72 C20 in-cab fuel tank head scratcher (non-EEC, vented cap)

[jocko]

Bought this tank from Classic Parts for the 72 C20 and the ad stated that “thou shalt” use this gas cap (for a 71 non-EEC).

From the ad: Gas Tank- w/o EEC-New for 1968-72 Chevy Truck:
***Note: Does not work on 70-72 with EEC*** Includes sending unit gasket and lock ring. Must use our number 23-108 gas cap.

So.. I bought that cap to go with it, ugly warning sticker on it and all.
The ad states the cap is vented, which is what I need with this 72 C20 non-EEC setup (and I was under the impression ALL C20/30 were non-EEC tanks?? - regardless, the one I removed was identical to the one I installed). Tank plumbed correctly with return line, etc.

All is installed and truck is running again – BUT.. it does not appear to be a vented cap – when I remove the cap after the truck has been running for a bit, you can hear the tank hiss loudly and release the vacuum and “de-imploding.” So, I HAVE a vented cap (which may be defective, gonna try another) – not sure what else my issue might be.
Any thoughts?

[Sheepdip]

1/8" drill bit centered thru the internal wall will make it vent. Be sure and blow out the metal shavings.

[ochremo]

You can take the cap apart and shorten the spring to take some tension off to allow it to vent at a lower pressure. It took me two tries to get it just where I wanted it.

[jocko]

Thank you both for your replies, very much appreciated. I'll give these a whirl - both make sense to me and I have an extra cap to experiment on. Was really hoping to make it work like it did 50+ years ago with out of the box parts - also wanted to make sure I wasn't missing something simple in the process, but it sounds like this may be a more common problem than I assumed. Thanks again gents!

[lil hoodlum]

Both of my 68's have the vented gas caps. My C10 is my daily driver and I have replaced several times with new vented gas caps from various auto parts places, all of the new gas caps have held vacuum as indicated by the wooshing effect by taking off the gas cap when refueling. Didn't really have any real issues but I knew it wasn't supposed to be holding vacuum as well. I finally just used an old gas cap from the salvage yard and no more whoosing of the gas tank when taking off the gas cap.


I'll investigate further with a new gas cap about the spring to soften it up.

Basically I'm saying, the new gas caps aren't functioning as they should.

[ochremo]

I bought several supposedly vented caps for my '72 thinking I'd get a correct one but they were all the same. I couldn't blow through any of them that's when I decided to take one apart as I had several. I shortened the spring until I could get some air through it.

[jocko]

Thanks again for sharing your experiences with these - had no idea how prevalent this was. lil hoodlum, I have an old beat up cap (matches the truck...) that I removed from the old tank - all the rubber, what's left of it is worn out - and it "vents" just fine - I stuck that back on there the other day as an interim measure while I pinged the crowd to see what other folks may have seen. Just might be the easy button, new certainly isn't always better. Appreciate all the good ideas, have a few new caps (all have the same problem...) and one old original, am going to try some of these ideas just to force the new caps to work.

[jocko]

Thanks again for the great suggestions, really appreciate the help. I ended up trying something else that seems to have worked so far. But will keep the spring and drill approaches in my hit pocket in case it doesn't. I was fortunate enough to have the original cap, but figured I'd "follow the instructions" and do what Classic Parts recommended in regard to the cap for the replacement tank. I bought a couple of them so I'd have matching caps with the ugly borderlie unremoveable stupid sticker so my NL2 aux tank could look equally hideous. Anyway, I tried both caps on the main tank, neither worked. My original cap had a completely hardened and useless original seal on it. So I took a shot at getting that crusty old seal off and finally got it all out - then pulled the rubber off the otherwise useless new cap and transferred it to the old cap. Boom, seems to be working like a champ. The seal isnt' a perfect fit, but it's close enough for government work - and I'm glad I got to keep the original cap in the process.

[ochremo]

Great to hear glad you found a solution. And yes mine has the ugly sticker also that I've not tried getting off yet.

[PbFut]

Good to see you have a solution. Also, if the cap is a bit difficult to turn, a little bit of silicone grease on the seal will free things up nicely. I had a cap the PO installed. Did the trial and no work on a couple caps as others have done. My caps had 2 springs in them. I removed the stiffer spring and it seems to work well. Venting as needed without any spillage.

[jocko]

Originally Posted by PbFut

Good to see you have a solution. Also, if the cap is a bit difficult to turn, a little bit of silicone grease on the seal will free things up nicely. I had a cap the PO installed. Did the trial and no work on a couple caps as others have done. My caps had 2 springs in them. I removed the stiffer spring and it seems to work well. Venting as needed without any spillage.

Thanks for that, good tip. My old one is snug but not bad at all. The new (bad) ones were way, way too tight - had I modified one of them, the grease would have been a big help. Good to know for the future, thanks again.

[zicc1835]

I had the same problem PO installed new tank and with that cap with the warning on it and wasn't venting this one solved my problem

CJ Pony Parts

Description Price Qty Total

1938-1971 CHEVY/GMC C10 TRUCK CHROME NON LOCKING VENTED GAS CAP
Part #: GC113
$12.99 1 $12.99

[jocko]

Thanks for sharing!