Gas cap: Vented vs. Non-vented

[Mytwojeeps2020]

With my new gas cap, I’m getting a lot of pressure built up in the gas tank like I’ve never seen before.

Apparently, my old cap was vented, and this new one is not.

Now, whenever I take the cap off, a solid rush of fumes blows out - even if the Jeep has not been running for for a week or so.

If I put the cap back on (with the engine not running) for 15 seconds or so, when I take it off again, I still get a small rush of fumes.

My engine has also been running rough, and will not hold an idle.

I’m guessing that with the tank pressurized like this, the fuel pump can’t draw gas fully into the carb.

Any thoughts?

[Bruce_W]

If it’s building pressure in the gas tank, it would actually do the fuel pump’s job for it, and possibly overpower the carb’s inlet valve and flood the carb, resulting in a rich mixture and a poor idle. You need a vented cap
  Another problem with a non-vented cap is that the fuel pump will pull a vacuum in the tank and then you get the “failure to deliver fuel “ problem. Air must enter the tank to get fuel out.
BW

[Rus Curtis]

With my new gas cap, I’m getting a lot of pressure built up in the gas tank like I’ve never seen before.

Apparently, my old cap was vented, and this new one is not.

Now, whenever I take the cap off, a solid rush of fumes blows out - even if the Jeep has not been running for for a week or so.

If I put the cap back on (with the engine not running) for 15 seconds or so, when I take it off again, I still get a small rush of fumes.

My engine has also been running rough, and will not hold an idle.

I’m guessing that with the tank pressurized like this, the fuel pump can’t draw gas fully into the carb.

Any thoughts?

If it’s building pressure in the gas tank, it would actually do the fuel pump’s job for it, and possibly overpower the carb’s inlet valve and flood the carb, resulting in a rich mixture and a poor idle. You need a vented cap
  Another problem with a non-vented cap is that the fuel pump will pull a vacuum in the tank and then you get the “failure to deliver fuel “ problem. Air must enter the tank to get fuel out.
BW

I was about to post the same thing as Bruce.  The only thing I'd add is try unscrewing the cap to break the seal while running to see if there's a difference.  My suspicions are that there's a separate issue with your carb and it's just coincidence that they are overlapping.

[Mytwojeeps2020]

Bruce & Rus. Good advice, thanks!  I can’t get back to where it’s stored for two weeks, so I’ll post the results then.

Rus, you’re aware of my previous issue about the gas pump pumping gas into the crankcase, I believe I’m onto a conspiracy.

I bought the new fuel pump (that turned out to be bad) at the same time I bought my new gas cap. This may be a foreign entity trying to disrupt our country: make Jeep owners miserable and thus bring down the country...

[Rus Curtis]

Bruce & Rus. Good advice, thanks!  I can’t get back to where it’s stored for two weeks, so I’ll post the results then.

Rus, you’re aware of my previous issue about the gas pump pumping gas into the crankcase, I believe I’m onto a conspiracy.

I bought the new fuel pump (that turned out to be bad) at the same time I bought my new gas cap. This may be a foreign entity trying to disrupt our country: make Jeep owners miserable and thus bring down the country...

Hmmm.  Interesting.

[Bruce_W]

Don’t tell me, let me guess - Omix-ADA?

[Mytwojeeps2020]

Don’t tell me, let me guess - Omix-ADA?

Good question, but I don’t know.

[Chuck W.]

I am not sure I understand how a gas tank can build pressure with only one line running from it to the fuel pump.  Is the jeep sitting in the hot sun or something that is causing the gas to expand? I would've thought you'd have the opposite problem, a vacuum in the tank if it was un-vented with the engine running.

[PercyUK]

Hi, does it idle and run OK with the cap off altogether?

[Mytwojeeps2020]

I am not sure I understand how a gas tank can build pressure with only one line running from it to the fuel pump.  Is the jeep sitting in the hot sun or something that is causing the gas to expand? I would've thought you'd have the opposite problem, a vacuum in the tank if it was un-vented with the engine running.

No, it doesn’t sit in the sun, and here in Central Florida it is cool most of the time now. It still continues to pressurize even with the engine off and cold.

[Mytwojeeps2020]

Hi, does it idle and run OK with the cap off altogether?

I don’t know. I can’t get back to it for another week to test this theory.

[SteveKfl]

Are you using ethanol by chance?  I've had that happening in gas cans inside my garage with plastic cans that aren't vented.  They can really get puffed up.  I switched to non-alcohol and it isn't so bad.     

[Mytwojeeps2020]

Are you using ethanol by chance?

Nope. I’ve been using ethanol-free gas for a number of years.

I should have more info in a few days (Jeep is stored out of town)

[Mytwojeeps2020]

Problem solved!

Like you suggested, I took off the gas cap and let her run. A little rough for a few minutes, but then the idle returned to normal. I then put my new, vented cap on, and gave it a good run. The pressurization problem is no more. All is right with the world!

My egg timer is back in action!

Thanks, all!

PS: On the “Original technical questions, answers, observations” page, I posted a thread called “Oil filler tube upchucking oil into my oil bath air filter.”

Question for possible discussion: Could the unvented gas cap have caused all those problems? I had replaced the old vented cap with the non-vented cap at the same time I bought the first “new” fuel pump. It could be just a coincidence, but who knows?

[SteveKfl]

Quote... PS: On the “Original technical questions, answers, observations” page, I posted a thread called “Oil filler tube upchucking oil into my oil bath air filter.”

Question for possible discussion: Could the unvented gas cap have caused all those problems?
 
I can't wait to see what the "experts and experienced" respond to that.  Great question.