If the tank has an inspection hatch, loosen it a little and watch for fuel squirting past the seal. If it does not squirt past the seal, take it off and have a look inside. If it squirts past the seal, the tank is full right into the filler hose.
If it's not full, have a friend fill the tank from the filler line and have a wee look in to see if the fuel is entering the tank OK.
I fitted a quick-release inspection hatch to the tank top, and I fill from there, in the cabin. Personally, filling right to the deck fitting can give an old tank a hard time as it raises the average pressure in the tank by about 1.5 psi (4ft head diesel), and 1.5 psi over the top area of the tank... maybe 4 x 4 ft, that's (2304 square inch).... will be a fair old load on there, close to 1.5 tons, on the roof of the tank, and probably the floor too.
Very few boat owners would park a 1.5 ton automobile on the roof of the tank, but plenty of them use 1.5 tons to try to lift the roof off the tank, or blow the floor out of it. All they have to do is fill their 4x4 ft tank right to the deck filler entry, 4 ft above the tank top.
Not for me that one.
Last edited by Rockter; 06-23-2008 at 02:17 PM.