Hi guys,

I'm honestly not sure which sub forum this was supposed to go in but here we are.

Periodically I smell gas fumes in the car.

One of the main culprits was the old-style hose clamps weren't sealing the hoses to the vapor tank well enough.  I changed them to modern belt-clamps which helped a lot but not fully.  I've been testing the leak by blowing in the hardline that connects the carbon canister to the vapor tank and scampering back there to sniff around (which is how I originally found the clamps were a problem) but I'm still smelling some fumes while driving so it's not fully fixed.  At this point I can't seem to pin down the smell point and even used a smoke machine but found no leaks.

Could the check valve that goes from the vapor tank to the carbon canister have a bleed-off function?  Also, any idea where that check valve is? I can't seem to find it.  If it sits outside of the cabin then that might not be the leak though.

Note: my carbon canister is disconnected because it doesn't hold vacuum.  If I was able to use that system, I'm assuming this wouldn't be as big of an issue but who knows.

 

Any ideas?

chase

Edited November 7, 20231 yr by chaseincats

The "check valve" isn't a traditional check valve. The way it operates is that it passes flow both directions (unlike a "real" check valve), but the trick is that it passes flow much EASIER in one direction than the other.

Here's some pics that help explain operation and location:

Not sure why my pics are showing up sideways, but that's what I got.

2 hours ago, Captain Obvious said:

The "check valve" isn't a traditional check valve. The way it operates is that it passes flow both directions (unlike a "real" check valve), but the trick is that it passes flow much EASIER in one direction than the other.

Here's some pics that help explain operation and location:

Not sure why my pics are showing up sideways, but that's what I got.

Thanks for providing this!

Kind of odd, I noticed it went from easy to blow through (from the engine side) to hard with each successive time I blew through that line in the engine bay.

According to this however it should be tough all the time?  Also, it is kind of confusing as to why it should be hard to blow through when going from the engine to the vapor tank. You'd think that should be easy since the vapors are coming that direction (from the tank to the engine), no?

 

-chase

You don't want to over pressurize the tank. You want the carbon canister catching the vapor, not venting to atmosphere when someone pulls the gas cap.

2 minutes ago, SteveJ said:

You don't want to over pressurize the tank. You want the carbon canister catching the vapor, not venting to atmosphere when someone pulls the gas cap.

Gotcha.  If I smell near my gas cap, I do smell vapor - is that normal?

15 hours ago, chaseincats said:

Note: my carbon canister is disconnected because it doesn't hold vacuum.  If I was able to use that system, I'm assuming this wouldn't be as big of an issue but who knows.

Nissan knows.

What did you do with the open end of the hose?  And, to be clear, which hose is disconnected.  And, more clarity, the canister itself doesn't "hold vacuum".  It's wide open on the bottom.  The cap on the top controls vacuum to open the purge line that pulls through the canister.

1 hour ago, chaseincats said:

Gotcha.  If I smell near my gas cap, I do smell vapor - is that normal?

Yes, it is normal. The only way to prevent any fuel vapors would be to chill the tank below the vapor point of all of the volatile organic compounds (VOCs) in the fuel.

Since it isn't practical to refrigerate the fuel tank and the ambient temperatures in the summer are well above the vapor point of the VOCs, Nissan has the carbon canister to capture most of the fugitive emissions from the fuel tank. On the other hand, you have to allow some make up air into the tank to allow the fuel to flow to the fuel pump. That is why you need the two-way check valve.

Here is how the purge system works in modern cars: https://www.samarins.com/glossary/purge-valve.html

Actually, the gas cap is sealed and should only let air in, through the "vacuum relief valve".  Gasoline fumes from expansion go to the charcoal vapor canister, fresh air is let in through the valve on the gas cap to replaced fuel burned or volume contraction..

I found that the seal on the relief valve on my cap was shot.  The cap design was changed for unleaded fuel cars and is still available.  The older big-mouth caps are hard to find and very expensive.  I just lived with the faint smell.  Can't remember what CIC is working with.

https://www.classiczcars.com/settings/signature/

3 hours ago, Zed Head said:

Nissan knows.

