Howdy...
I've got a 69' Camaro with a 454.
Note 1: Over the past year or so, on occasion, I will run the car to town and back, and when I shut it off, it doesn't shut down cleanly, then after rattling around for some time, it finally shuts down and spews a lot of fuel down the carb (for about 30 seconds).
Note 2: Apparently the previous owner had backed into something to the point where the gas tank had a small dent in it. Last week, I took the car to town, and when I got back, I noticed a small crack in the gas tank where the dent had been. I shut the car down (note 1 above happened). I looked under the car at the gas tank while opening removing the gas cap... there was a mass amount of pressure released and I could visually see the tank supress back into shape where the crack was in the tank as if it was imploded/expanded abount an inch or so.

My question is this... Outside of temperature change, or gas fumes from driving around town... what would create that much pressure in the tank? Should there be some kind of vent/vapor release out of the sending unit? The gas cap I have is vented (one way only ~ into the tank). How does the pressure escape the tank to not force fuel into my carb after I shutdown the engine?

Thanks in advance
Terry

 

You're correct, the gas cap only vents in, not out. It's natural for the tank to build up pressure, and that's what keeps the fuel pump primed. But the gas cap should release if the pressure inside the tank gets too high. I once tried replacing my old gas cap with a shiny new (still vented) one from the parts store. But, my tank built up so much pressure that gas was coming out past the sending unit. I ended up putting the old gas cap on, and lost trust in the replacement cap's "venting" ability.
As far as your engine running on after you switch it off, that's not a gas problem. It wouldn't run if you weren't getting spark, or detonation. The problem could be your timing setting, hot spots from carbon build up, or an ignition issue. What ignition are you running, and do you happen to be running an electric radiator fan? If an electric fan isn't wired the way it should be, it can back-feed power to the ignition while it's still spinning down and cause run-on. Mine absolutely will run-on like clockwork if I don't switch the fan off before turning off the engine (yes, it's not wired the way it really should be).

 

Hi, actually... I found out the gas caps do vent out if there is a specific amount of pressure to open the diaphram. I could suck in on the cap pretty easily, but had to blow out with a good amount of pressure for it to open. So, I finally recieved the tank this week, so I'll be replacing it this weekend. I've got a new gas cap too. The old one won't allow air out (turned purple in the face trying to get it to open).
I'm running a Mallory electronic ignition/coil.
The fan does shut right down when I kill the power... no run on at all.

Thanks for your feedback.
Terry