I recently recharged my dead battery in anticipation of taking the car for a spin and was talking with the previous owner of the car who told me to release the gas cap if the car didn't start up immediately. The car didn't so I did and was surprised at how much pressure was released.

This doesn't seem quite right and I've been curious what would be causing the build up or is this something that should be happening?

well did the car start after you released the gas cap?

the fuel system is pressurised so there should be a release of pressure when you open the cap. If there isn't a whoosh - not imo an excessive whoosh but definitely noticeable - you should investigate whether the two check valves in the return air line are functioning. One is plugged in the inside of the left front fender (looking forward fron the seat in my car, which is rhd), the other is connected to the fuel tank under the car. The way it works is there's an airline that returns to the tank after the fuel pressure regulator via a charcoal air filter to reduce emissions. the return line has two check valves to allow one way passage of air to maintain a positive pressure in the system. Something like that, anyway. try a search on fuel tank pressure to find out more.

the cap is designed to release pressure at an excessive level. if it isn't doing so and you're worried about the pressure then you could try swapping it with a new one. Support this site and buy the proper one from Fastlane, don't try buying a cheapo from your local parts store, get the one that's right for your car.

if you get round to investigating the check valves, take a look at your charcoal filter inside the front left fender. they are supposed to be replaced evey four years but apparently rarely are. I've just had some major issues to deal with because mine was rusted and full of water due to a broken run off hose from the windshield drain. i was getting a full load of water and vapour dumped into my tank which contaminated the whole FI system, Believe me, you do not want that to be happening.

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1987 300e manual 250,000 km (sold)