Fuel tank venting

[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.