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Leaking gas tank

I just bought a 1985 Volvo 740 wagon with 110,000miles on it. But when I top off the tank it leaks gas and this has caused my car to fail it’s smog test in California. I have the Haynes manual. the tank is just under the floor. Is it safe for me to open the floor and see if there is a leak in the tank? Can I fix it myself. I am not a professional. Thanks, Mary Alice

" when I top off the tank it leaks gas"
first don’t top off tank, not a good practice leak or not…if it only leaks after a fill up then there could be a leak in filler tube…or in overflow tube?

Why not get acquainted with gauge levels and gas needs and if you know 15 gallons will fill it put in 12? I would think it is not the gas tank itself but a bad connection between the tank and the gas cap, or as posted above.

If there is an access panel for a tank mounted fuel pump on the floor or under the seat, it’s perfectly OK to pop it open and look at as much as you can see of the tank. If there is a tank mounted fuel pump (possible, but by no means certain on a 1985 vehicle), I suppose that it might have a gasket that is leaking gasoline when the tank is full. Doesn’t seem likely unless you have had the fuel pump replaced recently, but it’s an elderly car and all sorts of things happen when cars are in their dotage.

If you have had a fuel pump replaced and the gasket is leaking, take it back to whoever did the work. Either they did not tighten the screws (They are often numerous and small) sufficiently or the gasket is misaligned. If one or more screws are loose, you can tighten them yourself, but be careful not to break the heads off or strip out the threads.

Some gas tanks have a rubber coupling to the fuel fill pipe. If yours does, that could be the source of the leak. If the tank and the fuel fill pipe are all one metal piece, the join of pipe to tank could be the location of the leak. I’ll leave it to you, or your mechanic, to devise ways to stop the leak.

I should have explained that when I took it in for the smog test it failed because they pressurize the tank to find leaks and that test is why I failed. But I appreciate all of your suggestions and I’ll take it to my mechanic.The manual warned an electrical charge could sparka disaster when working with the fuel tank. Thanks for your help, Mary Alice

Taking it to the mechanic is a smart move. A leaking gas tank is also known as a “bomb”.

The place where the fuel is leaking is also causing the leak which fail the emissions EVAP test. Fix one, and you fix both.