Fluctuating fuel flow

[Ron Franks]

My problem: Fluctuating fuel flow, manifold pressure, fuel pressure. On takeoff, I switched the electric fuel pump off at 1,000’. Almost immediately I got a low fuel pressure warning on my JPI 900. I switched the electric pump back on, and the fuel pressure climbed back into the green, but was still very low until I leveled off. After level off with the fuel pump off, the pressure climbed to middle green, but continued to hunt +/- 1 psi. At steady level flight, the manifold pressure “wanders” between =/- 1 psi. The fuel flow wandered on this setting between 12.0 gph to 14.2 gph. I can’t hear the engine or the prop surge.

Attempted solution: I have rebuilt the engine-driven fuel pump (which did raise the pressure, but did not solve the fluctuation), and I have put new seals in the fuel selector block. The electric fuel pump checks good on the preflight checklist.

This is spooky to me! Anyone have any ideas on what I should be checking?

[N3322G]

Ron,

Suggest checking grounding, transducer routing, JPI website for advice and pull JPI data to see if the fuel flow variance is actually being shown in EGT or CHT changes. Hope this helps.

[Pat Elliott]

[quote="Ron Franks"] but continued to hunt +/- 1 psi. At steady level flight, the manifold pressure “wanders” between =/- 1 psi.[/quote] this very well might be normal. You'd never see +/- 1psi on an analog gauge.

[quote="Ron Franks"] The fuel flow wandered on this setting between 12.0 gph to 14.2 gph. [/quote] I'd look at transducer placement. sharp bends near the inlet or outlet? If I recall they want 6" of straight on either side to prevent bubbles and turbulence tht will cause fluctuations in the readings. Is it adequately isolated from vibration? I once saw one that was hard mounted to an engine mount, when you hit a certain RPM the thing would go crazy..