Fuel tank venting

[72GMC51]

If I plugged the Vapor return lines on my 72 GMC and didnt have a vented cap, could the pressure build in the tank to a point that it would be able to push fuel passed the fuel pump (edelbrock that is supposed to have a preset pressure of under 5psi) and push gas pass the float seat and flood the engine?
Thanks
John

[oldblue1968chevy]

Gonna jack ya thread..

when I loosen my gas cap to pump gas i can hear the pressure BLOW thru the cap and the gas tank expand again..

could this be bad "

[GASoline71]

The '70 to '72 trucks had an EEC system that vented the tank externally and not from the cap. So if you pulg all the lines (like I did in my '72 GMC) you will ned to use a vented cap.

Gary

[68gmsee]

Originally Posted by GASoline71

....... So if you pulg all the lines (like I did in my '72 GMC) you will ned to use a vented cap. Gary

Yes, the fuel tank cap needs to be a vented cap. On the later vehicles the tank was vented into a charcoal canister rather that into the atmosphere, hence the vent lines.

When you buy a cap, let them know you want a vented one. Usually the NAPA and CarQuest people will carry it or can order it. Not sure about Auto Zone or O'reilly.

In response to oldblue1968chevy:

You say it "blows" when you remove the cap and tank "expands" again. That's contradictory. It probably "contracts".

Anyway, a "whoosh" of air being released when the cap is removed, that too is an indication that the tank is not venting properly. Too much pressure can also create too much vacuum and the fuel pump has to actually work harder to overcome it. In some cases, if you have weakened areas in the tank due to rust, the pressure can cause failure of the metal and create fuel leaks.

[oldblue1968chevy]

Soo...

I need a vented cap?

(Darn I scored my current one for a dollar a at yard sale brand new)

[GASoline71]

Sounds like it. Could also solve all the fuel pressure probs you've been havin' too.

A cap at a yard sale? Meh... go buy a new vented one.

Gary

[72GMC51]

So now that we have settled the question of the guy who jacked my post does anyone have an opinion on my original question?

[68gmsee]

Originally Posted by 72GMC51

So now that we have settled the question of the guy who jacked my post does anyone have an opinion on my original question?

Sorry buddy, you're right, we got involved with the venting post to the extent we forgot to answer yours.

Anyway, yes, it is possible to force gas into the carb with too much pressure but worse, it could cause a leak into the engine oil pan to occur. In this hot weather the tank could create enough pressure to rupture the fuel pump diaphragm especially if it was aleady weak. As I mentioned in the other post, it could also lead to gas tank failure if the insides were already weakened from rust.

[72GMC51]

Originally Posted by 68gmsee

Sorry buddy, you're right, we got involved with the venting post to the extent we forgot to answer yours.

Anyway, yes, it is possible to force gas into the carb with too much pressure but worse, it could cause a leak into the engine oil pan to occur. In this hot weather the tank could create enough pressure to rupture the fuel pump diaphragm especially if it was aleady weak. As I mentioned in the other post, it could also lead to gas tank failure if the insides were already weakened from rust.

Thank You, I thought that it could happen but wasn't sure I seem to have a very rich condition and have had the carb rebuilt and still is the same, I also had the diaphragm in my fuel pump go bad. I moved the tank to the rear of the truck and went through the work of installing the canister venting and all of the lines, so I will go back to using the system. I would just use a vented cap but the way I had the filler door done it is very difficult to get a cap on and off so I went with the push access. I have a cover so the lines and filler neck are not exposed just have not installed it yet.
Thanks Again
JC