauge pressure, i.e. Absolute fuel pressure - Ambient pressure. and the regulator regulates relative to manifold pressure. So to read the pressure that your regulator is actually regulating, you need to add the ambient pressure back in, and then subtract off the manifold pressure to get a differential pressure between absolute fuel pressure and manifold pressure.

It looks to me like your display is showing the gauge pressure.
-Before start, your MAP is as high as it can be (ambient). Since your regulator maintains some specific setpoint above that, it too is at its highest value.
-After start and at idle, your MAP is about as low as it can be, so you see a huge drop as the regulator does its job.
-Fuel pressure increases again as you increase power because the regulator is maintaining pressure relative to MAP.

If you can make your G3X do what neville's AFS is doing, you should see pretty constant fuel pressure at any power setting and altitude. An "offset" to me sounds like a constant bias, and not what you want, but I'm completely unfamiliar with the G3X.

[JoeRuscito]

Yes totally agree with everything said.. The g3x only allows you to set an offset, everything else is done in the background and I do believe it is doing what you both describe. The interesting part is if I set a 0 offset I see about 15 psi low readouts throughout the range... of course coincidentally 15 psi is about ambient pressure at sea level (Im at sea level). So it makes me think I need to set the offset to ambient pressure when the engine is not running. In that way the fuel pressure reads 0 when engine stopped. ~44 before start and then ~44 through the whole range. This seems great to me... but then begs the question what happens at altitude? When the ambient pressure is not ~15 psi. Im really interested to hear what Eddies offset settings are.

Thanks for thinking this through with me guys.

[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]

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.

[rosslr]

HI Joe,

I no longer have my plane but I consistently saw 40-41psi at idle and 48psi at TO rev with both pumps. turn one pump off and 5000rpm and I had 46-47psi. As this cruise fuel pressure was on the high side it worried me at first but after talking to people smarter than me I decided to monitor fuel pressure for changes rather than absolute readings - an increase of fuel pressure above the norm indicating time to change out the fine filter.

By the way, in your earlier post you said the you were concerned about your idle pressure being below the min fuel pressure of 46.5psi. That is the max. The min fuel pressure for the 912is is 40.5psi

cheers

ross

[aviator79]

if I set a 0 offset I see about 15 psi low readouts throughout the range..

I think you've got it Joe. This would indicate that you're seeing the difference between the raw sender measurement and the MAP. Neville's AFS adds back in the ambient pressure, which would make your readings perfect at sea level. I would guess that if you offset by your home field elevation pressure, you'd see artificially high readings as you climb, but they'd be more in line with the limitations in the Operating Manual. If you can confirm that is the case, that's probably what I'd do.

If you were really serious and you can't configure the G3X to compensate for both MAP and ambient pressure, you could tee into the line from the regulator to the airbox and install a differential pressure sensor. That's basically the recommended way to measure fuel pressure on the 914. If it were mine though, I probably wouldn't go to that much trouble. I think the G3X can get you a good enough value to make smart maintenance and flight decisions even if it's not absolutely what Rotax wants you to monitor.

[efwd]

The information Ross has is precisely what I was told by Brian at California Power Systems. I have been monitoring for changes to date.