Fuel Pressue Oscillation

My fuel pressure gauge recently began to register about a 10 psi oscillation (with a median pressure of about 7 psi) and at a frequency that looks comparable to that that of its drive. The needle is a blur. It was much more stable inthe past. Is this indicative of the gauge losing it's damping or an issue with the pump? It is a standard issue Lycoming low pressure mechanical pump.

Type of gauge?

I had a similar problem, although the movements of the needle were slower. My gauge was the type with a sender (transducer) fwd of the firewall and a wire through the firewall into the back of the gauge. The problem turned out to be a loose ground wire on the back of the gauge. I crimped it a bit tighter - problem solved!

I had a similar problem in a Cherokee that was also a ground connection problem. Another thought might be a bad sender or air in the line to the sender. Try bleeding the fittings using the boost pump.

Roberta

Try draining the hoses between the engine and the FP gauge or transducer. Then re-hook them up and pressurize the fuel system. The trapped air in the lines will become compressed and will act as a shock absorber and stop the vibrating needle in a lot of cases.
Good Luck,
Mahlon
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