EFIS and Eng Monitor Mfrs. Please Comment

[AlexPeterson]

There is yet another thread regarding fuel pressure excursions currently running, which seems to be a very commonly reported problem on this forum. By excursions, I mean atypically low pressures seen from time to time.

The fuel pressure on my injected O-360 C/S has had these low pressure excursions (down to 12-18psi) for as long as I can remember, across two pumps, two different engine monitors and two different transducer designs. They are seemingly random, and are corrected by boost pump. I've never had the engine cough. I do not turn the boost pump on any more during these events, as I am curious how they resolve. Usually within perhaps 10s the pressure recovers.

Dan H. and I have discussed this mystery in the past. One theory Dan suggested is that there is a beat between the actual pressure pulses, due to something mechanical (pump stroke freq, mechanical vibrations, etc.), and the transducer sample frequency/signal conditioning algorithm.

What I have been able to demonstrate is that at certain RPM's, I get an oscillation in the pressure indication of perhaps 3 or 4 second period, with about a 2-3psi amplitude. Change the rpm slightly, and this stabilizes (2400rpm will cause this, 2300 won't). It is worth noting I see a similar oscillation on the MAP. (I've running GEA-24 into a G3X Touch).

What we don't know is how the various engine monitors condition the signal before converting them to digital. If any of the manufacturers are reading, please comment. Just knowing sample rate is not helpful.

My homework assignment is to connect a high frequency voltage recorder to the output of the transducer, and record enough flights to get the excursion. The excursions down to 12-18 seem like real pressure drops, but I'm not totally convinced. Additionally, I can see if the oscillations are real or not.

Has anyone seen these excursions with a mechanical, or a non-digital electrical, fuel pressure gauge?

[jonweisw]

Alex,

Very interesting. I will be watching this thread carefully. It still doesn't answer why there is improvement with the new pump or why (as you acknowledge) the observation resolves with aux FP use.

Here's a screenshot of the data from a flight that had a dip to 16.2psi from 22 avg. There are some oscillations in it that you allude to in your post.