Fuel Pressure Reads Low

[ncq6c1eq4u]

Rotax 912ULS in Vans RV12, Dynon 180...   Fuel pressure readings on low side of normal.  Got low pressure alarm during climb.  Data Download showed FP at 1-2PSI during cruise.  Always had minimum FP for takeoff.

Ground check showed following: 0.2 engine off, boost pump on
2.7 engine and boost on
2.3 engine on boost off

I installed a separate pressure gauge: 2.5 engine off, boost on
  4.5 engine on high idle
  4.3 engine on, boost off

Re-installed to Dynon FP sender from RV12 kit: no improvement.

There seems to be an issue with sender functioning or Dynon calibration.  Can you please help me out?

Thanks,  Jim

 

[dynonsupport]

We've checked the calibration tables in the past and have found them to be correct. The sensitivity of the fuel pressure sensor gets a bit tough at very low pressures - we're investigating using other sensors in the future, but do not have a solution here yet (the main issue is finding a reliable, accurate, and inexpensive sensor). You can try another sensor if you like. We don't offer up-front warranty replacements, but if you buy one, find it fixes the problem, and then send back the original (and you're within 1 year of purchase - our warranty for most sensors), we'll refund your replacement sensor purchase price.

 

[Roger_Lee]

I work on Rotax 912's all the time. (A Rotax Repair Center) The fuel pressure senders go out more frequently than most think. If it were me buy a new one from Lockwood (about $35) or on line and if it is mounted on the engine move it and remote mount it off the engine like the firewall. Lockwood even has a kit for doing this. I just had this same issue with another gentleman. A sender can last for years or go out in a few days if it was bad.

 

[dynonsupport]

We do sell the fuel pressure sensors here, by the way. Also note that per our installation instructions, neither the fuel pressure sensor nor the oil pressure sensor should be mounted directly to the engine. We recognize that some ASTM'd Rotax configurations will have mounting methods that are contrary to this. That is Rotax's prerogative, but it will shorten the life of the sensors, in our experience. Some other thoughts that we've obtained from Vans:

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Verify that your engine-driven pump is not in the process of failing. If it is failing, there should be evidence of fuel coming out of the "weep hole" in the pump. Verifying function of the airframe mounted pump is simple, just turn on the master switch and listen for the sound, look for fuel flow indication of around 2 gallons per hour, and look for a pressure indication. Then actuate the fuel shut-off valve and listen for a different pump sound, look for the flow indication to drop to zero, and look for the pressure to drop to zero.

If everything checks-out, look at the rest of the fuel system for faults.
This should include cleaning-out the gascolator screen. Even though Vans' Production Acceptance Procedures checklist goes into detail on flushing-out the fuel system and calibrating the flow transducer two things can and do happen. First, not all the debris in the fuel system gets flushed immediately but accumulates in the gascolator screen. Second, some builders prep their aircraft for flight without including a fuel system flush, contrary to Vans' explicit recommendations.