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

 

Test procedure is the same for the 77.

Gary, you'll see it as soon as you look up towards the damper along the body. 

I did find it. When I blow in the line with my mouth at the carbon canister, air will flow through it fairly easily and pressurize the gas tank. However, when I try to suck air through it I can draw air but it seems more difficult to do.  But the again it is easier to blow air though a small tube than suck air. If I disconnected it (which I'm not going to bother doing at this point) and then tried blowing through it from the tank side as outlined in the test procedure it may be easier to do. But anyway, it does allow air to pass in either direction which sounds to be correct

Thanks

Gary

 

Gary, that sounds perfect.

It should be easy to blow air into the tank and difficult (but possible) to pull it out of the tank.

Enjoy the whoosh.

 

Something I don't understand, and maybe someone can explain it to me, is why and how does the tank get pressurized. For example, as  the fuel pump pumps fuel from the tank to the engine to get burned, I would think the tank would develop vacuum and hence the need for a fuel filler cap with the vacuum relief valve. But I have trouble understanding why pressure develops in the tank. (just trying to educate myself)

 

Gary

 

Fumes from all the bouncing would be my guess?  Like shaking a beer before handing it to your friend to open.

EDIT: My reasoning is my backpack blower.  The check valve that releases the fumes from the fuel tank got all clogged up after a few years of use.  I used it one weekend and the next time I go to get it fuel was dripping out the air filter compartment, filter was soaking wet.  The pressure had built up so high it pushed fuel backwards through the carb then through the filter.  Until I figured it out I would loosen the fuel cap after each use to prevent the leaking.  My experience only, I'm no mechanic just a guy with a computer and some figuring out sense.  Thank you Dad!

Edited October 1, 20186 yr by siteunseen

I haven't studied it, but off the top of my head I suspect it has mostly to do with "vapor pressure" and temperature increase.

Vapor pressure is basically the pressure at which a compound will exist in both liquid and gaseous phases at the same time. In your gas tank, you have liquid fuel, and fuel which has evaporated into a gaseous phase. When that whole thing reaches equilibrium, the pressure in the tank will be the "vapor pressure" of the gasoline (at that temperature).

I think the biggest contributor to the tank pressure is the increase in temperature as you drive the car. Most times you start it in the cool morning and everything is the coldest it's going to be all day. Then you drive the car and everything heats up. When the temperature of the fuel and vapors in the tank goes up, the vapor pressure goes up too. Exponentially.

So, yes... You are taking fuel out while the engine runs. But I think the volume reduction (as fuel is removed) is a smaller contribution to the overall tank pressure than the vapor pressure is.

Then when you shut the car off at the end of the day and everything cools off, the pressure will decrease and it will pull air in through the gas cap as necessary to make up the difference.

5 hours ago, siteunseen said:

Fumes from all the bouncing would be my guess?  Like shaking a beer before handing it to your friend to open.

The fuel bouncing around would speed evaporation by increasing surface area. In other words, it would reach vapor pressure more quickly as the temperature goes up, or after something like a fuel fill up. When you stop for gas and woosh the cap, you release the vapor pressure. But it will re-attain that pressure when you put the cap back on.

About the backpack blower... It's my understanding that the check valve in the cap allows air INTO the tank, but doesn't let anything out. My understanding is that entire system is supposed to operate under a vacuum as fuel is removed from the tank. If one would ask the question "so why doesn't the tank pressure go up on that" I don't have a good answer. I don't know why your blower ends up in a puddle of fuel when you shut it off.

And maybe even more important... I don't know why mine doesn't.

Maybe because you're taking fuel out so fast? Maybe because the float chamber is atmospherically controlled? Don't know, and now it bugs me. Thanks a lot.  

My Stihl has a two hose valve at the top of the fuel tank. The cap seals so tight I can run it and my weed whacker upside down.

I took it apart and cleaned that release valve and it's all good now.

Don't lose any sleep over my comments. The nice rape kit with chewing gum and Tampax still has me laughing. I must admit I did not see Greg's post on the MacGyver kit. Sorry Captain.  You know me...

Edit: for the rest of the forum I will move to PMs.  I am twisted in my thinking and 99% are not. Thanks you other 1 percenters!  

Edited October 1, 20186 yr by siteunseen

LOL. MacGyver is still looking over his shoulder after all this.

So the tubes on the small engine two-cycle thing... One is supply from the tank to the carb, and the other is the return from the carb back to the tank. The system operates just like the Z. The fuel pump pulls more then necessary from the tank, some of it is used by the carb, and the excess is pushed back into the tank. Continuously circulating. And the closed system operating under a slight vacuum is what allows it to be used in any position without spillage. Like chain saws.

And as the fuel is used up, the duck--bill valve on the cap allows air into the tank to replace volume of consumed fuel.

I've had that duck-bill check valve plug up on some of my small engine stuff and what happened to me is it runs great for a little while and then will stall and not restart. Too much of a vacuum in the tank and the fuel pump can't pull fuel out anymore. Loosen the cap and it'll run for a little bit and then quit again. So a little vacuum is good, but too much vacuum will starve the engine lean.

I don't know why your clogged tank valve resulted in a puddle of fuel. (And it still bugs me.     )