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Petrol is a lot thinner than water. Petrol could leak where as water may not... plus would take quite some time and effort to dry it out...

Just a thought...

I left water in my tank for a few days before installing it in the airplane. It proved I had no major leaks, but I still ended up discovering a small leak once I added 100LL (an extra quarter turn on the fitting stopped the leak).

Bryan Cotton wrote: On the C140 I was restoring I used air pressure but that is scary.

Bryan. I agree, air testing with too much pressure would no doubt be scary.

I would still be checking with air, just at an extremely low pressure. It is in my opinion the best way to test for leaks. Tape a balloon or glove to one of the fittings, you only need enough pressure to inflate the balloon which will also act as a pressure relief valve. Spray soapy water around the fittings to check for leaks.

Steve
Sonex 892

I tested with water. It at least gave me a base line. There were a few months between doing my fuel system and flying the plane, so drying out was not an issue. My plane was in the basement, so I did not want to test with fuel. I did another check with just a couple of gallons of fuel once I had the plane at the airport.

Thanks for the thoughts guys. Steve, I did just as you described on the 140- balloon and soapy water. Still due to the flow restriction to the balloon it swelled the tank a bit. Scarier in a 65 year old aluminum tank that is made by welding top to bottom as compared to our plastic tanks.

If you test with water do not do it with the Princeton fuel probe installed. The probe does not like water even if you wait a year to let it dry out. The first probe I had was destroyed by water when I tested for leaks. The newer directions tell you to "dry" out the probe with low heat. Mine was completely corroded.