Vented fuel cap for non-vented gas tank - '72 C20

[540ChevyC20]

Hi All,

My '72 C20 (originally 402 BB) has a non-vented gas tank. I've been trying to find the correct fuel cap for it - it needs to be vented. I've purchased fuel caps both from AutoZone and LMC truck - both look identical - see pic below. Neither seem to be vented as every time I go to fill up with gas there's a loud "whoosh" as I remove the fuel cap. Would somebody be able to point me to the correct, vented fuel cap for a non-vented gas tank? A pic would be hugely appreciated too if you have one. Thanks!

[72c20customcamper]

NOS originals show up from time to time on the sale forum I bought two last year

[70STOVEBOLT]

Your pic looks like a vented cap to me. Same style as on my '70.

[sixty8HDC30]

Like 70stovebolt said, it is a vented cap. however it's only vented one way meaning the tank can build pressure but it wont hold vacuum. This is correct for our trucks and most vehicles. when you take a newer vehicle for a smog test they always check that the gas tank and the gas cap hold pressure, hearing a whooshing sound when you remove the gas cap is normal.

[72c20customcamper]

I had one of those “vented “ caps on my 68 it built up so much pressure I heard the tank bulge . Made a hell of loud bang I took it off and found some NOS ones
This is the one I had I tried two of them same result way to much pressure.

[71CHEVYSHORTBED402]

I had the impression 1972 models had "vented" caps. I know my 1971 EEC didn't, vapors captured by the canister and so forth.

The 70-71 EEC called for a different cap than 1972, however, that doesn't mean 72 weren't built using up 71 stock.

FWW, I had a 72 402/400, but I didn't study the tank. It did blow vapor after installing an aftermarket, I just got a dependable aftermarket instead, with no problems. It was a driver, so I only cared it worked. It was a "vented cap". If it had a canister, which I'm positive it once did, everything vapor had been removed. Regardless as mentioned, 1971 Vs 1972 caps differed. For at least some applications, anyway, because the 72 caps "vented".

Check your vapor lines and canister BTW?

Here's what I think is the best explanation for the "unvented" 1970-71 caps:

"the cap has a pressure check valve; the cap allows air into the tank as fuel is used, and only releases air when pressure rises above 3-4 PSI. Fuel vapor otherwise expands into the vapor canister.".... Note trucks with lower side moldings call for a stainless fuel cap. Otherwise the caps were painted, though perhaps not as a rule.

[1970cstblazer]

The aftermarket caps are trash. The only ones that work as designed are GM NOS. The 1972 3/4 tons and up use the same caps as the non-RPO NA9 evap emission control 1970 and older trucks.

[HO455]

This thread should help you ID the cap you need. Member ClassicBowtie may have one in his stock.
Good luck!

https://67-72chevytrucks.com/vboard/....php?p=9186244

[72c20customcamper]

My 72 c20has a vented cap, no canister

[540ChevyC20]

Thanks for all the replies and useful info guys! In fact I found what I believe is an original vented cap on Ebay - it's on its way to me as we speak. It's not NOS and the rubber gasket does appear to have some cracks so I'll just have to see if it leaks. I do like the look of the locking cap in the thread referred to above - that's definitely my fallback plan.

[Steeveedee]

Every locking cap I've bought, the gasket turned to feces in a couple of years and leaked badly. Something to think about. I have a "vented" cap that does hold some pressure. I recall a thread around here where a member reduced the vent spring pressure.

[PbFut]

I think the vent spring was me. I opened one up as I was having issue with the tank bulging. You could hear it pop out. Opening the cap proved too much pressure. Upon opening the cap I had, there were two springs in it. Set up sort of like a engine valve with the inner vibration spring. I removed the stiffer of the two springs and left one remaining. It seemed provide good result. No leaking cap coming up and down the twisty hill to the house and no pressure or vacuum build up. I would be careful modifying these caps and pay very close attention to tank pressure after doing so. Things can go really bad pretty fast if the cap is not working correctly. Think of how bad it would be to build pressure and have an old rubber line split.
Also, the gasket on new ones can make removing and putting cap back on difficult and likely damaging to the gasket. A very light thin swear of silicone grease on the neck and gasket works wonders.