69 350 2bbl no AC. Should be vented gas cap, i'm pretty sure? Cap says vented if that means anything. After a few minutes of driving if I open the cap there is a whoosh like it was sealed up and built up pressure. It sucks in. Is it safe to assume that my vented cap is no longer venting both ways properly? The venting valve in the cap moves freely. But the default position appears to be closed, so preventing blowing out as a default. My problem appears to be sucking in. Cap won't help with that? Also I don't see any vent tube near the filler. I have really good records for this car and I don't see that the tank was ever replaced.
I appears to be the opposite? Valve closed. Would let air out if tank built pressure. That is to say it "opens" when you push in on it. Unless there is something I am missing. My thought is to drill a tine hole somewhere in the cap. Bu I know these original vented caps are rare. I would hate to modify one if it doing what it should and I just don't get it. Which wouldn't be that rare of an event.
One of the 69 mid year changes. Early cars had the vent in the filler and later cars had no vent tube but used a vented cap. Sounds like maybe your cap is not working like it's supposed to. Why not buy a new cap? Maybe there are different designs of caps? Some vent air in, some out, some both? I don't have your answer. Just more questions.
It appears there is only one vented cap available. A new cap is $30. That's a bit steep for something that may or may not be working. I hate the "just buy a new one" fix I guess what I need to do is make sure it's vacuum and not pressure out. I thought it was a sucking in, but will confirm. Out valve seems fine. I tested it was some light air. Released easily.
Attached is the description of a vented cap from the '68 Pontiac Diagnosis Manual. Maybe the description of it's function and internals diagram it will help you figure out yours. Note that there is an error in the book. It incorrectly states that only station wagons used vented caps in '68.
That is perfect. Thanks. It also shows one way venting... out. I keep hearing that the cap vents in both directions but this clearly shows that it does not which seems to match my cap exactly. Once again I need to make sure that my "woosh" is air out, and not air in.
The description does not describe the venting into the tank but it must be a function of the cap to allow free air flow into the tank to displace the fuel being pumped out. What they seem to describe is only the relief of excess tank pressure at a threshold level and seal off fuel loss with a full tank sloshing at the cap. Thus, it has to be bidirectional.
I almost regret posting the manual page because it may confuse the issue more than answer it.
That certainly makes sense. There MUST be a system for replacing lost fuel volume. But what I don't understand is how it lets air in? I clearly see the "out" mechanism? I am always fascinated by simple effective, and usually older, engineering.
In any event my cap seems to be working. However on occasions I have opened the cap a felt what I would consider to be too much pressure. I have not associated this with poor performance. Only noticed when getting gas.