Ok I need help it seems my gas gauge is now not working correctly as it is almost always sitting on empty but when I try to fill it the gas spews out of the tank. Mostly my concern is the fact that when I open my gas tank there is so much pressure being released and I have no idea what is causing it. The car runs and drives fine but the pressure and the gauge are worrying me. I also just replaced the original gas cap with a locking ventilating one from the parts store. 

77' Datsun 280

just bought the car, looks stock to me no mods on the car

The gas cap is only supposed to let air in as gas is used.  The vent for the tank, to relieve pressure, is supposed to run through a hose up to a charcoal canister in the engine bay.  People sometimes remove them as "emissions junk" not knowing what they're doing.  That might the cause of the pressure buildup.  Maybe somebody removed the engine bay vent and was using a vented cap in its place.

Shouldn't affect your gas gauge though.  It's just a metal float on a lever that moves up and down with fuel level.  Floats, in general, do sink sometimes, although I've never heard of the gas gauge float doing that.

Or the vent line could be plugged up with rust as mine is. FSM has a check for "stuffiness" of the vent line. Think there is a check valve in there as well, that check valve #8 can be hard to find.

Edited April 18, 20178 yr by Dave WM

The sending unit to the gauge is pretty easy to get to and if you touch the two wires together it should show EMPTY on the gauge (or FULL, I forget). That will let you know if it's a bad gauge. Mine was sticking at Empty because the previous owner had tapped the glass so hard the needle was up against the lens.  I replaced the sending unit and fixed the lens. Now it works right.

Thank you I will try these and see how it goes


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