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Fuel leak isolation questions

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Aug 21, 2010
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So last weekend I pulled my pilot's side panel on my '73 PA28-180 to get a look at the fuel selector valve and the fuel plumbing around it (I've been catching whiffs of fuel in the cockpit that seem to be associated with manipulating the selector knob, and I'm trying to track it down.)

Of course, when I turned on the boost pump and cycled the selector knob all over the place I couldn't induce any leaks or seepage, and even with my head down in the foot well I never smelt a hint of avgas. (Yes, I also pumped the primer a few times and inspected that plumbing too, with no joy there either.)

First question(s): I noticed that the fuel inlet lines to the selector spider were aluminum, but the outlet to the fuel pump was copper. Is this how my '73 came from the factory? Why the different materials?

While I did not find active leaks during my investigation, I did find three suspicious areas:
  • Some crusty crud on top of one of the AN fittings going in to the fuel selector spider, and some light blue staining of the insulation right behind it.[/*:m:glyz8a79]
  • Rubber stripping wrapped around the selector outlet fuel line where it crosses the port brake line up in the foot well, to keep the lines from chafing each other where they intersected--that rubber was soft/sticky and stained a deep blue that would come off on my finger when I touched it. Nothing else around it was heavily stained, but there was a faint blue stain on the insulation below the fuel line between the rubber and the firewall.[/*:m:glyz8a79]
  • The copper fuel line had a fair bit of green oxide ON TOP of the line just INSIDE of the firewall in the foot well. I also peeled the lower cowl away from the pump/gascolator area to see what the line looked like on the other side of the firewall, but the line goes straight into the pump/gascolator and wasn't really visible. The box around it looked like ~40-year-old engine...metal surfaces were covered in gray residue, but hard to say if it was copper oxide, blue dye powder, aluminum oxide, fiberglass dust, etc., etc. No real useful info there to my eye.[/*:m:glyz8a79]

While it was all open, I grabbed my mechanic and let him look at all the same stuff. He ended up scratching his head and said, "I don't see any active leaks. Pay attention if/when you smell it again and see if you can localize where it's coming from." Gee, thanks... : ) (He also suggested checking the lines coming in from the wings, but I'll be tearing the plane down for annual in 6-8 weeks so I'll wait and do that then.)

Do any of those "suspicious areas" above raise any red flags warranting further attention? Any other suggestions for identifying the leak?

(Now that I type this, I have the idea to wrap some white paper towels in key locations to see if they'd pick up any blue dye from active leaks. Sure wish I'd have thought of that before I put the panel back on!!)


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