Chevy Blazer Gas Cap Vent

[Kyle K]

Hey folks, simple question here. I have a 1973 Chevy Blazer, standard tank with main hose to fuel filler, and secondary hose to fuel tank vent hose (see image when it was on the ground a year ago). I put in gas about three weeks ago, been driving around, no issues.

Yesterday I'm driving a decent amount (20 so start/stop miles), and notice this whistling sound, almost like a train. Could figure it out for the life of me, till I get out of truck when it's turn off and I hear the gas cap venting very loudly.

I have a locking cap which I like (see image). Shouldn't this vent to release pressure? I'm not sure if that's what it was trying to do because it's supposed to, or because it was forcing it. The cap says it fits a 73.

I understand the vent when filling (no issue there). I'm trying to understand this pressure building up in the tank when driving, and what the cap should be doing to release pressure.

Appreciate the help! Images attached.

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[bucket]

My '77 would do that. The exhaust ended right before the rear axle and it would heat up the tank. I dropped the tank to replace the rotten fuel hose and found my tank vent (mine was mounted right to the sender basically) was clogged up from a mud dobber nest. I cleaned it out and put it all back together and there were no tank pressure issues after that. Even with the same exhaust.

 

[PrairieDrifter]

Had the same type of issue on my daily. The evap vent and probably whole box was plugged and started to create a vacuum on the tank.

 

[Kyle K]

Thanks @bucket and @PrairieDrifter, I'll take a look at the vent hose. I replaced the hose last year, but maybe the metal section was clogged there.

Question on the cap...is it supposed to vent out, and let pressure out? Because when that cap is on, it looks like a full seal all the way around. And then even if the vent tube is clear, it still has nowhere to go after that. Does it vent out through the center hole when looking at the backside?

 

[PrairieDrifter]

Thanks @bucket and @PrairieDrifter, I'll take a look at the vent hose. I replaced the hose last year, but maybe the metal section was clogged there.

Question on the cap...is it supposed to vent out, and let pressure out? Because when that cap is on, it looks like a full seal all the way around. And then even if the vent tube is clear, it still has nowhere to go after that. Does it vent out through the center hole when looking at the backside?

I have no idea with locking gas caps. I've never liked em, factory caps are supposed to vent I believe, some don't I think as well.

 

[Kyle K]

Ok, I feel that I have learned a few things after some more googling. As you see from my tank pic, I have one fuel line going to fuel pump (green zip tie) and one return line from the fuel pump (red zip tie). The other large hose is obviously the fill tube. The other hose connects to the metal filler tube. I was calling this my "vent", but I think I stated incorrectly. I know this hose is needed when filling up gas (gas in, so air can come out). But when rubber seal of cap is on, both the main rubber filler hose and this hose are essentially sealed off to the outside world.

I do NOT have another hose or line running up to a charcoal canister, or any other check valve or connection to the carb. I'm feeling like that's my problem. I'm bet the new Stant cap I've got is "venting", but it only vents one way (allowing air in when gas gets sucked up by fuel pump). I don't think it is allowing any vapors to escape out when the gas is expanding and creating pressure due to heat and such. Hence the train whistle coming out of the cap...I'm betting that was safety measure of cap, but probably not how it wants pressure to be relieved. Correct me if my working assumption is wrong here.

So if this is correct above, is anyone NOT running charcoal canister? From what I've read, the ones that aren't have drilled a hole in their gas cap to vent out some.

@PrairieDrifter and @bucket, when you said your "tank vent was clogged", was that vent going to a canister or somewhere else? Or going to the metal filler neck like in my image? Thanks all.

 

[bucket]

The cap should not be vented, I don't believe.

My '77 does not have a return hose. It simply has the supply line and a vent. I don't know if that is original or not, but I'm pretty sure it's original. It's a vent like GM used for for axle housing breathers.

 

[sirweesarunch]

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Heres my tank. The filler neck and hose go to where you see the two bungs with blue tape. On the sender unit if you zoom in you will see the 3/8” deliver to pump , 1/4” return line back to tank and the 5/16” center one has a plastic vent trap. Tanks need air in as fuel leaves so my guess is you require a vented cap because you dont have the vent tube on your sending unit.


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[Raider L]

@Kyle K,

Kyle, I do have a locking cap on my truck, but I also have a cannister going back to the tank.

But here's the question does your tank have three tubes coming off it or two? Your '73 may, may only have two and that would be for one hose going to the fuel pump and one coming back from the carb. Or if you have a three tube sending unit it will have three tubes, one for the fuel going to the fuel pump, and two coming back from the cannister. The cannister will draw vacuum from the carb which sucks vapor from the carb back to the tank. Actually the vapor would be coming from the intake manifold but with a third hose coming off the carb. back to the cannister, and two of those will return vapor back to the tank. It's all a bunch of EPA bull crap.