Where is the Fuel Vapor Check Valve Located

[Gary L]

This is on my 76 280Z.  After running the car for awhile, my fuel tank gets pressurized ---- When I take off the gas cap I get a huge rush of air coming out of the tank, a big whoooosh. Looking at my service manual, I can't tell if if the check valve is under the car or behind the passenger side rear panel near the vapor separator in the hatch. I saw in an old post where a guy had a similar problem which was corrected by replacing the check valve. Of course, I'm never going to find another check valve for sale, but I first just want to know where it is located. 

 

Thanks

Gary

 

[Zed Head]

I think it's behind the panel.  But you might check the end of the vent line in the engine bay, where it connects to the carbon canister, before you go too crazy.  It might be clogged or blocked there.  You should be able to suck some gas fumes through the hose after you disconnect it from the canister.

[Gary L]

So when I try sucking through the vent line at the carbon canister, it is difficult to suck hard enough to tell if I am drawing any fuel fumes through it. However, if I hook the vacuum cleaner to the vent line for a couple of seconds with the gas cap on, when I then take the gas cap off, there is a considerable vacuum present, hearing the air rush in to the tank. So I don't know if that is good and indicating proper operation or not. I hope that makes sense.

 

 

[Zed Head]

I wouldn't do the vacuum cleaner again.  The motors are not sealed and could ignite fumes.

Some pressure in the tank is not a bad thing.  The system is designed to be sealed to the ambient atmosphere, to keep gas fumes in, and moisture out.  Both ends are spring-loaded, with a spring in the check valve and a spring in the vacuum relief valve in the filler cap.  Your fuel rail works on pressure differential so doesn't even know that the tank has pressure.

If it was mine, I'd be happy to have some pressure.  My filler cap vacuum relief gasket is shot so I don't get any whoosh, and I'm sure that my carbon canister isn't getting the workout it should.  Sounds like everything is working right with your system.  You could take a look under the car at the tank to see if the tank is bulging.  If it's not, the pressure probably isn't very high.  The whoosh might be more volume than pressure.

[rcb280z]

On my 77 its close to the fuel pump, in the vapor vent line that runs between the liquid / vapor separator and carbon canister. I think its the same for the 76 (?)

Edited September 26, 2018 by rcb280z

[Captain Obvious]

Second. On my 77 it is under the car close to the fuel pump and I would also assume that 76 is the same.

That said... It's a check valve, but it will build up pressure in the tank until the "cracking pressure" of the check valve is reached. I don't know how big of a woosh you get, but I get a significant woosh on my car. And I believe it's working properly.

[rcb280z]

12 minutes ago, Captain Obvious said:

I get a significant woosh on my car. And I believe it's working properly.

As do I. Every time I pop the gas tank cap. Sometimes I think it working too well.

[Captain Obvious]

Yeah, me too. But it's consistent. By that, I mean,  it seems to build up pressure to a consistent pressure and then level off. Based on that, I believe it's working.

Well that, and my tank hasn't ballooned out from overpressure.

[Gary L]

Thanks guys. It's reassuring others get the same "whooosh" and that its normal. I guess I'll leave well enough alone. 

It's getting near peak color here in the UP of Michigan and so I'm hoping to take it on a couple of Fall Color road tours in my area.

Gary

 

[rcb280z]

4 hours ago, Gary L said:

It's getting near peak color here in the UP of Michigan and so I'm hoping to take it on a couple of Fall Color road tours in my area.

I envy you. I bet it's pretty!

[Zed Head]

Just a loose end.  This is why I haven't bought a new cap.  Trying to figure out how to fix that gasket instead.

https://www.courtesyparts.com/oem-parts/nissan-cap-filler-17251n4501

[Captain Obvious]

So I had forgotten that I had these snippets from the FSM that relate to that check valve. Had posted them somewhere on some thread prior. Details have faded. In any event, mention of the check valve started in 76. I don't know if it existed in 75 and they just didn't mention it, or if it was a new device added in 76.

Here's the location on the car:

 

Here's how it looks in the system schematic. This is from the 78 manual. Previous years had a similar pic, but 78 is the most accurate:

 

And here's how you are supposed to test it. Note that this test procedure didn't appear at all in the FSM until 1978. It describes a device which allows air to flow easily ("smoothly directed") from the carbon can back to the tank, but dictates that it should be harder to push air from the tank towards the carbon can. They say "considerable resistance" towards the carbon can. My interpretation is that "considerable resistance" could cause a woosh: