First time poster here.
I have a 1991 Subaru Legacy L Wagon, FWD and within the last 2 weeks, I haven’t been able to completely fill the gas tank. Today, for instance, I was on empty and was only able to fill the tank 7.37 gallons when the auto-stop on the pump kicked in. The capacity of the fuel tank is 15.9 gallons. I’m in RI and we have had a cold snap or two but I’ve never had this problem before. The car runs fine otherwise.
The most likely problem is that the gas tank isn’t venting correctly when gas is being introduced.
Vehicle manufacturers use two methods to vent gas tanks as they’re being filled. One method is to run a seperate hose from the gas tank back up to to the fill neck where the gas pump nozzle is inserted. The other method is to run a hose within a hose. This has an inner hose that’s attached to the fill neck that points into the gas tank. The outer hose has one end attached to a neck on the outside of the gas tank and the other end is attached to a neck on the fill nozzle. So as gas is introduced into the tank via the inner hose, the tank vents via the outer hose. The problem with this design is, if the inner plastic hose should break off from the fill neck and slide down towards the gas tank, you then starting filling the tank using the vent hose. And this will cause the pump to shut off when it shouldn’t.
So take a peak under the vehicle where the gas door is, and see if there’s one or two hoses going from the fill neck to the gas tank. If you only see one, it could be that the inner hose has broken off the fill neck, and now you’re filling the tank with the vent hose.
What about the possibility the gauge is reading wrong due to a faulty sending unit?
An old worn sending unit can develop a high resistance and a higher resistance means the gauge will read artificially lower than what it should.
Maybe the tank is nowhere empty when the needle is on E.
I’ve never seen this on a Subaru but have seen a weird problem on several other cars; mostly VWs.
Sometimes a vacuum glitch can cause the gas tank to get sucked flat. Take a look underneath and make sure the tank is not caved in.
(Don’t laugh. I’ve personally seen this half a dozen times and in some cases the tank capacity was about 2-3 gallons, maximum. It’s doubtful this is the problem, but a quick look won’t hurt.)