Vapor lock or something else?

I have a Quadrajet carbed 454, basic factory setup: stock mechanical fuel pump with heat shield-wrapped rubber fuel line (away from the block), factory-style tank and factory style sending unit with 2 fuel lines running the frame rail of the truck.

Two issues/questions:

- When the outside temperature is hot, after driving the truck for 20 mins or more and stopping, I can hear hissing coming from the tank/sending unit. The sending unit is the factory-style, 3 outlet kind, with the middle outlet as the vent. Is the hissing sound normal for hot days, or is something not working right?
- Also on hot days, after driving the truck for awhile, sometimes the engine will just die. Generally happens after driving, and then sitting in a drive-thru, sitting at a stop light, etc., and then when I try to accelerate again, it dies. I can immediately start it again by cranking the engine and pressing the accelerator pedal about half-way; when it starts, I rev it a couple of times, and I’m good. Is this vapor lock, or something else?

Any tips on either of these?

Sounds like blockage in the vent line to the charcoal canister. I could be all wet as 74’s don’t have that emission device.

Thanks. I should have specified my year, it’s an 84 C20, and I don’t have the charcoal canister.

If you hear a hissing sound from there, you need to drop the tank and remove the sender/pickup and check for cracks at the seam and install a new O-ring seal. Also, when the tank is out, clear your return and vapor line.

If charcoal can is gone that's a problem if not converted correctly. Vented and non-vented caps, return line from pump 2- or 3-line senders. It might be easier to install a charcoal can than pulling tanks and changing the pump. Other than adding the complexity of a few hoses those cans do not affect HP. Really don't understand why some emission items are yarded off.

By chance was the fuel pressure and volume checked? Could be more than 1 problem.

Thanks. I don’t remember ever having the charcoal canister (it may have been removed prior to my getting the truck), but I don’t think it had one. What is its purpose? I haven’t checked the fuel pressure/volume; is there an easy way to do that?

84 would have vapor recovery: charcoal can. To install one need to reference a vacuum diagram and install the missing parts. Some diagrams are available here.

Need a pressure gauge, check wherever accessible usually at the carb inlet. 5-6 lbs is decent. Not sure what the GM spec was. Volume, disable ignition to prevent spark. Using a hose in a small container crank the engine for 10 -15 seconds. Should have at least a cup or more fuel and a good fuel pulse.