Why does my gas tank build up so much pressure ?

[Gary L]

This is on my 76 with stock EFI. When I remove the gas cap I get a rush of gas fumes coming out. How can I test my vapor canister? I did put a new filter on the bottom. Do vapor canisters go bad (eg, the charcoal inside)? I do have a new fuel pressure regulator on it and a new fuel pump.  My newer vehicles don't build pressure or vacuum in the gas tank. Can someone explain how the vapor system works on these cars?

Thanks

Gary

 

[Zed Head]

It's described in the Emissions chapter.  Basically the tank is vented through a line with a check valve to the carbon canister.  The check valve is lightly spring-loaded so there could be a slight pressure build under normal conditions.  If the tank cools and a vacuum is created the cap has a vent to let air in.  You can remove the line from the tank at the carbon canister and suck on it.  Remove the gas cap to let air through easily.  There could be a clog in the vent line anywhere from the back to the front.

[Gary L]

Thanks Zed, I found the diagram in my SM also. Could it also be the purge control valve on my canister?

[Zed Head]

The purge control valve just controls when the line to the intake manifold opens up.  The line from your tank is supposed to be open to the charcoal all of the time.  The charcoal absorbs the vapors until the engine is running and can suck fresh air through it, pulling the vapors in to the engine.

I would imagine that there are a few areas that could get clogged.  You can pull the vent line from the tank and blow in to the canister to see if it's clogged.

[Zed Head]

Funny, I just realized why the area around my fuel filter always has the faintest smell of fuel.  It's right next to the charcoal canister.  I've checked for leaks several times.  None found.  Deeper thought on the canister function made it clear.  Bonus.

[Captain Obvious]

Hmmm... I thought the whole point of the carbon can recovery system is that you should NOT smell gas fumes. If you're smelling fumes, I suspect the carbon in the canister is saturated and cannot retain any more fuel molecules.

Are you sure your purge system is working properly?

 

[Zed Head]

37 minutes ago, Captain Obvious said:

 If you're smelling fumes, I suspect the carbon in the canister is saturated and cannot retain any more fuel molecules.

Certainly possible that the carbon is full.  It is 40 years old and the material doesn't last forever.  It's not a real concern, just an interesting to me observation.  That's why I added I added "est" to faint.  I have to get my nose right down by the filter to smell the fuel, which I do to check for leaks since there are several hose connections there.  Otherwise I wouldn't even notice.  Everything works except that my gas cap doesn't hold pressure.  Might make the fuel smell more noticeable of course, but I can't follow-up until I find a good cap.  The early 280Z caps are not cheap.

On the other hand, the system is open and those gas molecules will exit the carbon canister eventually if the engine is not run at speeds off-idle.  Another odd factoid - it's a designed vacuum leak.