Wrong Fuel Cap:

[40ford]

I took a friend of mine to pick up a super nice 1950 Mercury coupe yesterday he had purchased..When we got back to his house and looking at car I noticed fuel cap which I thought was not correct.When I took it off there was a lot of pressure released.What kind of problems does this create?I got some photos I will try and post,i know everyone like pictures!

[Flathead Fever]

Was it pressure or was it a vacuum when you released the cap? I would think that after driving with a non-vented cap you would create a vacuum in the tank. It should be a vented cap so that it cannot build up pressure on not days when the car is sitting and the fuel heats up and the vapors need to vent. Then when the engine is running the cap needs to be vented to allow air to flow into that tank to replace the fuel being pulled out by the pump. If air cannot flow into the tank the fuel pump will not be able to pull fuel out. You need a new "vented" cap. If the tank did have pressure in it after driving, make sure the exhaust is not to0 close to the tank and heating up the fuel.

Modern cars have non-vented gas caps, which are actually vented by a valves in the caps. The valves will open when the vacuum or pressure exceeds the spring tension of the valves. It only takes something like 1 lb. of pressure and the cap will open and vent excessive pressure. By being closed most of the time it prevents hydrocarbons (unburned fuel) from escaping into the atmosphere

[40ford]

It felt like a lot of pressure to me.Thanks FlatheadIMG_3120.JPG

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[petehoovie]

[QUOTE=40ford;2225759]It felt like a lot of pressure to me.Thanks FlatheadAttachment 516059

[Karl Wescott]

Immaterial. The fuel system HAS to be vented whether through the cap or other arrangement. Cool gasoline gives off vapor and if contained increases pressure when warming, and condenses vapor and creates a vacuum when cooling, and the fuel pump creates a vacuum when drawing fuel out of the tank. If you have pressure when you release the cap you have a potentially explosive cloud of flammable vapor looking for a source of ignition (cell phone, cigarette, static electricity spark etc), or you could blow a hose off a fitting with equally catastrophic results. With vacuum and a strong enough fuel pump you could collapse the tank, or just quit running because fuel quits moving.
Whether its the wrong cap, the vent hole in the cap is blocked, or another vent system blocked there is a problem that needs to be fixed.

[APDAVE]

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Nice 50 Mercury! Dave

[JayChicago]

Karl
You said " a source of ignition (cell phone, cigarette, static electricity spark etc)"

I am curious how a cell phone can be a source of ignition. I use one often, don't want to blow-up something some day.

[deuce_roadster]

Good explanations by Flathead Fever and safety info from Karl. You need to identify what the problem is and I vote for either a plugged vented cap or a non vented cap on a car that needs one.

[paul2748]

You and your friend were lucky. Due to the buildup of pressure the engine might have quit (no gas being pulled through)or sucked in the gas tank

Originally Posted by 40ford

I took a friend of mine to pick up a super nice 1950 Mercury coupe yesterday he had purchased..When we got back to his house and looking at car I noticed fuel cap which I thought was not correct.When I took it off there was a lot of pressure released.What kind of problems does this create?I got some photos I will try and post,i know everyone like pictures!