6T7L71CPE

After a 20 minute drive and only 70 deg. outside, I opened my gas tank and got a blast of pressure release. I don't recall this happening before esp.since it's a "VENTED" cap. I've been having a problem with my carbs base gaskets getting soaked since I started using the car this year. And, this time the gaskets didn't get soaked! So, about these midyear "VENTED" caps. I searched this topic and found some discussion but nothing detailed. I tested the cap: Right side up it appears to draw air into the tank freely and out of the tank with some resistance. It will largely shut off flow outward if the pressure is too high. Upside down it shuts off the flow out of the tank, I assume this is the rollover safety feature. It basically acts like the ball valve in the Shop Vac. My question is, should it have more resistance EXHALING than INHALING?? My guess is it's rusting inside and not functioning as designed.
Thanks

SWCDuke

All C2s use a vented cap. The non-vented cap with pressure and vacuum release valves did not come into being until the advent of the evaporative emission control system that I think was first used on the 1970 model year.

I'm not aware that any GM non-vented caps have a "rollover check valve". I checked my SWC and it flows about the same either way - right side up or upside down. It's just a simple labyrinth vent - no valves or other moving parts.

Try soaking the cap in mineral spirits or carb cleaner It may just be clogged with dirt.

Duke

6T7L71CPE

Thanks Duke, I examined the cap again and it's more than corrosion or dirt in there moving around. Maybe the baffels have collapsed?? I'll just order a new one and then dissect the old one to see what's up with it.

6T7L71CPE

I should have done my home work first... Noland Adams page 93! It shows the late 63, and forward I guess, has a pressure valve and vacuum valve in it.

SWCDuke

Well, that's interesting! There is one cap shown in my '77 vintage parts manual for all 63-69, 3952708 with a note that it is vented, which is later than the design with valves shown in Noland's book.

I always thought a cap with valves was considered "non-vented", but I guess not.

Having pressure and vacuum relief valves is a better design because it will help prevent loss of the light end fuel components for cars that aren't driven much and will also help prevent moisture from enterning the system if the car when temperatures drop to the dew point.

Duke

6T7L71CPE

The gas door on my '67 is a late '63 door. The CORVETTE script on the back is pre '67. This gas cap may have come from a '63. E-bay here we come, "EXTREMELY RARE, LOW ORIGINAL MILES, NUMBERS MATCHING, EARLY CORVETTE gas cap, once owned by......."

SWCDuke

Well, if you end up doing a "destructive failure analysis" by cutting it up, I hope you take photos and post, so I can see how they look on the inside and whether or not yours has the pressure and vacuum relief valves.

In looking at the drawings in Nolands book there did not seem to be any identifying marks to distinguish a cap with valves to one without valves.

Looking at my '63 cap (no valves) I can only see one identifying mark - a small stylized "WS" inside a circle on the bottom side, which I assume is the manufacturers logo, but I don't know who it might be.

I wonder if there is any visual way to distinguish them or if you have to actually check whether they have valves with a vacuum or pressure tester.

Duke

nassau66427

The small stylized "WS" could be "SM". It all depends on how you look at it. I'm clueless as to the meaning. S.... Mfg. ?
Just shook my cap and it rattled. I held it up over my head and lightly blew into the hole in the bottom of the cap - you could hear the air escaping. Gave it a hard puff and you could hear the rattly thing moving and no air came out.

clem zahrobsky

most caps marked "vented" are air in only and cause pressure build up in the tanks causing the 3X2 setups on sharks to fill up the engine with gasoline because the heat from the mufflers added to the problem. we drilled a 1/8" hole on the caps to prevents this

nassau66427

Getting back to your original question - the cap should have equal resistance "inhaling" and "exhaling" provided you do so gently. The blast of pressure release you experienced indicates your cap is not venting properly. As an aside, a non venting cap on a mid-year can also lead to fuel starvation .

SWCDuke

Not necessarily true. The early '63 cap just had a labyrinth vent to the atmosphere with no valves - a true "vented" cap. Late '63 and later caps were "vented" but only above a certain pressure or vacuum.

All cars with evaporative emissions systems have pressure and vacuum relief valves - usually built into the cap, but they can be on the tank with a non-venting cap. The specs are not easy to find, but they are generally in the range of one psi, but not necessarily the same for both pressure and vacuum.

Duke

nassau66427

Dear Sir; please elaborate - what part of my statement was not neccessarily true. Also, aren't the caps providing a vent "below" a certain pressure and vacuum as opposed to "above". Larry

clem zahrobsky

duke the caps marked "vented" used on the 68 sharks if you would "suck" on the inside of the cap would let air in but if you "blew" at the same location the air would not go out. i also noticed the the new mid years ones i owned when you removed the cap you would get a rush of air from the tank. i never checked these for what type of venting till some people with 68 had trouble with the engine crankcase filling up with gasoline. this also happened to 67s with 3X2 carbs but we never checked anything out with them as we just thought it was a carb problem

6T7L71CPE

The flooding is exactly what is happening to my 3x2's. (Why this is worse for the Holley 2bbl vs the 4bbl escapes me) For whatever reason it seems worse this year on mine. It doesn't appear from the discussion that a new repo cap would cure this problem. I'll look into the "vented" locking caps to see if they have the valves in them or are like the early caps with a labyrinth. Or drill a hole in my cap as Clem described. And it's time to change the oil since it may be a little dilute!

Thanks

SWCDuke

The drawing of the late '63 cap in Nolan's book clearly shows both vacuum and pressure relief valves, but the values are not necessarily the same. My understanding is that the range is typically one to two psi and the pressure relief delta is a little higher than the vacuum relief delta.

The only way to test them is to have both vacuum and pressure pumps and the specification for the valves, which is not generally published.

Part of CA emission test procedure is testing the cap's pressure holding capability, so there is a "one for all" minimum pressure holding spec.

A lot of people notice a "whooshing" sound on modern cars when they crack open the fuel fill cap and think it's escaping pressure, but in my experience, it's always been air flowing into the tank due to a slight vacuum.

Duke

nassau66427

Yes, a functioning non '63 "vented" cap will allow an outflow of air.

JohnZ

The new/NOS late "Vented" cap on my '67 rattles nicely and will allow air to flow IN (so the tank doesn't collapse), but will NOT let air flow out at all. The "SM" stamped on the inside is for Stant Manufacturing, who made them.

SWCDuke

John - look at the cap drawings on page 93 of Noland's Vol. II. No specs listed, but the late '63 cap shows both vacuum and pressure relief valves. Just from observation of the spring geometry I would opine that the delta for pressure relief is greater than for vacuum relief.

The part number changed again in later years, but I just can't imagine there is no pressure relief valve built into later caps, unless it was incorporated somewhere else in the fuel supply system.

Yeah - Stant - I should have figured that one out.

Duke

clem zahrobsky

unless you race your car i would change to .097 dia needle seats from the .110 that is stock because they are easier to seal because of the smaller area being pressurized.

JohnZ

The same second-design cap (3843576) was used from late '63 through '67. I see the pressure/vacuum relief valve in the drawing too, but mine has no pressure relief function (at least blowing as hard as I can on the opening). The rusty old cap I replaced was the same way.