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Fuel leak only when full or shortly after driving

1994 F-150, 4.9L I6, 177K mi. Equipped with twin tanks.

For the last two fill-ups, the truck has produced a strong odor of fuel when parked with more than 1/2 tank; it also produces fuel odor just after running at lower fuel levels. Observation of the tank show that it is wet with fuel, coming from the top…exactly where is obscured by its mounted location.

Today I dropped the tank. There are no obvious leaks, and I’m beginning to suspect the fuel lines are leaking at the quick-connects.

Is it feasible to put in new washers/o-rings? I plan to diagnose things by leaving the tank dropped, fully connecting everything, and idling it while looking for leaks. As bad as it’s leaking, it should not be hard to observe. What other options are there–add on some compression fittings?

Is the top of the gas tanks rusty?

Tester

It’s almost old enough to drink…everything’s rusty! However, everything has a reassuring metal ring when tapped on by a combo wrench, and briefly inverting the tank does not produce leaks anywhere.

You have to use compressed air to see if the rust has turned the tank porous.

And I don’t mean 90 PSI. 20 will do.

When there’s gas in the tank pressure builds. This will force out vapors and liquid fuel if the gas sloshes around in the tank.

Gravity alone won’t show pin hole leaks on a rusty gas tank.

Tester

I’ve had leaks at the sender ring seal (on the tank) on my truck before. Don’t forget to check there too. If it is a fuel line (I’ve had those leak too on my truck) I’m guessing the problem isn’t a connector per se, but that a line is leaking near a connector, b/c that’s where they get the most bending stress. Replacing the fuel line with a new one, connectors and all, is probably the best solution.

On a vehicle this age, when chasing a leak, it’s safe to assume that everything rubber needs replacing.

And I agree with Tester. You need modest pressure to test it.

I had a fuel leak on my 2002 Town & Country with a plastic gas tank. Turned out to be a crack in the metal plate that holds the fuel lines and pump in the tank.

Just to make it aggravating. you can buy the pump without the plate, but not the plate without the pump.

do both pumps work ?
if not, the tank that doesn’t work can be getting filled by the pump that does.

I have a leaky filler neck hose on mine due to age. If I fill up too muck the slosh will leak out there.

The o-rings are replaceable in the quick connect fittings. They do not have part numbers but take them out with a pick and match by size.

you have a great idea to be watching as you run tests.

Update to this: upon dropping tank, saw that a metal grille from a barbecue I scrapped had dropped between the bed and cab. For obscure reasons, Ford has it so that the fuel line quick connects line up perfectly with this gap.


Removed object, re-connected fuel line, problem seems to have resolved itself.

Obscure causes and fixes are the best! Thanks for posting back. None of us would have guessed this one.