95 taurus 3.8 gas smell

Dad says he smells gas ever once in a while,no pattern.

2 questions

#1 If charcoal canister is saturated will it cause a gas smell? I do not think so but am not sure.

#2 Say you stop for gas engine off and take cap off and hear a whoosh a little pressure in tank,say 2 pounds. Does this increase when driving down the road?

dad thinks if cap goes whoosh it is ok and he maybe right.

Any problem in the evap system can produce gas smell - can dad tell whether it is the smell of fumes or the smell of raw gasoline? A smell of raw gasoline would not be good and I’d want to stop driving the car until I figured out the leak.

Other than that find & inspect the evap hoses. They’ll come poking thru the firewall somewhere and lead to the canister.

When sitting or when driving down the road gas will evaporate - the fumes should be fed thru the evap system. A little pressure in the tank will occur, but it should never build up to high. If it does build up a lot then there is a blockage somewhere in the evap system.

#1 Yes, a saturated charcoal bed in the canister will cause a gas smell. The “bed” if what the gas tank breaths through, and it is exposed to the outside air.

#2 Yes, a small amount of vacuum and a whoosh is normal. The system is designed to maintain a small amount of vacuum to prevent gas funmes from escaping to the outside air. It does not increase when driving down the road. As gas is pumped out of the tank, it’ll allow a slight vacuum to exist in the resulting air space above the fuel. That is the design intent. As a matter of fact, when your dad starts the engine a “purge valve” is opened that allows the engine’s vacuum to breath in any fumes entrapped in the charcoal canister and to create a slight system vacuum.

Your dad’s car also has a monitoring system that’ll trigger a Check Engine Light if the pressure becomes positive.

There are numerous possible reasons why he might smell gas occasionally, none of them normal. My wild guess would be a bit of flooding from a falky injector.

Thanks for the good info.
Does canister have a relief valve for when it gets full or gets to much pressure?
it would seem if not it would smell all the time.
I had a 1980 280z and it had a short rubber gas line from metal tank line to injector and 1 of them had a pin hole but have not been able to get to dads yet to check.

The canister does not have a “relief valve”. It does not build pressure.

The canister is simply a path that allows the gas tank to breath in air as the gas is pumped out and forces any fumes that might eminate from the gas tank to vent through an activated charcoal bed. “Activated” charcoal is acid-treated to make in pourous, dramatically imcreasing its surface area per volume. Since carbon bonds very well to carbon, the hydrocarbon molecules stick to the surface of the charcoal in the canister as the fumes move through, becoming entrapped. Charcoal is carbon.

When the engine is started, the “purge valve” is opened and allows the fumes in the charcoal bed to be drawn in by the engine and burned. An engine’s normal operation of the pistons pulling air in for their “intake strokes” creates the necessary vacuum.

While an operating engine normally maintains a slight vacuum in the system, the charcoal bed is needed to allow escape of fumes when the tank is filled and when the gas in the tank expands due to thermal expansion or agitation. Gas, like all matter, expands as it warms, and agitated fluids take up more space than stationary fluids. The EPA requires that virtually no hydrocarbon molecules be allowed to escape into the atmosphere during any of these situations, Thus, the fume-catching charcoal.

A “relief valve” that allowed escape of fumes would violate the EPA mandate forbidding escape of hydrocarbons into the atmosphere.

If canister is bad I still cannot get in my head where gas smell “leaks” out?

Gas is highly aeromatic. The charcoal bed is exposed to the outside air. A gas saturated charcoal bed smells of gas just as a gas saturated sponge would.

Ok that helps.
Thank you all a bunch!