Gas Tank Gasping for Air

I ran my 96 RMW pretty low on fuel this evening which I rarely do because the gauge doesn't read very accurately. When I removed the gas cap there was a large gasp of air that lasted several seconds. I replaced the cap with a Stant cap shortly after getting the car because the old cap was one of those with a trap door that you don't have to remove to refuel. The spring had let loose and was venting fumes. Anyway, I've heard gas caps do this before but not as severely. Is it likely that the cap is just junk and I need another one, or might there be something else going on?

If it's just the cap, can you recommend a good brand? I thought Stant was a good brand but looks like maybe not.

Mines doing this on the 92 OCC, Only filled it up three times and haven't really thought about it. I think the owner's manual says something about it, or maybe the cap itself about the release of vapor.

Old cars in the pre smog days had caps vented to the atmosphere. Vapors could come and go as they pleased. But this isn't good for pollution, so they started using caps with a one way vent valve in them. Air could come in but vapors couldn't get out. Instead, those vapors get pulled into the charcoal canister under the battery to be stored and later burned thru the engine.

It seems like the cap isn't letting air in though, so the pump is pulling a vacuum on the tank as gasoline gets used up. Unless There's some sort of situation where the venting system leading to the charcoal can is causing something weird, then it has to be the cap. My question is what word thing could be happening if not the cap.

Maybe your wagon cant handle spicy gas anymore!LOLThat corn gas makes my exhaust smell like egg salad!

It is normal for the tank to be pressurized to a small extent. When you open the cap it should vent the pressure. Stant is a good cap. OEM caps were designed for the car.

That's the thing: it didn't seem "to a small extent". I heard a large hiss for probably 4 or 5 seconds that's never hastened before. I've heard gas tanks do this before but not this much. Thinking back though I'm not sure if it was air rushing in or air rushing out. It's there any way a mis-routed or plugged vent hose could cause this? Is there a check valve in the canister system that could be malfunctioning?

First figure out if it was pressure or vacuum.

Assuming it's a pressure build-up, bear in mind that at this time of year stations are still pumping winter grade gasoline, but temperatures may be warmer. Winter grade gasoline is more volatile with a lower vapour pressure to improve performance in cold weather.

If it was vacuum, I'd think you'd have performance issues, like the fuel pump was failing.

4 or 5 seconds is a REALLY long time...some of mine would hiss when I open it as it was letting air in, but usually around a second tops...

Exactly! It seemed very strange to me as well, hence the question. I suppose I might have unscrewed it slower than normal once I heard the hiss, which might have prolonged it. I'm going to run the tank down and see what happens next fill up.

I never gave it much thought until I read this, but my 96 Roady wagon also does a 4-5 second hiss when I fill. I just filled it today, so I remember clearly. I also occasionally get a gasoline smell in the cabin, not really strong, but still there. I can't find any leaks on the engine or under the car, but it has me curious/concerned.

Mark, I was getting a strong gasoline smell in my car too. It happened most frequently when the outside temperature was really cold. Turns out that my fuel pressure regulator would leak externally when cold. It was a small leak but the fumes would get in the car easily because the FPR is so close to the cowl.

The gas smell I get is intermittent. And, since I'm in south Florida, cold weather is not a factor for me. Where is the fuel pressure regulator? I'd feel better if I could find out where the smell is coming from, especially since in the next 5-6 weeks I'll be driving the Roady back north for the summer to Pittsburgh.

It's doing it again. I drove about 45 miles in the interstate today. When I got out of the car I noticed a fuel smell. I didn't see any obvious leaks under the car. I unscrewed the gas cap quickly and it spewed out vapor for several seconds. So it's definitely under pressure and not vacuum. After replacing the cap I let the car sit for about 3 hours in the sun. I then moved the car about 50 yards and left it running to cool off while I was loading up all the kid's junk. I opened the cap again while the engine was running and it was under pressure once again. Drove home 45 miles and it was pressurized again. I'm going to try to get a video of removing the gas cap so you guys can see that I'm not exaggerating about the amount of time it belches.

So, there may be something to the winter gas theory, since it was in the high 70's today. However, I'm still wondering if something else is wrong. My gut says yes. Isn't there a vent solenoid near the tank and also a purge solenoid somewhere up front? Could a malfunction with one of these, or a clogged line be causing this?

One last clue, and it might be meaningless, I seem to remember looking at the gas gauge before I left and thought I had plenty of gas left. But after driving it that first 45 miles the tank was reading considerably lower than what I'd expect for that distance.

HoosierDaddy wrote:

Isn't there a vent solenoid near the tank and also a purge solenoid somewhere up front? Could a malfunction with one of these, or a clogged line be causing this?

Yes, yes and possibly, but on a 96 (OBDII) it should throw a code if there's a problem. The Evap system is supposed to capture vapours and burn them before they can escape. The EVAP vent solenoid controls the release of vapours from the tank, the purge solenoid admits vapours into the manifold and the gas tank pressure sensor monitors everything.

When I stuck my hand over the filler neck to check if it was pressure or vacuum I noticed the vapor coming out was warm. I suppose it's warm because of heat from the pump itself and maybe it gets warmed near the engine then gets returned, plus the warm ancient temperature. The tank was reading less than 1/4 full at the time, so there's lots of headroom to build up pressure.

FWIW, an internet search yielded this result. The responder seems to say it's normal.

Would the appropriate OBD2 codes be the "silent" type that don't throw the check engine light on?

Oops! Forgot to attach the link

http://buickforums.com/forums/threads/36696-gas-tank-pressure-build-up

(And I meant ambient not ancient)

Sadly, there are some Darwin Award aspirants who might actually try that!

HoosierDaddy wrote:

Would the appropriate OBD2 codes be the "silent" type that don't throw the check engine light on?

No, EVAP codes are emissions related, and therefore should trigger the SES light.

I thought that's how it was supposed to work. Mine didn't start with the pressure release until last year when I replaced the vent hoses to the tank. Car runs fine otherwise.

I think I have had this problem as well. Sometimes I smell fumes around the back of the car, then I can crack the cap open and have it vent for 30 seconds or more. I replaced the gas cap last year and it seemed to help, but it is doing it bad again. The clogged tank vent makes sense. Is there a way to fix this that doesn't involve dropping the tank?