ok...this is kgb's answer. it's kinda long. did i miss anything? here goes.......
the EVAP system was put in place to prevent HC emissions.
the fuel tank venting to the atmosphere was found to be the biggest single cause of hydrocarbon emissions from a motor vehicle(actually putting gas in your car was #2, now#1).
gasoline is a naturally cold liquid. the reason for this, i admit that i don't know, but i know it stays cold. it evaporates rather easily, and quickly. this also caused motorists to waste considerable money, as it was literally evaporating out of their cars.
enter the EVAP system. early systems were pretty simple. a charcoal canister, a purge control valve, and a vent control valve. they were also operated by vacuum. it worked, but not always as it should, as evidenced by the early 80's cadillacs that had a tendency to suck the tank in on itself when something failed in the system.
the EVAP systems main purpose is to prevent system leaks, resulting in hydrocarbons entering the atmosphere. as the fuel evaporates(from whatever cause, be it temp., moving the vehicle, etc), it is captured into the charcoal canister. it is then stored there until such time as it can be released into the engines air intake system, and burned. again, early systems worked, but not nearly as efficiently as they do today.
they were, and still are prone to leaks. the older systems really had no way of self testing for this. the modern systems test for this often. they can/will test while the car is driving, and then can/will also test while the car is turned off and parked.
the engine control system(OBD2) has several codes for the EVAP system. it can detect a leak down to smaller than .020 of an inch. these leaks are classified as small, medium, large, and gross. i forget the exact numbers, but i'm doing this out of memory.
as fuel is taken out of the fuel tank, something must replace it. air is vented in as this happens.
again....on older systems, this was done differently. fuel filler caps were sealed to prevent leaks, but would allow air(much like a 1 way vacuum check valve) to enter. these were always the culprits on the old cadillacs i keep referring to.
on our modern cars, this is done through the charcoal canister, hence the vent control valve on this canister.
when the EEC wants to run a pressure or vacuum test on the system, it closes this vent valve so no air can enter, or escape.
as much as i say bad stuff about engineers.......these engineers that have designed these systems, are top of the line in my book. they overcame a LOT of problems to figure this crap out.
the designers on the other hand(sorry shirly) that decide that under the car directly behind the drivers side back tire is a nice place to mount this vent control valve.......well...i got a few customers that'd like to have some words with him.......
anyway.....now for a few pics......
this one is an early ford set up. i had forgotten about TAB and TAD. those things used to drive me nuts.....but i liked working on that stuff, and most other mechanics i've worked with HATED working on fords, due to the lack of data stream...so i got them.....
this one is from an 8 mustang gt......
this one is from an f-150.....
this one brings me to something i forgot above......the LDP. some makes use the LDP(Leak Detection Pump) in order to pressurize the EVAP system, and i believe this also can pull a vacuum in it. chrysler is big on using this set up. they also go bad a LOT.
this is just a generic pic to show a system overview.
now......i fully understand how one can make the mistake you're making. it's very easy to do. but you have to remember.......something has to enter the tank as something is removed. if not, the tank will go to a negative pressure, and a couple of things will then happen. first off, the pump will become unable to overcome the vacuum in there, and the engine will shut down due to fuel starvation. the tank will cave in on itself. the worst one i saw, required replacement of the tank. the more common occurrence was that once you allowed air to enter, then the tank would kind of "pop" and resume its normal shape.
and finally.....earlier, you mentioned that you were staying civil towards me, and i do seriously appreciate that. these kinds of discussions can take bad nasty turns rather quickly.
one final note too......on the fuel pump issue. look at the size of the frame mounted pumps as compared to the in-tank units. they're nearly double the size. the smaller pump cannot shed heat fast enough to be mounted outside of the tank. this is also why you'll note a higher failure rate among customers that tend to run their tanks below 1/4 of a tank.
