It has to be positive pressure. I think the vent line is completely closed probably in the canister. This causes fuel to be pushed out of the tank through the return line from its tube in the bottom of the tank to the canister.
Let the gas tank pressurize up and disconnect the tank vent line at the canister.
If it hisses as you slide the hose off the tube, the canister is bad and shoud be replaced.
If it does not, there is an obstruction or kink at the tank or the vent line.
Tom and Ray may be right about the need to drop the tank. They went off target about it being a vacuum issue.
My bet it is a bad evaporative (condensing) canister.
They don’t last forever
I wonder if the leaking exhaust system is heating the fuel tank causing the gasoline to boil and get into the evaporative canister. I’ve seen this many years ago working at a gas station. The can drove from the interstate and I heard a rumbling from the rear of the car and when I opened the gas cap it vented hot fumes! The exhaust pipe was leaking directly onto the gas tank.
The new exhaust does not have necessary heat shields, and is heating the fuel. The evap sys can’t keep up, and is now ruined. It is likley to ruin the tank, i.e. melt or burn a hole in it. Problem is extremely dangerous.
Listening to the “answer” provided to Betsy brought back to me Andy R.'s immortal words:
“the intricacies, the gives and takes, the wherefores and why-nots, can become a veritable pas-de-deux of breathtaking speculation, interwoven in such a way that apologies or gestures of doubt are rendered unnecessary.”
All of the above comments about the canister are good and should be checked. There is another possibility; it was never said weither the vehicle was carburated or fuel injected. On the outside chance that it’s fuel injected,. most high pressure and electric fuel pumps have a return line that goes back to the tank. One function of this line is to keep from overpressurizing the fuel injection system. If the return line came loose or is split, broken or blocked; this would also account for a fuel leak at idle or low speed. I’m trying to think outside the box a bit. When troubleshooting you start with the most likely and work to the least likely until you find the problem. Good luck with this one.
The gas cap has a one way vent valve which is to relieve VACUUM. This is to prevent tank collapse if there is a vacuum in the tank. Relieving PRESSURE at the cap would completely negate the entire purpose of the EVAP control system.
Positive pressure is relieved through the purge valve in the charcoal canister. If the purge valve opens, the pressure is then blocked by the TVV (Thermo Vacuum Valve), which opens when the engine temp reaches about 130F. If the purge valve and TVV are both open, then the pressure is blocked by the purge port in the throttle body which opens when the throttle is opened past a certain point. If all of these valves are open, then a blockage in any of the hoses between the charcoal canister and TVV, or the TVV and the throttle body, could be the problem. There is a very remote chance that the throttle body port valve could be plugged.
It might be possible to work back from the throttle body, removing one hose at a time, to locate the source of blockage.
Re. “mud chiggers:” I found no such thing in a Google search. I question the existence of “mud chiggers,” never mind how they might get into a fuel line.
Could it be that the cannister is just full of fuel from overfilling (or exhaust heating as previously suggested) and therefore not working as expected to vent fuel? I can’t imagine how fuel would leak from the cannister unless it’s full of fuel, vs. just capturing the vapor it’s designed to capture.
I believe that she said there was pressure coming out of the tank. I think the boys mentioned the canister. She was not specific about exactly where the fuel was leaking. I think the clue was in the exhaust being replaced and her moving to New Orleans. If the exhaust is missing heat shields or was routed too close to the gas tank, the gas in the tank will expand and pressurize the tank. The fuel will leak out the weakest link. Also this condition would be much worse in the warm weather of New Orleans.
I think this was a mid-90’s Toyota pickup, right? I had a Toyota pickup once and it had fuel starvation issues at freeway speeds. It ran fine otherwise, but would often have a fuel smell. Turns out the lines on the fuel pump had been reversed by a previous repair job. If I recall correctly it was the lines coming from the fuel tank to the fuel pump and cannister that had gotten reversed. It was trying to draw in the vent line and vent through the fuel intake line. Interestingly this was enough fuel at low speeds to keep everything working, just had problems at high speeds.
It could be that the mechanic pulled the fuel lines to clear them and then put them on backwards. The only way I figured out the problem on my truck was to pull the lines and stick the air compressor nozzle on them one at a time. I was troubleshooting it as though it was a blocked line, and was surprised to hear the air coming out of the wrong place. Might be a similar issue here, if the mechanic hooked them back up wrong after removing the “mud chiggers”. (I suspect it was a different source of fuel line or vent hose blocking, but either way the same mistake could have been made.)