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I'm sure everyone else already knows this but I figure I'll confess my ignorance anyways:
I just finished my annual and was taxiing to the fuel pump to top off. (I had not replaced the cowl yet.)
once I shut down I found a puddle of 100LL under the carb. Apparently the fuel line from the engine-driven fuel pump to the carb inlet was leaking (well, I'm not sure if it was one of the fittings or the braided stainless covered fuel line) either way, the leak was only evident when the fuel pump was on and the line was pressurized.
Since it was only leaking with the fuel pump on there was no fuel stain on the hanger floor, and since I only have the fuel pump on when I'm in the plane and the engine is running, there's no way I would have seen it any other way. ( I did not see any staining on the inside of the lower cowl upon inspection)
It sent a cold chill down my spine wondering how long it had been leaking.
Make sure your fuel system is pressurized when you leak check
Thanks for posting this. It may seem obvious after the fact, but I would have never thought of it.
Unless you have a liquid cooled engine I would strongly recommend against running the engine for longer than a minute or so uncowled. The cowl is a critical part of the engine cooling system (assuming a tradtional air cooled engine setup) and you can fry your cylinders in remarkably short time running it uncowled. It's ok to run an uncowled engine for a minute or two in order to check for leaks, but much more than that is taking a pretty big chance. To run it long enough to taxi to a fuel pump (assuming a tradtional aircooled aircraft engine installation) is scary, to me. And then you're gonna have to run it again to taxi away from the fuel pumps to your hangar or tie down spot. Scares me again.
For longer engine runs one can construct and install a cooling shroud on the engine that allows extended running time without the cowl installed, but without it you are inviting trouble.
And, as far as the initial idea in the posting -- great suggestion! Pressurizing the fuel system during inspection is a great idea and I thank you passing it on. At the moment every plane I work on is gravity flow and therefore always pressurized but that will change when my 601XLB is done.