Hissing from inside gas tank

Once again, '96 S-10 2.2L, 85K miles

I was changing out the spark plugs and wires tonight, and did a test startup to make sure everything was good. It was, so I turned it off and a few seconds after doing so, I noticed a strange noise from under the truck. A bit of crawling led me to the fuel tank, where I heard was sounded like a distinct hissing noise coming from inside the tank. It quieted and went away on its own in around 10-15 seconds.

Some of you might’ve seen my previous posts about how the truck does not hold fuel pressure when the engine is off. I wonder if this noise could be the source? If so…what the heck is it? Fuel pump check valve? Do they make this sort of noise?

The fuel pump is connected to the steel fuel line (inside the tank) by a short length of rubber (silicone?) fuel line. I have seen these flex hoses split and leak. You get under the truck and have someone turn the key on and off. Do you hear the sound? If so, that’s it…Usually, the vehicle still runs, but with a dramatic power loss…

Yup, I know the small hoses you’re talking about. One of the more interesting ways to lose fuel pressure.

There’s no power loss at all, just occasional long crank times and a moment of stumbling when started. Once it’s running, it’s just fine. Fuel pressure holds perfectly steady once the engine is running, so if this is from a flex hose it must not be a very big leak at the moment.

Soapy water slathered on the gas lines? Might show where the leak is. Or might not. Never tried it for anything other than tire leaks.

Hissing might indicate vapor rather than liquid gasoline? So maybe a return line or some such instead of the main fuel line? Or maybe a vacuum leak in a line going to an emission control of some sort rather than a gas leak?

“Fuel pump check valve? Do they make this sort of noise?”

Yes when they’re leaking fuel from the (temporarily) pressurized fuel line back into the tank.

You would smell an external leak.

I believe it, but there is no smell of gas or anything from the outside.

There’s also no hiss of gaseous fumes when the fuel filler cap is taken off; I don’t know what that does to the thought that it might be a vent line making this noise, but there it is.

It’s an internal leak, is what I’m saying.

Oops, gotcha.

The truck runs just fine, and actually starts a whole lot better since I changed out the plug wires (one of them had a rusted terminal inside one of the boots). I’m inclined to just not worry about this since it runs and drives completely normal.

Maybe at some point I’ll get around to changing the fuel pump, but on this truck it’s a $200 part. The pump itself isn’t something that can be purchased separately (at least, not that I’ve found); however he entire sending unit w/pump can be had. There’s plenty of pumps available for the 4.3 V6, but can’t find a standalone pump for a 2.2 anywhere on the internet.