if your Charcoal Canister is bad..when you go to put gas in..and you unscrew the gas gap and a LOT of pressure is vented.

 

Before I got a locking cap which I believe is vented I could never tell if the air was rushing in or out. I never spent much time on it. Dan

 

I've seen this question asked quite a bit over the years.

Never have seen a definite answer to it.

Lots of word salad but never an outright, definite answer.

 

If the system is functioning correctly, any pressure differential would be to flow into the gas filler neck.Everything in the vent system is via vacuum not positive pressure.

 

the gas cap can let air in but not out its a one way check in most older vehicles
The fumes I believe head to the charcoal canister which is regulated via solenoid.

On most old cars its a simple on/off solenoid (flow or not flow) triggered to flow during specific situations where the fuel vapors could be ingested by the engine without any consequence to operation.

I am not sure onthe C4 though and Iam unclear what happens when the pressure is very high for a long time during engine shut off, what would happen in that case.

So could your symptom be related to canister? sure. or the solenoid controlling it. Or some other check valve I don't know exists (Check the diagrams)

 

Pointer: Your choice of color for your text does not work in Dark mode, ends up gray on gray.

See:

 

There is a Tank Pressure Control Valve. It opens at .7psi pressure to allow vapor to flow to the cannister. The cannister purge valve opens at 2MPH, throttle not zero to allow the cannister to evacuate into the engine for combustion.

.7 psi doesn't seem like a lot, but with a low fuel level there is a large volume of vapor at .7 psi that vents by the cap when released. With a higher liquid level, there is not as much vapor volume to vent by the cap. Tank level affects how severe the 'woosh' is when the cap is released.

When the engine is running a large volume of fuel is returned to the tank after being heated by pressurization to 45-ish psi and absorbing heat from the fuel rails. This heated fuel is the primary source of the tank's pressurization.

The above, including the ",7 psi" setting is true for the 91-96. The 84-90 also has a Tank Pressure Control Valve located adjacent to the cannister, but the FSM does not stipulate a pressure for it. The 84-89 strategy has a purge solenoid, but there is also a requirement for ported vacuum to operate the vent and pressure control valve. The throttle plate must be open enough to present the port to manifold vacuum. Also, at WOT, ported vacuum will too low to allow the valves to remain open.

If the engine has been running, or the car driven, or conditions present to pressurize the tank to .7psi, and there is a release of this pressure when the cap is removed, there is nothing wrong with the system. It is operating normally. The only thing "wrong" is a misunderstanding of what is normal and how the tank vent and cannister system is designed to function.

 

Thanks!

 

The issue is ... a *lot* more than 0.7psi?