My truck has dual tanks, I only use the rear tank because the fuel pump is bad in the front tank. I noticed that I have been using more gas lately. Today I was washing it off and smelled gas and noticed it seemed to be coming from the front tank, loosened the cap and gas started leaking out, took cap off and tank is full. I have not put any fuel in that tank in months
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Very common issue referred to as cross-flow. Since the front tank is filling up that means it has a bad check valve in the front Fuel Delivery Module. The short story: as the truck is running from the rear tank the bypassed "return" fuel is filling up the front tank due to this bad check valve in the front tank.

The fix is to replace the front Fuel Delivery Module (FDM).