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Posted by D. Sherman on Tuesday, January 08, 2008 at 22:17:34 :
I took gage sender and fuel pickups out of the gas tank on my M43 just to see what was going on in there, test the sender, clean the filter and general go through everything. This tank has what appears to be a vent line going into the tank through a check valve. The check valve is such that it will relieve pressure in the tank. That is, if I suck on the vent fitting, I can suck air through it. If I blow into the vent fitting, however, the check valve blocks the air. This may be part of the provision for fording, to keep water out.
My question is this: How does air get into the tank to replace the gas as it's used? I would think that after a while the suction would build up and the fuel pump wouldn't be able to suck any more gas, or maybe the tank would collapse. Does enough air leak in through the filler cap even though the filler has a gasket on it? If the filler cap was leaky enough, then why put a one-way vent line on the fuel tank? I even took the check valve apart to make sure it wasn't stuck or if there was something I was missing about it. This isn't an idle question -- the truck acts like it runs out of gas while the tank is still 1/4 to 1/2 full. If I dump a few gallons more in, it runs for a while, and then acts like it's running out again. Could it be the suction that's stopping the fuel, and opening the filler cap to put more in simply relieves the suction? The in-tank fuel filter looks fine and there's absolutely no crud or water in the bottom of the tank.
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