Erratic Fuel Pressure Gauge PA-28-181

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techflight

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techflight
I'm back with another problem on my Piper Archer II that seems to have stumped the local mechanics unfortunately. My fuel pressure gauge needle has always tended to oscillate rapidly, but sometimes it oscillates around the bottom, including around the red of 0.5 PSI. Turning on the electric boost pump tends to raise the needle slightly. I captured a video of this occurring: https://www.youtube.com/shorts/JOZdW4MITf4

The engine runs fine. I read many recommendations from previous posts on this forum, and per some suggestions I asked a mechanic to connect an externally calibrated fuel pressure gauge which read normal, indicating no issues with the fuel pump. He then cleared the line of air, and the gauge functioned a lot better with the needle consistently higher in the green and far more stable! However, it would still occasionally read in the red when ascending at full power, and at around 2200 to 2300 RPM the needle would oscillate wildly again as I captured in this video: https://www.youtube.com/shorts/h6dv_CW-2X4

So I brought the plane back to the same mechanic per his recommendation to try to clear the line one more time, and now it's worse - perhaps roughly the same as before he cleared it for the first time (consistently oscillates but does occasionally stabilize, seems to read overall lower including occasionally in the red).

He doesn't know what the problem could be at this point. I've received recommendations to send the gauge in for overhaul, but not sure if there is anything else I should check first? I'm really not sure it's a problem with the gauge itself given these symptoms.
 
At WOT and maximum fuel flow in the climb, your pressure is going to drop because the fuel flow through the fuel bowl in the carb is more or less unrestricted. As long as you have more than .5 psi you're in spec. As far as the oscillation goes, there's supposed to be a measured orifice in the line to dampen those pulses, perhaps it was removed?
 
techflight said:
I asked a mechanic to connect an externally calibrated fuel pressure gauge which read normal, indicating no issues with the fuel pump.
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techflight said:
but not sure if there is anything else I should check first?
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Given the external test indicator shows no fluctuations then that leaves the instrument supply lines and the installed indicator. I would verify the instrument supply lines are serviceable especially any flex lines then have the indicator replaced or OH'd. Or if you prefer rig the test indicator at the panel using the existing supply lines and compare readings. Regardless, it appears to be the indicator.
Chip Sylverne said:
there's supposed to be a measured orifice in the line to dampen those pulses,
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FYI: the orifice fittings are there to reduce excessive fluid leakage if the line fails. Its a certification requirement and when working properly do not affect the pressure readings at the indicator. A snubber is used to dampen pulses and is a separate fitting with some an OEM option.
 
I am a member of the "LOW FUEL PRESSURE READING" club PA28-235. Very similar to your issue. I have watched the Fuel Flow during and always rock steady. engine never misses a beat and I am at 18 gl +/- in a climb . Replaced the engine pump ( was under warranty after engine OH ) blead any air in the line. Problem still there.... I have owned this AP for 25 years , and It not been a problem still Engine OH... Right now its a guessing game as my AI has NO IDEA , acts like a Fuel Pump , but its not . Acts like air in the line , but its not. Next step is to replace the fuel primer o-rings and change out the filter and gasket on the electric pump . These are easy and inexpensive - worth a try. After that its gauge OH and ?????
 
Those gauges were and are junk, get a new gauge. A Piper combination fuel pressure / amp meter is the best option I have seen.
 
Clip4 said:
Those gauges were and are junk, get a new gauge. A Piper combination fuel pressure / amp meter is the best option I have seen.
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Any suggestions on some other gauge I could get rather than overhauling the Rochester gauge?
 
Chip Sylverne said:
As far as the oscillation goes, there's supposed to be a measured orifice in the line to dampen those pulses, perhaps it was removed?
Click to expand...

Bell206 said:
Given the external test indicator shows no fluctuations then that leaves the instrument supply lines and the installed indicator. I would verify the instrument supply lines are serviceable especially any flex lines then have the indicator replaced or OH'd. Or if you prefer rig the test indicator at the panel using the existing supply lines and compare readings. Regardless, it appears to be the indicator.

FYI: the orifice fittings are there to reduce excessive fluid leakage if the line fails. Its a certification requirement and when working properly do not affect the pressure readings at the indicator. A snubber is used to dampen pulses and is a separate fitting with some an OEM option.
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Got it, so it sounds like I should have the shop ensure that the measured orifice and snubber are in the line. If they are and the line itself otherwise appears fine, then the best course of action is to send the gauge in for overhaul.
 
techflight said:
I should have the shop ensure that the measured orifice and snubber are in the line. If they are and the line itself otherwise appears fine, then the best course of action is to send the gauge in for overhaul.
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The book shows only a restrictor installed (item 36 below). The reference to the snubber was in reply to a different post. Also make sure the flex line in the fuel pressure indicating system have not internally collapsed (item 15). Maybe check prices on indicator exchange vs overhaul?

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Honestly, in my opinion the best course of action, if the guage is TU, is to explore installing a remote sender and electric guage and keep the fuel out of the passenger compartment and on the engine side of the firewall.
 
Bell206 said:
The book shows only a restrictor installed (item 36 below). The reference to the snubber was in reply to a different post. Also make sure the flex line in the fuel pressure indicating system have not internally collapsed (item 15). Maybe check prices on indicator exchange vs overhaul?
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Okay, so it sounds like if the flex line is intact then it's probably indeed an issue with the gauge itself. I would love to exchange instead of overhaul, but haven't been able to find anyone offering this.
 
techflight said:
Okay, so it sounds like if the flex line is intact then it's probably indeed an issue with the gauge itself. I would love to exchange instead of overhaul, but haven't been able to find anyone offering this.
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Airparts of Lockhaven sometimes has exchange units available.
 
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