As I approached 100 hours Hobbs time, I began smelling fuel and eventually some staining occured on the lower fuselage immediately aft of the exhaust tunnel. I looked around the fuel lines as they penetrated the floorboards, and found three that showed some evidence of seepage which didn't respond to additional tightening. I installed Del seals, which are used by NASA, and that didn't stop the smell or staining with additional flights although there was no local seepage at the fittings. Since there is a considerable amount of plumbing under the floorboards, I went ahead and removed all but two of the floorboards. The limited amount of plumbing under those floorboards showed no evidence of seepage when inspected with a flexible video borescope, and so they were left alone. I found seepage from the side screws of the Andair fuel valve which responded to tightening. Pipe fittings on the fuel flow sensor cube were likewise seeping; they didn't respond to tightening, and so I replaced the AN nipples with new with copious thread sealant, and there was no evidence of leakage using a pressure balloon test. I found one additional coupling nut that was leaking that responded to a Del seal. I then rechecked for seepage after pressurizing the system with the fuel pump and it appears that there is no more seepage. I reinstalled the floorboards.
