so my gas tank has pop'd in out and whatever after kill car or while driving car etc. and has done so for many a years now. but now that have extra cash and working on the car ready to fix this thing.

ive read alot.. and so far my only conclusion for me etc is charcoal canister
i read on the repair manual etc what it does and what its connected to etc with the haynes manual. and i mean it seems like my best bet that after 21 years or even less this thing got clogged or is needing to be replaced.
or... could it be these as well which i havent checked..
The lines connecting canister to fuel tank..
ported vacuum and intake minifold vacuum..



So its 120$ to get a new one.
could go ahead see if it works and if not check those other things like the lines. though i just fixed all my vacuum lines i thought...

 

what year of car is it? I am by no means a professional but here is some information you may already have and if not it may be handy...I have a 89 camaro and I have been looking at the canister also and have tested the system using the gm shop manual. Here's what they say... fuel vapor canister purge valve... apply short length of hose to lower tube of purge valve and attempt to blow through it. Little or no air should pass into the canister. With hand vacuum pump apply 15"hg to upper tube the diaphragm should hold the vacuum for at least 20 seconds. If not the canister must be replaced. If the diaphragm holds the vacuum again try to blow through the hose connected to the lower tube while vacuum is still being applied. An increased flow should be observed. If not the canister must be replaced... ALSO there is a fuel tank pressure vavle in the line going to the fuel tank just a few inches from the canister it can also fail you can test it by putting vacuum to its small tube (15"hg) and it should hold for 20 seconds if not replace it. If it does hold the vacuum you should be able to blow through it with the vacuum applied from the tank side. As well your gas cap acts like a pressure vacuum relief and it may have to be replaced as well.

 

This is an interesting one. With what you described with the gas tank popping in and out make me think that a vacuum is being created in the tank. That being the only reason for a tank to pop inwards. So before you think about the charcoal canister and purge valve, I'm betting they are working fine, I would look at the vent valve an the gas tank. It is a one way vent that allows air only to enter into the tank, not out of.
This vent valve is located in front of your fuel tank next to your fuel lines.

 

Do yourself a favor, search for bulletins on your specific make and model. There is a bulletin out there, from GM, in regards to the vent or purge valve (I don't remember specifically which one it was). The bulletin pertains to your exact symptom. I have the same issue with mine and I remember coming accross the bulletin years ago and just never printed it out. My car is a constant project and all of my parts have been changed in the process anyway. Good luck.

 

Haynes manual needs a round hollow binder, so you can hang it in the 'Library', where the throne has a BIG hole in the middle.

Tear a page, read, wipe. Repeat if [since] necessary.



'Krinkle', between 'read, and 'wipe', if you're tender in the hindquarters

ed.:
and it's good enough, regardless of your 'style' - front to rear, rear to front, sideways, pinch, blot, swirlie, etc.,