Gas seems to be leaking onto the top of the tank from the fuel line going to the engine. How can I find the source of the leak and repair it?
It sounds like you can see the area where the fuel seems to be leaking. If that's right you can start by inspecting the area and cleaning up any fuel that is sitting there. Be careful gasoline is highly flammable – don't have any ignition sources nearby as you're working. With the area cleared off you can do a few things to get started:
- If there is a hose clamp where the line connects to the tank check to see if it is loose.
- Inspect the fuel line for any signs of damage.
- Watch for new fuel to appear when the engine is running, if there is a fuel pump in the tank the line will be under pressure and may be leaking.
- In all likelihood your engine is fuel injected rather than carburated and there may be two fuel lines one delivering fuel to the engine and one returning unused fuel from the engine. Be sure to check both of them.
- If fuels isn't leaking at idle try driving the truck for a bit and then inspect again for leaked fuel.
As @spicetraders pointed suggested the leak may be dependent on the level of fuel in the tank or in the filler neck. So if you don't find fuel right away a good thing to do would be to fill the tank and check again. Fuel could also be leaking for the filler neck, the pump mounting flange, or the inlet and outlet fittings.
See if you can get a better idea of where the fuel is coming from and the conditions when it leaks (driving, at idle, full tank, etc.) and update your post with more details.