Fuel Pressure Readings

by Russ Pinder in 912iS Technical Questions 5 years ago

New owner with 6hr 912iS. On startup with a single fuel pump on I'm getting 3.12bar, adding the second fuel pump in at 2000rpm gives me 3.22/3.23 bar and a RED warning on my dynon EMS. At 4000rpm the pressure drops back to 3.18/3.2 but stays as a yellow warning. My POH states fuel pressure 2.8-3.3, Rotax owner manual says 2.8-3.2. Are these readings worthy of investigation / concern? If not I'll adjust the rotax warnings slightly, my plane test pilot tells me this is normal but I'd like a second opinion. POH and the permit authority in the UK both recommend leaving both fuel pumps on at all times (except startup) and this seems to make sense.

Re: Fuel Pressure Readings

by Bill Hertzel » 5 years ago

Bill Hertzel

13 1.95k 7.96k

Completely Normal!
Set the Yellow at 3.4 and the Red at 3.5

If you do not have an actual Differential Fuel Pressure Sender plumbed to both the Fuel Line and the Air Box, what you are displaying is the Fuel Pressure Sender reading minus the Airbox MAP Pressure Sender Reading.
Both Sender readings are subject to both Sender Accuracy and Tolerance variations and these are accumulative.
The Fuel Pressure as displayed could easily be off by 1/4 bar.
What is important is that it remains consistent in the future.
The actual pressure is not as important as that it remains constant.

If it ever goes bad it will go really bad, like 4-5bar bad.
The fuel pressure sender should be located BEFORE the Fine Fuel Filter.
You are Not displaying the fuel Pressure at the Engine Fuel rail.
You are displaying the pressure before the Fine filter which will always be higher than the Rail due to the filter.
If the filter begins to become restricted, the gauge will indicate a higher fuel pressure.
The fuel rail will still be at 3 bar and the engine will operate 100% normally but you will be seeing the warning long before it becomes a problem.
The pumps are able to compensate for up to ≈6bar.
You are supposed to have a bypass valve that opens at 4bar.
Many installations omit this recommended feature.

If the coarse filter plugs or the Fuel Pumps fail, the pressure will drop. This will become an immediate problem.
The good news is that if you replace the filters annually, it is unlikely that you will ever see either problem in your lifetime.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

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Re: Fuel Pressure Readings

by Roger Lee » 5 years ago

Roger Lee

125 5.12k 4.53k

Your engine only has 6 hrs., but lately I'm seeing pressure fluctuations from too small an orifice size through the fuel shut off valve and main filters that need to be replaced after some time. Flight Design is having some issues with too small a shut off valve in the system. The 3/8" ID hose going through a small shut off valve orifice. I hope who ever built your craft used 3/8" fuel supply line. Some I'm seeing is only 5/16" and that is causing issues.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Fuel Pressure Readings

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