Fuel Pressure ‘dances’

RyanB

RyanB

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The fuel pressure gauge needle in the Archer isn’t completely steady. It holds the general position around mid-range but dances just ever so slightly. I’ve flown in a similar Archer and the fuel pressure indication held completely steady and I’ve also observed others in various YouTube videos that seem to have a little dance to them as well. The fuel pressure gauge on the Archer has been this way for as long as I can recall since starting to fly it. If I encounter turbulence, it will also move a bit more abruptly at times, so I’m not sure if maybe it’s just gauge error or not. The engine runs fine and all else seems to be okay. Am I being paranoid or should I investigate further?
 
not something I personally would worry about. but if you lose all fuel pressure, then I would worry about it.
 
There’s a whole discussion point about this on the FB PA-32 page.

Let me find the information for you later today. But the short answer is that nothing is wrong. It’s an anomaly having to do with how Piper plumbed it. Owners have thrown thousands of dollars at this only to reach this decision.
 
Rgbeard said:
There’s a whole discussion point about this on the FB PA-32 page.

Let me find the information for you later today. But the short answer is that nothing is wrong. It’s an anomaly having to do with how Piper plumbed it. Owners have thrown thousands of dollars at this only to reach this decision.
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I tried to join that, but they rejected my request. Guess they found out I’m not a -32 pilot, but a lowly -28’er. ;)
 
My fuel pressure gauge used to fluctuate and I was told not to worry about it. Later I replaced the fuel pump while chasing another problem and now it's stable. Red herring? Dunno. Of course my io runs a good bit more pressure than a carb'd engine.

I do think it's pretty common on pipers, and as long as your electric pump is working the risk is really small.
 
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I found this. Seems similar to the air gaps installed in homes for water-hammer issues.

“Need to drain the fuel pressure sense line. Crack it open under the cowl and let it drain from the gauge. If it doesn’t then you will need to take it off the back of the gauge and blow it out. The idea is to get an air gap in the line so that there is a compressible area to absorb the pressure pulses from the engine driven pump.”​
 
Jim K said:
My fuel pressure gauge used to fluctuate and I was told not to worry about it. Later I replaced the fuel pump store chasing another problem and now it's stable. Red herring? Dunno. Of course my io runs a good bit more pressure than a carb'd engine.

I do think it's pretty common on pipers, and as long as your electric pump is working the risk is really small.
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Yeah I figure replacing the pump entirely would probably solve it, and that’s more or less the only ‘big ticket item’ that hasn’t been replaced firewall forward. Mechanic says they do well as far as lifespan is concerned and there’s no need to replace it for funsies unless failure is imminent. Wanted to get some opinions from other’s, but I’ll probably just chalk it up and move on. Thanks!
 
Turbulence makes it worse? Think of the fuel in the carb's float bowl. It's being vibrated constantly by the engine, and tossed around much more in turbulence, moving the float up and down and opening and closing the float valve so that the fuel flow is jerky. The pressure will rise when the valve is closed, and drop when it opens again. It's one of the numerous disadvantages of carburetors.

Some systems have a restrictor at the pressure line from the pump or carb to the gauge. That slows the fuel movement in the line so that changes are damped out quite a bit. Getting more air in the line also helps.

You might want to get your mechanic to check to see if there is supposed to be a restrictor in your system. Stuff gets changed over the years, and sometimes the wrong fittings are installed. Often find that on engines, where the oil pressure fitting at the engine is usually supposed to be a restrictor fitting. It gets chewed up when mechanics try to remove it from the old engine to put in the new, so they use a normal AN fitting, no restrictor.
 
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