What did you do with the open end of the hose?  And, to be clear, which hose is disconnected.  And, more clarity, the canister itself doesn't "hold vacuum".  It's wide open on the bottom.  The cap on the top controls vacuum to open the purge line that pulls through the canister.

Sorry, that's what I meant - the valve at the top doesn't hold vacuum, so it pulls in fresh air/vapor at all times, including at idle.

When I started it after re-connecting the canister to the purge/throttlebody trigger vacuum lines, it ran poorly with the rich/lean number on my a/f gauge bouncing between 12 an 16 and it drove equally as poorly until I disconnected both vacuum lines (capped the purge hose on the intake and connected the throttlebody directly to the distributor like before.

1 hour ago, Zed Head said:

Actually, the gas cap is sealed and should only let air in, through the "vacuum relief valve".  Gasoline fumes from expansion go to the charcoal vapor canister, fresh air is let in through the valve on the gas cap to replaced fuel burned or volume contraction..

I found that the seal on the relief valve on my cap was shot.  The cap design was changed for unleaded fuel cars and is still available.  The older big-mouth caps are hard to find and very expensive.  I just lived with the faint smell.  Can't remember what CIC is working with.

https://www.classiczcars.com/settings/signature/

I changed my cap to a generic one from oreilly's since I wanted one with a lock.  The original cap was on there when I got it and did not seal the tank vapors at all - with this one you can only smell them if you really put your nose to it so that should be fine I'm thinking.

To add a little more detail to what Zed Head mentioned... The gas cap (original cap) has a "real" check valve built into it such that it will allow air INTO the gas tank in the even that the tank is drawn down to a vacuum, but it will not allow anything to escape.

Now, I'm not really sure why they thought they needed to build a second valve into that little check valve device in the EVAP system. Seems like they only needed one way for air to get into the tank and that was handled by the gas cap. but there are actually two ways for air to get into the tank... Through the gas cap valve, or through the carbon can and then through that in-line check valve device.  I wasn't there when they designed it.

9 hours ago, chaseincats said:

Kind of odd, I noticed it went from easy to blow through (from the engine side) to hard with each successive time I blew through that line in the engine bay.

The reason it got more difficult to blow through (from the engine side) each time was because you were pressurizing the fuel tank a little with each blow. When the pressure got to the point where your lungs couldn't do any more, it stopped and held. The system is designed to hold some pressure, but not enough to damage the tank. In normal operation, the fuel tank will most likely be running under positive pressure as you drive the car. Goes like this...

When you start the car with a cold tank, everything should be pretty much at equilibrium. The tank pressure should be right around atmospheric. Then as you drive, you will heat the fuel in the tank... Exhaust pipes close. Warm rear diff close. Gas recirculating up the the warm fuel rail and back again. All that stuff and the fuel heats up and expands causing the tank pressure to go up. 

If (when?) the tank pressure increases to the point where it will open up* the "check valve" device in the vapor line, it will burp into the carbon can. By operating that way. the check valve allows the system to run at a positive pressure, but not high enough to damage the fuel tank.

 

* known in valve terminology as "cracking pressure". The check valve has an asymmetric cracking pressure design. Lower cracking pressure pointed towards the tank and a higher cracking pressure pointed towards the carbon can.

Thanks @Captain ObviousBut that sure made my brain burp as well!!😂😂

Interesting read. I didn't think my 75 (CA market) had a vented cap of any sort, but since it is described as such here, I looked at the pic I took when I changed the gasket - it does appear to have a valve in the center. I'll have to look under for the check valve - I didn't see any sign of it when I did the fuel pump work. I will be dropping my tank to take care of the vapor hoses (and probably bypass the vent tank). If there's space, I'd also like to add a later charcoal cannister in the back, to free up the bay for when I do the V6 conversion.