Gas cap not venting

[71K20chevy]

I've got a vented gas cap, yet whenever I open it there's a good amount of pressure. Are they really supposed to build up a lot of pressure before they vent?


It's a '71 non-EEC system.


Thanks!

[domeier]

I don't know what the factory spec was, but my locking cap is vented.

[Lattimer]

In warmer weather with the tank about 3/4 full I get a little sigh of pressure when I open mine.


FWIW
I had a locking vented cap, but it leaked like crazy turning right. Bought a vented non-locking cap from Brothers and no more leak. Cap has only been on there a couple of weeks, but so far I'm happy.

[cdowns]

1971 from the factory were nonvented gascaps// the gastank vented thru the charcoal cannister thru the carb

sounds to me that somebody that doesnot understand the venting concept changed things

any pressure buildup means u either have a defective vented cap or a stock nonvented cap


search this forom this has been covered hundreds of times

[Lattimer]

Originally Posted by cdowns

1971 from the factory were nonvented gascaps// the gastank vented thru the charcoal cannister thru the carb

sounds to me that somebody that doesnot understand the venting concept changed things

any pressure buildup means u either have a defective vented cap or a stock nonvented cap


search this forom this has been covered hundreds of times

I think the C-20 and K-20 didn't change to EEC until later. I believe I have read that the 71-72 C/K-20 and C/K-30 still had a vented cap. Emissions changes usually effect the lighter trucks first and the heavier trucks get left alone for a year or so.

[71K20chevy]

Originally Posted by cdowns

1971 from the factory were nonvented gascaps// the gastank vented thru the charcoal cannister thru the carb

sounds to me that somebody that doesnot understand the venting concept changed things

any pressure buildup means u either have a defective vented cap or a stock nonvented cap


search this forom this has been covered hundreds of times

I've been through this with the EEC before, and my truck doesn't have ANY emissions junk on it. It's not a stock cap, it's the 67-70 vented cap from Classic Parts


And that is correct about 3/4 tons not having EEC.

[cdowns]

i'm sure nuttin has been changed in 45years
fact is you either have defective gascap or wrong gescap those are the reasons you will have pressure


change to a working vented cascap to solve problem

[msgdsrf]

There's a reason (many) gas caps state "caution when opening". Some pressure difference from outside is normal.

[54blackhornet]

I have a 71 GMC with Calf Emmission Crap, IE charcoal canister! I rebuilt the entire fuel system over the winter. New tank,sender, New non-vented cap from NAPA. Now when I crack the cap there is a lot of built up pressure in tank ? Any help in figuring this out would be appreciated ?....Thanks
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[Southpa]

I've been buying locking caps from the local Cdn Tire. They are not vented and on hot days you can smell fumes on occasion if the gasket is not seated properly. I usually buy a couple caps at a time because the gaskets rot, last maybe a year or two. Get the smallest drillbit you can find (I use 1/16") and make a hole, venting problem solved.