Gas tank vacuum

[rcb280z]

On my 77 280 I usually don't let my fuel level get below 1/2 a tank. However, this time I let it got down to just below 1/4 of a tank. When I removed the gas cap at the gas station I got this "strong" vacuum suction sound. It was strong enough that the guy on the other side of the pump heard it. I don't recall hearing this at all before. Maybe its always been that way and I just didn't really pay it any mind before? Who knows. Anyone experience this or something similar. Our 05 Altima doesn't even do this.

[mgood]

Are you sure it was a vacuum and not an inside pressure release. Mine has been doing that since I got the car. Mine is more pressure, like the gas cap pushes off when it is released. It seems to be harder when the tank is emptier then when it is full.
FYI Mike

[Captain Obvious]

There shouldn't be any significant vacuum in the tank. There's a check valve built into the gas cap that's supposed to prevent that. (Funny that there's another thread running at this exact same time talking about the same thing. )

Here's some pics of the check valve. If you're truly getting a vacuum in the tank, maybe your check valve is plugged up? Here's the cap where you can see the air intake passage bulges:


Here's the air intake passage from the side:


Here's the check valve. I pried it up with a screwdriver:


And here's the air intake hole on the under side of the cap where the air intakes get to the edge:

[Captain Obvious]

Mike beat me to it... I wanted to ask if you're sure it's vacuum and not pressure.

It would make perfect sense for there to be some pressure in the tank. As a matter of fact, it's all related to the vapor recovery system that includes the carbon canister in the engine compartment and it's designed to operate such that there could be some pressure in the tank.

Either neutral, or positive pressure in the tank. But there shouldn't be a vacuum.

[TomoHawk]

Mine usually has some pressure, especially on warm days. You could only get vacuum if the fuel return line was blocked and the fuel pump sucked out a lot of fuel.

[rcb280z]

Hey guys sorry its taken so long to get back to you. I didn't fill the tank like I wanted to this morning just so I could hear it again. I have been working a couple of things for most of the day, Z related which is in a different thread. You are correct, it is a pressure release. So that makes me feel better about it. Sounds like its supposed to be that way. I put a few miles on it before taking the gas cap off again and the same "pressure" release is there.

[Captain Obvious]

The good news is that I think that's exactly the way it's supposed to work for pressure. My read on the way it works like this...

The fuel filler cap is supposed to be a positive check valve in that it allows air to flow into the tank, but won't let anything out. So why doesn't it build up pressure uncontrolled when you park the car in the hot sun and heat up the fuel and vapor in the tank?

Because there is also a device that is part of the EVAP carbon canister system that they call the "check valve", and it's in line with the vapor line that runs from the liquid/vapor separator overflow tank in the rear wheel well up to the carbon canister in the engine compartment. Note that this "check valve" is not to be confused with the check valve on the output of the fuel pump. This one does vapor while the one on the pump does liquid.

Here's a layout of the items involved in the EVAP system. The check valve is #8:


It's not a simple normal "check valve" that allows flow in one direction, but blocks it in the reverse. Instead it allows unrestricted flow in one direction and restricted flow in the other. Here's the test procedure. Note that I had to go to the 78 manual to find this. Earlier manuals showed the check valve in the system, but didn't include test procedures:


It's unrestricted flow if the tank runs a vacuum, but it's restricted flow if the tank pressure is positive.

So I wasn't there when they design the system, but what I believe happens is that the pressure inside the tank will build up some until you reach the cracking pressure of the check valve and then the valve will "burp" into the carbon canister. Then the pressure will build up again until there is another "burp". This will continue as long as necessary until the pressure stabilizes right below the cracking pressure, and that's the residual pressure that's in your tank when you take the cap off to put gas in.

[rcb280z]

Great info Capt! Was going to look it up, inquiring minds need to know So thanks for posting the info. I'm just glad all my lines, hoses are in good shape, knock on wood

[Captain Obvious]

Glad to help. Of course, I can condense all of my above ramblings above into the following...

"My 77 behaves the exact same way yours does, and after reviewing the system and the documentation, I believe that's the way it's designed to perform. If you find that the tank has a VACUUM, you have an issue, but a little pressure is normal."

The difficult part for me is that my non-Z daily driver never develops any tank pressure. Whenever I get gas, there's no "woosh". No nothing ever. I dug into it a little and it seems that it's about 50/50 with that car. Half the drivers get a woosh and half (like me) don't. I've not been able to figure out who's is normal. My 50 or their 50.

It's OBD2 and I'm not throwing any EVAP codes, and that car isn't intended to be the project, so I just drive it.