Strange Fuel Flow and Pressure Oscillations

[KSMooniac]

The slow response probes suck IMO...terrible for diagnostic work like this and precision leaning so perhaps they can swap them for you.  

[romair]

I asked my avionics shop about the fast response probes....apparently they need a smaller caliber hole than the slow response ones, so they would have had to weld the holes already made, then make new holes. I decided not to go that way...

[KSMooniac]

Good luck with that...the slow ones won't be able to help with diagnostics like spotting valves failing, or help you find peak easily. I'm surprised they still sell them in fact.

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[microflight]

I have a JPI EDM 900 and no problem with ther fuel flow. However, I get the same oscilation in the fuel pressure. I even got a snubber from JPI and after we installed it,it seemed that the fuel presssure was very steady but after a few hourts the oscillation returned. Using the maual fuel pump without the engine running showes a very steady fuel pressure. Also,we connected a manual fuel pressure guage while the engine was running. Again the fuel pressure was rock steady.

Yesterday, I disconnected the fuel line from the transducerr and ran a small amount of fuel through it with the manual boost pump. I then reconnected the fuel liine to the transducetr and ran the airplane including a mag check run-up to 2000 rpm. The result was rock steady fuel pressure at 25 psi.

My belief is that the fuel pressure sensor is too sensitive and there is a small pulse from the engine driven fuel pump that the transducetr is able to pick up. Having said that, I am trying to find a solution thaty does not require disconnect/reconnect.. Also, when i reconnect the fuel line there is an air gap. I believe this air-gap acts llike a baffel and damps the small pulse so that the transducer does not pick it up.

Problem still not solved but I think I understand the issue.

[N601RX]

I had the same problem last year. The fuel pressure was noisy and continually went from 13 to 34psi. Putting the snubber at the outlet of the fuel servo and draining all the fuel out of the line before reconnecting it fixed the problem. It has been noise free for almost a year now. I had the same problem with MP also.

If the problem goes away and the comes back in a few hrs after reconnecting is seems like there may be a small leak around the transducer that slowly lets the air out.