I have 1990 chevy pickup 1500 4 wheel 350 eng. throttle body inj. the truck started missing and stalling i changed fuel filter and it ran good for 2 days then started the same thing so i changed fuel pump it ran good for about 2 miles then it sat for about 2 hours when i started it started doing the same i barely made it back home. when i took off gas cap it had worst vapor lock i've ever seen (it did the same before i changed the pump) also there was gas gushing out of charcoal cannister any help please. frank.

It sounds like the engine is pulling fuel from the tank, and not replacing it with air, causing a vacuum to build in the tank. The vacuum, may build to the point that it resists the fuel pump's efforts to **** fuel from it. If the vacuum is sufficient, it may collapse the tank, or expand the fuel enough for the engine manifold vacuum to **** raw gas into the evaporative emissions control system, hence the cannister.

Replacing the fuel pump, and filter will both break this vacuum, and make it run well for a few more miles, until the vacuum builds again.

The tank is usually vented to relieve pressure and vacuum through the gas cap.

Basically, sounds like a bad gas cap.

Try driving with the cap loose or removed. If that fixes it, buy a new cap. Preferably a locking one, as the amount of fuel these trucks hold is pretty tempting to less than honest folks. My 1990 C2500 has a 34 gallon tank.

Coil?? with the other symptoms, how'd you come up with that?

Sounds like you need a new coil get it checked its free. they sometimes stop workin when they get hot just before they die
Jay