Whaleman

My 96 had a lot of pressure build up. Replaced the cap with an Ebay locking gas cap and no more pressure buildup. Dan

PatternDayTrader

Lol ... yeah I'm sure.

aklim

IOW the cap is venting. Assuming that the pressure is caused by gasoline vapors I believe that you are venting slightly more gasoline without the pressure being on it.

OlrustysGhost

I wonder, I buy a cap and install a gauge in it , I could check it that way? 0-40 maybe?

aklim

You could but I am not sure what pressure is normal

Ziggy*

My 86 was building up a bunch of pressure in tank. The vent hose on top left of tank that runs along the left side of car up to the Evap canister was clogged. I pulled the vent hose off tank, unhooked hose on left side of engine bay an blew compressed air from front to back. mine turned out to be small 4" piece of hose was collapsed at rear in front of left rear wheel where it comes from tank to line that runs along frame....Pressure problem fixed

auggy

Funny thing is releasing the pressure causes a vapor lock. When you release it quickly it lets the warm/hot gas boil just like opening a soda bottle. Happened to me once in the summer but the car started after 20 seconds or so. I open cap more slowly and that vapor is scary and a real concern. Perfect for an explosion. I would try running the car with the cap slightly opened and see what happens. Might be a clue. Then again quite possible the tank pressure has nothing to do with your problem which I am inclined to believe.

Tom400CFI

How can that happen? The fuel from the pump to the regulator should still be under pressure.

BrandensBeast

My 89 has a ton of built up pressure in the tank no matter the temp or weather, kind of let it slowly let out, almost seems like the top will pop off if I just let it go! My truck never has any pressure in the tank.

Cliff Harris

I've only seen pressure in the tank when the car/tank is hot. I always drive directly from my house to the gas station when I fill up and I have never seen pressure in the tank.

Maybe this thread would be helpful:

https://www.corvetteforum.com/forums...nk-normal.html

RWDsmoke

This may sound crazy but it's really hard to type wearing a straight jacket. When you loosen the gas cap and hear the whoosh of air equalizing the pressure, it's really hard to tell if the air is going in or out. After reading all of this and checking things on my 86 I have a better understanding of this system. I removed my tank pressure valve and pulled a vacuum on the tank side with my mityvac. I now realize air can not go into the tank through the tank vent line so it has to enter through the cap. They must let air in but not out. I looked up gas caps at oreilly auto parts and they list the gas cap as vented but we know they are not. On advance auto they list two caps as correct for my 86. Both say non vented but one says vacuum: yes and the other says vacuum: no. I'm confused with their terminology.

If you have the wrong gas cap or a defective one it would cause a high negative pressure in the tank, it looks to me like fuel pressure would drop because of the negative pressure on back side of the fuel pressure regulator diaphragm. The differential pressure would still be the same but discharge pressure would be less. That could explain why loosening the gas cap to equalize the pressure would make the car run good again. Does anyone understand what I am trying to say or did I eat too many sugar cookies and I'm hallucinating?

PatternDayTrader

The gas cap is vented in such a way as to allow air in but not out. There is a threshold that must be reached before air is allowed in. So depending on the circumstances, you may get a very small vacuum in the tank, provided the purge solenoid functions properly. The other 99% of the time you end up with some amount of pressure in the tank. The question then becomes how much is to much, and is it possible for this to cause the OP problem. In my opinion the answer is no it will not cause the OP problem. The fuel pump is a (for lack of a better description) a positive displacement pump, therefore neither negative pressure or positive pressure in the tank, will change the pump performance in a meaningful way. In the event there is actually a relationship between this guys complaint and the twisting of the fuel cap, then it is much more likely to be a bad connection at the fuel sender/pump assembly, either in the tank or on the outside, and that connection is being disturbed in an almost imperceptible way once you start messing with the fuel cap. The pressure in the tank is pure happenstance.

aklim

That would be true EXCEPT for the fact that we are not talking about an air tank but a gas tank. If it pushes air out, you can bet it has a lot of gasoline fumes so you will smell it. Want to be absolutely sure? Put a match and release.

PatternDayTrader

On the surface this idea seems to make sense, the pressure regulator diaphragm has manifold pressure on the opposite side of the fuel and not return line pressure or tank pressure. I guess I'm not clear on which side of the regulator diaphragm your referring to. It may not be impossible for excessive pressure in the tank to affect fuel pressure.

maddogwyatt

This could be the answer. I once had a new Alfa Romeo that drove fine until you drove at freeway speeds for a few miles. It would die but recover after a few minutes. If I removed the gas cap, air rushed in to the tank indicating a vacuum. The vent was 90% blocked by underseal.

Whaleman

I am sorry, I do not understand this response. Would you be kind enough to explain? Thank You, Dan

TA

Look at the posts by 1fatcat. I would bet the same hose is clogged on your car. There's been many threads about it here over the years.

PatternDayTrader

Your locking gas cap is basically vented in both directions. Meaning, it will allow air in, and vapor out. Its not supposed to do this, and probably says something to the contrary on the box, but I've never seen a locking cap that works the way its supposed to. Don't take that the wrong way though; if you need to be using a locking cap for some reason, then use it without question. I would.

biggiefl

I have seen 2 gas tanks implode from vacuum. One was on a 1995 Jag XJ6 and the other a 200 Benz CLK430. I think they were plastic but the tanks had to be replaced they imploded so bad. Get the vent fixed. Checked mine twice on Monday, not a noise in both sun or shade.