What's the function of vacuum in a gas tank?

Or is it pressure? I kow I hear a "whoosh" whenever I open the tank.

It is not a vacuum. It is pressure. It is not particularly regulated as it is produced by the fuel vaporizing which is a characteristic of temperature and fuel blend. In modern cars, the pressure is allowed to build with the excess vapors being absorbed by the fuel vapor emission system. The vapors are routed to the engine when running to be burned along with the normal fuel provided. The fuel evaporative emission control system "can" be damaged (over filled) if the fuel tank is over filled. That is why most, if not all, manufacturers recommend filling the tank until it clicks off the first time.

Lugnut

=============== ===============

google is yer friend.............

and yes Virginia ....there is a vacuum....

.

lugnut wrote in news: snipped-for-privacy@4ax.com:

It's my understanding that late-type EVAP systems apply vacuum as part of the on-board testing process. And I'd think that during purge you could have below atmospheric in the tank, since the engine is drawing vapors at that time.

There is probably a vacuum drawn while the engine is running at least for a short time to test the integrity of the retention of the system. I may have misunderstood the OP or drawn an unwarranted conclusion, but I was thinking of the whoosh you sometimes hear when you remove the cap. The main concern of the system is to entrap the fuel vapor which produces positive pressure. If the vapor did not produce pressure, there would be no concern for evaporative emissions. A vacuum may lend to evacuation of the vapors, but it would be a pretty weak vacuum. A continuous strong vacuum would boil off more vapors which are more difficult for the system to control with respect to fuel charge compensation. You do not want to hinder the work of the fuel pump or cause the tank sides to collapse which may result in fuel being introduced into the vapor system. IIRC, the vapor system has a device to prevent this that works something like a floating ball valve. As I understand, one of the big vapor problems is the increased Reed vapor pressures with modern fuels. Carbs can become very finnicky about high vapor pressures along with an increased propensity to vapor lock.

I hope my response is not too senile. I sometimes confuse myself.

Lugnut

Mopars with ldp use pressure to test. All systems should be open to atmosphere (through the charcoal canister) except while the obd2 testing is being done by the evap monitor.

It is not desirable to have either pressure or a vacuum in a gas tank. The reason that both do OCCUR under real world conditions is because there are evaporative emission controls that try to limit how much gasoline vapor escapes into the atmosphere. As a result, the systems prevent free exchange of vapor in and out of the tank, instead forcing vapors to flow slowly through an activated charcoal canister where they are absorbed, to later be drawn off and burned by the engine. Under certain conditions, this slightly pressurizes the tank. Under others, it places a slight vaccum on the tank. There is a relief valve, typically in the gas cap, which prevents too much pressure differential in either direction.

lugnut wrote in news: snipped-for-privacy@4ax.com:

The late "intrusive" systems do that (~'03 and up).

Earlier systems simply expect something other than atmospheric (up or down from atm.). Early OBD-II EVAP does not deliberately create a vacuum as part of its testing.

Yes.

Yes. Gas sitting all by lonely self, as well as when it's heated by ambient temperatures, generates all kinds of footloose vapor. EPA bureaucrats figure this is worse than death or vampires, and so must be fanatically controlled almost to the very molecule. Nobody else in the entire world thinks like the people working for the EPA.

It is. It's just intake manifold depression relative to atmospheric. That's how the purge cycle is effected. I guess if you were to get really pedantic about it, you could say atmospheric PUSHES the vapors towards the below- atmospheric intake. IOW, you can't really suck, but you can blow. Think about that one for a sec...

Steve Austin wrote in news:49426b36$0$12987$ snipped-for-privacy@news.westelcom.com:

So those vehicles _pressurize_ the EVAP, then measure pressure _drop_?

I thought they all pulled a slight vacuum, then checked for pressure _rise_.

I thought all later "intrusive" types closed off the vent entirely when not actually purging.

The latest type systems are natural vacuum that tests after the car shuts off.

Steve Austin wrote in news:494542c0$0$12958$ snipped-for-privacy@news.westelcom.com:

Hm. Right now I'm looking at a Toyota document on EVAP. The document covers models up to 2001.

According to this paper, vacuum retention/leak detection on Toyotas is tested immediately after the purge operation, while the engine is running. IOW, purge operation is what creates the vacuum used by the PCM when it checks the integrity of the system.

Personally, I'd rather do evap work on anything than a Toyota. The mechanical part of the system is too complicated, and the computer controlled part of the system is too simple.