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My 1975 F250 4x4 has been overhauled with a new motor and the old one junked a long time ago. My current fuel system is the factory in cab 20 gallon tank which runs directly from the hard line up into the engine bay to a fuel filter and directly to the carburetor.
I assumed I needed a gas cap for a non-evap emission truck, so that's what I ordered. I put on the new cap which causes some pressure in the tank (large purge of relief when I unscrew the cap).
I only ran the truck for 30 seconds or so but stopped as the vacuum was a concern. I attached a picture of my filler neck and old vs new cap. Should I be using a cap for a truck with evap emissions even though I have no smog equipment on this truck? Thanks.
Is it building "pressure" .... or is it creating a "vacuum"? The fuel pump is drawing fuel out of the tank, so it'll create a vacuum unless air is let in. If the tank is builing pressure inside, then likely is cool gasoline going into a tank and warming up. A fuel pump isn't gonna suck a tank to implosion, nor is it gonna cause it to explode.
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